12. Aromaticity – Advanced Organic Chemistry

12

Aromaticity

LEARNING OBJECTIVES

By the end of this chapter you should be familiar with

  • The concept of aromaticity, non-aromaticity, anti-aromaticity and homoaromaticity.
  • The annulenes and their aromatic character.
  • Aromaticity in charged rings, fused ring systems and heterocyclic rings.
12.1 INTRODUCTION

Organic compounds have been classified on the basis of their structure. Open-chain structures containing carbon–carbon single (C–C), double (C=C) and triple bonds are said to be aliphatic compounds. The term ‘aliphatic’ is derived from the Greek word for fat or oil. In alicyclic molecules, the carbon atoms form a cyclic structure. The generic name aromatic was given originally to a structurally diverse collection of compounds, which had one common property, a fragrant odour. The discovery of benzene in 1825 by Faraday (isolated from whale oil) and 1845 by Hofmann (from coal tar) may be considered to institute the constitutional study of aromatic compounds. The term ‘aromatic’ acknowledges the fact that many fragrant compounds contain benzene rings. Aromatic compounds are cyclic molecules having delocalized π-electrons. Aromatic compounds may be carbocyclic (ring containing only carbon atoms) or heterocyclic (ring containing one or more heteroatoms). Heterocyclic compounds may be aromatic or alicyclic. For a compound to be classified as aromatic, it must fulfill both of the following criteria.

  1. It must have an uninterrupted ring of p orbital-bearing atoms so that it has an uninterrupted cloud of delocalized π-electrons above and below the plane of the molecule, meaning the molecule must be cyclic and planar.
  2. The π cloud must contain an odd number of pairs of π electrons.
12.2 CONCEPT OF AROMATICITY

The meaning of the word aromaticity has evolved, as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C–C, and C–H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules.

Aromaticity is usually described in molecular orbital terminology. Cyclic structures that have a particularly stable arrangement of occupied p molecular orbitals are called aromatic. A simple expression of the relationship between a molecular orbital description of structure and aromaticity is known as the Hückel rule. The German chemist Erich Hückel was the first to recognize, in 1931, that an aromatic compound must have an odd number of pairs of π electrons. It is derived from the Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2π electrons. The Huckel rule can be applied to charged as well as neutral systems.

Simple Huckel calculations on benzene place all the π electrons in bonding molecular orbitals. The π-electron energy of benzene is calculated by summing the energies of the six π electrons, which is 6α + 8β, lower by 2β than the value of 6α + 6β for three isolated double bonds. Thus, the HMO method predicts a special stabilization for benzene.

Figure 12.1 Energy levels of HMOs of cyclic π system

The lowest-energy α+2β bonding orbital has no nodes, and the two degenerate α+β bonding orbitals each have one node. The degenerate α–β antibonding orbitals have two nodes, and the α–2β orbital, three nodes. The pattern of two half-filled degenerate levels persists for larger rings containing 4n + 2 electrons. In contrast, all 4n + 2 systems are predicted to have all electrons paired in bonding molecular orbitals with net stabilization relative to isolated double bonds. This pattern provides the theoretical basis of the Huckel rule.

Attempts to describe just how stable a given aromatic molecule is in terms of simple HMO calculations have centred on the delocalization energy. The total π-electron energy of a molecule is expressed in terms of the energy parameters α (the coulomb integral) and β (the resonance integral) that arise in HMO calculations. This energy value can be compared to that for a hypothetical localized version of the same molecule. The HMO energy for the π electrons of benzene is 6α+8β. The same quantity for the hypothetical localized model cyclohexatriene is 6α+6β, the sum of three isolated C=C bonds. The difference of 2β is called the delocalization energy or resonance energy. Although this quantity is often useful for comparing related systems, it is not a measurable physical quantity; rather, it is obtained by comparing a real molecule and a hypothetical one. Most estimates of the stabilization of benzene are in the range of 20–40 kcal/mol and depend on the choice of properties assigned to the hypothetical cyclohexatriene reference point.

There have been two general approaches to determining the amount of stabilization that results from aromatic delocalization. One is to use experimental thermodynamic measurements. Bond energies are nearly additive when there are no special interactions between the various bond types. Thus, it is possible to calculate such quantities as the heat of combustion or heat of hydrogenation of ‘cyclohexatriene’ by assuming that it is a compound with no interaction between the conjugated double bonds. For example, a very simple calculation of the heat of hydrogenation for cyclohexatriene would be to multiply the heat of hydrogenation of cyclohexene by 3, that is, 3 × 28.6 = 85.8 kcal/mol. The actual heat of hydrogenation of benzene is 49.8 kcal/mol, suggesting a total stabilization or delocalization energy of 36.0 kcal/mol. There are other, more elaborate, ways of approximating the thermodynamic properties of the hypothetical cyclohexatriene. The difference between the calculated and corresponding measured thermodynamic property of benzene is taken to be the aromatic stabilization. For benzene, the values obtained are usually around 30 kcal/mol, but the aromatic stabilization cannot be determined in an absolute sense because these values are established by the properties assigned to the cyclohexatriene model.

The second general approach to estimating aromatic stabilization is to use molecular orbital methods. This has already been illustrated by the discussion of benzene according to simple HMO theory, which assigns the stabilization energy a value of 2β units. More advanced molecular orbital methods can assign the stabilization energy in a more quantitative way. The most successful method is to perform calculations on the aromatic compound and on a linear, conjugated polyene containing the same number of double bonds. This method assigns a resonance stabilization of zero to the polyene, even though it is known by thermodynamic criteria that conjugated polyenes do have some stabilization relative to isomeric compounds with isolated double bonds. Using this definition, semi empirical molecular orbital calculations assign a value of about 20 kcal/mol to the resonance energy of benzene, relative to 1,3,5-hexatriene. The use of polyenes as reference compounds has proven to give better agreement with experimental trends in stability than comparison with the sums of isolated double bonds.

Both thermochemical and molecular orbital approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy that is assigned. It is very significant that molecular orbital calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated noncyclic polyene models are called antiaromatic.

There are also physical measurements that can give evidence of aromaticity. The determination of the bond lengths in benzene by electron diffraction is a classic example of use of the bond-length criterion of aromaticity. Spectroscopic methods or X-ray diffraction can also provide bond-length data. Aromatic hydrocarbons show carbon-carbon bond lengths in the range 1.38–1.40 Å, and the bond lengths are quite uniform around the ring. In contrast, localized polyenes show alternation between typical sp3-sp3 single bond and sp2-sp2 double bond lengths along the conjugated chain. The uniformity of bond lengths has been developed as a criterion of aromaticity.

NMR spectroscopy also provides an experimental tool capable of assessing aromaticity. Aromatic compounds exhibit a diamagnetic ring current. Qualitatively, this ring current can be viewed as the migration of the delocalized electrons in the aromatic system under the influence of the magnetic field in an NMR spectrometer. The ring current effect is responsible for a large magnetic anisotropy in aromatic compounds. The induced ring current gives rise to a local magnetic field that is opposed to the direction of the applied magnetic field. Nuclei in a cone above or below the plane of an aromatic ring are shielded by the induced field and appear at relatively high field in the NMR spectrum, whereas nuclei in the plane of the ring, that is, the atoms bound directly to the ring occur at downfield positions. Antiaromatic compounds show opposite effects. The occurrence of these chemical shift phenomena is evidence for aromaticity. The chemical shift phenomena can be treated on a quantitative basis by quantum-mechanical calculation of the chemical shift at the centre of the ring. The value of the chemical shift at a point in the centre of the ring can be calculated. These values are referred to as the nucleus-independent chemical shift (NICS). These values show excellent correlation with other manifestations of aromaticity. Benzenoid hydrocarbons such as benzene, naphthalene and anthracene show values of about −9 to −10 ppm. Heteroaromatic five-member rings show slightly more negative values (pyrrole, −15.1; thiophene, −13.6; furan, −12.3). The values for aromatic ions such as cyclopentadienide (−14.3) and cycloheptatrienylium (−7.6) are also negative. Those for antiaromatic species, including cyclobutadiene (+27.6) and borole (+17.5), are positive. Saturated compounds such as cyclohexane have values near zero.

Another property associated with aromaticity is magnetic susceptibility. Magnetic susceptibility is determined by measuring the force exerted on the sample by a magnetic field. Magnetic susceptibility can also be determined using an NMR spectrometer. It is noted that aromatic compounds have enhanced magnetic susceptibility, relative to values predicted on the basis of the localized structural components. Magnetic susceptibility can also be calculated by computational methods.

It has been argued that there are two fundamental aspects of aromaticity, one reflecting structure and energy, and the other, magnetic properties and electron mobility. Parameters of aromaticity such as bond length and stabilization appear to be largely independent of the magnetic criteria, such as diamagnetic ring current. However, there is a correlation between the two kinds of measurements. The more stabilized compounds exhibit the greatest magnetic susceptibility. Aromaticity is thus best conceived as a single property resulting from cyclic delocalization that results in both stabilization and the magnetic phenomena associated with electron mobility.

The experimental resonance energies of polynuclear aromatic compounds such as naphthalene, anthracene and phenanthrene are found to be 61.0, 83.5 and 91.3 kcal/mol, respectively. Naphthalene is like benzene and behaves almost similarly. Phenanthrene, however, undergoes electrophilic addition with bromine, suggesting that greater resonance energy does not necessarily result in more aromatic behaviour.

12.3 ANTIAROMATICITY

An aromatic compound is more stable than the analogous acyclic compound. The planar cyclic 4n systems are referred to as antiaromatic. An antiaromatic compound is less stable than the analogous acyclic compound. Aromaticity is characterized by stabilization, whereas antiaromaticity is characterized by destabilization.

relative stabilities aromatic compound > acyclic compound > antiaromatic compound

A compound is said to be antiaromatic if a planar cyclic conjugated system contains an even number of pairs of π electrons. Antiaromatic molecules possess negative value of resonance energy and a small energy gap between their highest occupied and lowest unoccupied molecular orbitals. In antiaromatic molecules, an external magnetic field induces a paramagnetic electron current. It has been found that conjugated rings with 2, 6, 10 and 14π electrons are aromatic, whereas those with 4, 8, 12, 16 and 20 surely are not. Thus, it was found that the cyclopropenyl anion (A) is less stable than the cyclopropanyl anion (B), although the former is an allylic anion. The delocalized square-planar cyclobutadiene (C) is less stable than the localized rectangular form (D). The antiaromatic molecule will thus be in its ground state, but this will be of higher energy than would be calculated or found for a model system. Antiaromatic compounds can be expected to have antithetical properties to aromatic compounds.

Cyclobutadiene has two bonding electrons, but the other two electrons are unpaired because of the degeneracy of the two nonbonding orbitals. The two electrons in the nonbonding levels do not contribute to the stabilization of the molecule. Furthermore, because these electrons occupy a high-energy orbital, they are particularly available for chemical reactions. The experimental evidence indicates that cyclobutadiene is rectangular rather than square. This modifies somewhat the orbital picture from the simple Hückel pattern, which assumes a square geometry. The two-nonbonding levels are no longer degenerate, so cyclobutadiene is not predicted to have unpaired electrons. Nevertheless, higher-level molecular orbital calculations agree with the Hückel concept in predicting cyclobutadiene to be an extremely unstable molecule with a high-energy occupied orbital.

The cyclooctatetraene with 8π electrons is assumed to be planar. Three orbitals are bonding, three are antibonding, and two are nonbonding, that is, they have the same energy α as the original atomic orbitals.

The simplest 4n system is cyclobutadiene (in which n = 1), which is an extremely unstable compound. It can be stabilized by metal complexation. One method of stabilizing cyclobutadiene is by means of well-chosen substituents, for example, diethyl 2,4-bis(diethylamino)cyclobutadiene-1,3-dicarboxylate (1) is reasonably stable, melting at 52°C, and the ring is square planar. Benzoannelation has also been used as a method of stabilization. Biphenylene (2) has long been known, and 1,2-diphenylnaphtho (b) cyclobutadiene (3) is now known.

X-ray diffraction studies indicate that the two bonds connecting the benzo rings in biphenylene are of approximately single bond length, and hence we might expect that the four-member ring is an anti-aromatic singlet. The NMR spectrum has indeed recently been interpreted as lending some support to an anti-aromatic structure for the four-member ring.

12.4 ANNULENES

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. Conjugated monocyclic polyenes (CnHn), in which n is greater than or equal to 10, are usually called annulenes. The number in brackets tells us how many carbon atoms there are in the ring. The synthesis of annulenes has been extended well beyond the first two members of the series [4]-annulene (cyclobutadiene) and [6]-annulene (benzene). Considering the properties of members of the annulene series can test the generality of the Hückel rule. The annulenes prepared have n = 12, 14, 16, 18, 20, 24 and 30. Of these only [14]-, [18]- and [30]-annulenes are (4n + 2) π electron molecules, whereas the rest are (4n)π molecules.

The smallest annulene of the (4n + 2) π type is [10]-annulene. Van Tamelen et al. (1971) have synthesized it by photolysis of trans-9,10-dihydronaphthalene. There are three possible isomers of [10]-annulene: (A) all-cis, (B) mono-trans and (C) cis-trans-cis-cis-trans. In case of isomer A, where all five double bonds are cis and planar, each internal angle would be 144°. Since a normal double bond has angles of 120°, this would be far from ideal. So this compound cannot adopt a planar conformation and, therefore, is not aromatic even though it has 10π electrons. By contrast, [18]-annulene with 18π electrons (n = 4) adopts a planar conformation and is aromatic. NMR spectrum indicates that it has a ring current characteristic of aromatic compounds. The resonance energy from combustion data has been estimated to be 100 kcal/mol.

[14]-Annulene (I) follows the Huckel rule (14π electrons), is non-aromatic and unstable. This is attributed to the steric hindrance of the four internal hydrogens, which twist it out of planarity. However, monodehydro[14]-annulene (II) containing 14π electrons is planar and aromatic.

The smallest member, cyclobutadiene, was the object of attempted synthesis for many years. The first success was achieved when cyclobutadiene released from a stable iron complex was trapped with various reagents. Dehalogenation of trans-3,4-dibromocyclobutene was shown to generate a species with the same reactivity. Various trapping agents react with cyclobutadiene to give Diels-Alder adducts. In the absence of trapping agents, a characteristic dimer is produced. This dimerization is an extremely fast reaction and limits the lifetime of cyclobutadiene, except at very low temperatures.

Cyclobutadiene can also be prepared by photolysis of several different precursors at very low temperature in solid inert gases. These methods provide cyclobutadiene in a form that is appropriate for spectroscopic study. Under these conditions, cyclobutadiene begins to dimerize at around 35 K. Whereas simple HMO theory assumes a square geometry for cyclobutadiene, most molecular orbital methods predict a rectangular structure as the minimum-energy geometry. With very high-level calculations, good agreement is obtained between the calculated minimum-energy structure, which is rectangular, and observed spectroscopic properties.

The rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. For example, cyclobutadiene is found to have negative resonance energy of −54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. A number of alkyl-substituted cyclobutadienes have been prepared by related methods. Increasing alkyl substitution enhances the stability of the compounds. The tetra-t-butyl derivative is stable up to at least 150°C but is very reactive toward oxygen. This reactivity reflects the high energy of the HOMO. The chemical behaviour of the cyclobutadienes as a group is in excellent accord with that expected from the theoretical picture of the structure of these compounds.

[6]-Annulene is benzene. Its properties are so familiar to students of organic chemistry that not much need be said here. It is the parent compound of a vast series of derivatives. The benzene ring shows exceptional stability, both in a thermodynamic sense and in terms of its diminished reactivity in comparison with conjugated polyenes. As was discussed earlier, stabilization on the order of 30 kcal/mol is found by thermodynamic comparisons. Benzene is much less reactive toward electrophiles than are conjugated polyenes. This is in line with the HOMO of benzene being of lower energy than the HOMO of a conjugated polyene.

The next higher annulene, cyclooctatetraene, is nonaromatic. The bond lengths around the ring alternate as expected for a polyene. The C=C bonds are 1.33 Å and the C–C bonds are 1.462 Å in length. Thermodynamic data provide no evidence of any special stability. Cyclooctatetraene is readily isolable, and its chemical reactivity is normal for a polyene. Structure determination shows that the molecule is tub-shaped and, therefore, is not a planar system to which the Huckel rule applies. There have been both experimental and theoretical studies aimed at trying to estimate the stability of the planar form of cyclooctatetraene. The results indicate that the completely delocalized D8h structure is about 4.1 kcal higher in energy than the conjugated planar D4h structure, suggesting that delocalization leads to destabilization.

Figure 12.2 HMO π-energy level diagram of annulenes

Larger annulenes permit the incorporation of trans double bonds into the rings. Beginning with [10]-annulene, stereoisomeric structures are possible. According to the Huckel rule, [10]-annulene should possess aromatic stabilization if it were planar. However, all the isomeric cyclodeca-l,3,5,7,9-pentaenes suffer serious steric strain that prevents the planar geometry from being adopted. The Z,E,Z,Z,E-isomer, which has minimal bond-angle strain, suffers a severe nonbonded repulsion between the two internal hydrogens.

The Z,Z,Z,Z,Z-isomer is required by geometry to have bond angles of 144° to maintain planarity and would, therefore, be enormously destabilized by distortion of the normal trigonal bond angle. The most stable structure is a twisted form of the E,Z,Z,Z,Z-isomer. Molecular orbital calculations suggest an aromatic stabilization of almost 18 kcal for a conformation of the E,Z,Z,Z,Z-isomer in which the inner hydrogens are twisted out of the plane by about 20°, but other calculations point to a polyene structure.

Experimental studies have indicated that all of the isomers prepared to date are quite reactive, but whether the most stable isomer has been observed is uncertain. Two of the isomeric [10]-annulenes, as well as other products, are formed by photolysis of cis-9,10-dihydronaphthalene. Neither compound exhibits properties that would suggest aromaticity. The NMR spectra are consistent with polyene structures. Both compounds are thermally unstable and revert back to dihydronaphthalenes. It appears that [10]-annulene is sufficiently distorted from planarity that little stabilization is achieved.

A number of structures have been prepared that do not have the steric problems associated with the cyclodeca-1,3,5,7,9-pentaenes. In compound 4, the steric problem is avoided with only a slight loss of planarity in the π system. The NMR spectrum of this compound shows a diamagnetic ring current of the type expected in an aromatic system. Thus, its carboxylic acid derivative 5 is also prepared.

Most molecular orbital methods find a bond alternation pattern in the minimum-energy structure, but calculations that include electron correlation lead to a delocalized minimum-energy structure. Thus, although the π system in 4 is not completely planar, it appears to be sufficiently close, providing a delocalized 10-electron π system. Resonance energy of 17.2 kcal has been obtained on the basis of an experimental heat of hydrogenation.

[12]-Annulene is a very unstable compound that undergoes cyclization to bicyclic isomers and can be kept only at very low temperature. The NMR spectrum has been studied at low temperature. Besides indicating the double bond geometry shown in the structure drawn here, the spectrum reveals a paramagnetic ring current, the opposite to what is observed for aromatic systems. This feature is quite characteristic of the [4n] annulenes and has been useful in characterizing the aromaticity or lack of it in annulenes.

[14]-Annulene was first prepared in 1960. Its NMR spectrum has been investigated and shows that two stereoisomers are in equilibrium.

The spectrum also reveals a significant diamagnetic (aromatic) ring current. The signals of the internal hydrogens (C-3, C-6, C-10 and C-13) are very far upfield (δ = 0.61 ppm). The interconversion of the two forms involves a configurational change from E to Z at least one double bond. The activation energy for this process is only about 10 kcal/mol. The crystal structure for [14]-annulene shows the Z,E,E,Z,E,Z,E-form to be present in the solid. The bond lengths around the ring range from 1.35 to 1.41Å but do not show the alternating pattern of short and long bonds expected for a localized polyene. There is some distortion from planarity, especially at carbon atoms 3, 6, 10 and 13, which is caused by nonbonded repulsions between the internal hydrogens. A 14-electron π system can be generated in circumstances in which the steric problem associated with the internal hydrogens of [14]-annulene is avoided.

Several derivatives of this ring system have been synthesized. These compounds exhibit properties indicating that the conjugated system is aromatic. They exhibit NMR shifts characteristic of a diamagnetic ring current. Typical aromatic substitution reactions can be carried out. An X-ray crystal structure shows that the bond lengths are in the aromatic range (1.39–1.40Å), and there is no strong alternation around the ring. The peripheral atoms are not precisely planar, but the maximum deviation from the average plane is only 0.23Å. The dimethyl derivative is essentially planar, with bond lengths between 1.38 and 1.40Å.

The Huckel rule predicts non-aromaticity for [16]-annulene. The compound has been synthesized and thoroughly characterized. The bond lengths show significant alternation (C=C, 1.34Å; C–C, 1.46Å), and the molecule is less planar than [14]-annulene. These structural data are consistent with regarding [16]-annulene as being non-aromatic.

[18]-Annulene offers a particularly significant test of the Huckel rule. The internal cavity in [18]-annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most molecular orbital calculations find the delocalized structure to be more stable than the polyene.

The properties of [18]-annulene are consistent with its being aromatic. The X-ray crystal structure shows the molecule to be close to planarity, with the maximum deviation from the plane being 0.085Å. The bond lengths are in the range 1.385–1.405Å. The NMR spectrum is indicative of an aromatic ring current. At about −60°C, the protons outside the ring of [18]-annulene are strongly deshielded (δ 9.3) and those inside are strongly shielded (δ −3.0, that is, more shielded than TMS). Demonstration of such a ring current is good evidence for planarity and aromaticity, at least at low temperature. As the temperature is raised, the signals broaden because of slow interchanges in ring conformations. At about 110°C, a single averaged peak appears at approximately δ 5.3 because of rapid interchanges in ring conformations, to give an averaged chemical shift. The chemical reactivity of the molecule would also justify its classification as aromatic. The bond lengths are 1.39–1.42Å, and a stabilization of 18 kcal/mol is indicated.

The synthesis of annulenes has been carried forward to larger rings as well. [20]-Annulene, [22]-annulene and [24]-annulene have all been reported. The NMR spectrum of [22]-annulene is consistent with regarding the molecule as aromatic, whereas those of the [20] and [24] analogs are not. In each case, there is some uncertainty as to the preferred conformation in solution, and the NMR spectra are temperature-dependent. Although the properties of these molecules have not been studied as completely as those of the smaller systems, they are consistent with the predictions of the Huckel rule.

Both clever synthesis and energetic processes leading to stable compounds have provided other examples of structures for which aromaticity might be important. Kekulene was synthesized in 1978. How aromatic is this substance? Both by energy and magnetic criteria, it appears that it is primarily benzenoid in character. Its energy is close to that expected from isodesmic reactions summing smaller aromatic components. Magnetic criteria, too, indicate that it is similar to the smaller polycyclic benzenoid hydrocarbons, such as phenanthrene and anthracene.

It has been pointed out that a different array of atomic orbitals might be conceived of in large conjugated rings. The array, called a Mobius twist results in there being one point in the ring at which the atomic orbitals would show a phase discontinuity. If the ring were sufficiently large that the twists between individual orbitals were small, such a system would not necessarily be less stable than the normal array of atomic orbitals. This same analysis points out that in such an array the Huckel rule is reversed and aromaticity is predicted for the 4n π-electron systems.

Figure 12.3 Change in resonance energies for annulenes

Chemical reactivity data suggest that larger rings may be less aromatic than the smaller ones. If we take a large delocalization energy as a measure of aromaticity, then the distribution between what is aromatic and what is anti-aromatic becomes smaller and smaller with increasing ring size, and for a ring of around 26π centres, even a member of 4n + 2 series no longer is calculated to be more stable than its acyclic analogues.

12.5 AROMATICITY IN CHARGED RINGS

There are also striking stability relationships due to aromaticity in charged ring systems. These energy levels are applicable to charged species as well as to the neutral annulenes. A number of cations and anions that have completely conjugated planar structures are shown below. The Huckel rule predicts aromatic stability for cyclopropenium ion (A), cyclobutenium dication (B), cyclobutadiene dianion (C), cyclopentadienide anion (D), cycloheptatrienyl cation (tropylium ion, E), the dications and dianions derived from cyclooctatetraene (F and G) and the cyclononatetraenide anion (H). The other species having 4nπ electrons would be expected to be very unstable.

There is a good deal of information about the cyclopropenium ion that supports the idea that it is exceptionally stable. The cyclopropenium ion and a number of derivatives have been generated by ionization procedures:

The 1,2,3-tri-t-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete species. The heterolytic gas-phase bond dissociation energy to form cyclopropenium ion from cyclopropene is 225 kcal/mol. This compares with 256 kcal/mol for formation of the allyl cation from propene or 268 kcal/mol for formation of the 1-propyl cation from propane.

In contrast, the less strained four-π-electron cyclopentadienyl cation is very unstable. It is calculated to have negative stabilization energy of 56.7 kcal/mol. The cyclopentadienyl cation is also found to be anti-aromatic on the basis of magnetic susceptibility and chemical shift criteria. The heterolytic bond dissociation energy to form the cation from cyclopentadiene is 258 kcal/mol, which is substantially more than for formation of an allylic cation from cyclopentene, but only slightly more than the 252 kcal/mol for formation of an unstabilized secondary carbocation. Solvolysis of cyclopentadienyl halides assisted by silver ion is extremely slow, even though the cyclopentadienyl ring is doubly allylic. When the bromide and antimony pentafluoride react at −78°C, an EPR spectrum is observed which indicates that the cyclopentadienyl cation is a triplet, but the ground state of the pentaphenyl derivative is a singlet.

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a pKa of 16.0. The pKs of triphenylcyclopropene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. Thus, the six-π-electron cyclopentadienide ion is enormously stabilized relative to the four-π-electron cyclopropenide ion, in agreement with the Huckel rule.

The Huckel rule predicts aromaticity for the six-π-electron cation derived from cycloheptatriene by hydride abstraction, and anti-aromaticity for the planar eight-π-electron anion that would be formed by deprotonation. The cation is indeed very stable. Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pKa of cycloheptatriene has been estimated at 36. This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-member eight-π-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-π-electron hydrocarbon, cyclooctatetraene. The cyclononatetraenide anion is generated by treatment of the halide with lithium metal. The NMR spectrum of the anion, however, is indicative of aromatic character.

Several doubly charged ions have been observed experimentally. Ionization of 3,4-dichioro-l,2,3,4-tetramethylcyclobutene in SbF5-SO2 at −75°C results in an NMR spectrum attributed to the tetramethylderivative of the cyclobutadienyl dication. It is difficult to choose a reference compound against which to judge the stability of the dication. That it can be formed at all, however, suggests special stabilization associated with the two-π-electron system. The dianion formed by adding two electrons to the π system of cyclobutadiene also meets the 4n + 2 criterion. In this case, however, four of the six electrons would occupy nonbonding orbitals, so high reactivity could be expected. There is some evidence that this species may have a finite existence. Reaction of 3,4-dichlorocyclobutene with sodium naphthalenide, followed a few minutes later by addition of methanol-d6, gives a low yield of 3,4-di-deuterio-cyclobutene. The inference is that the dianion [C4H42−] is present. As yet, however, no direct experimental observation of this species has been accomplished. Cyclooctatetraene is reduced by alkali metals to a dianion.

The NMR spectrum indicates a planar aromatic structure. It has been demonstrated that the dianion is more stable than the radical anion formed by one-electron reduction, as the radical anion is disproportionate to cyclooctatetraene and the dianion.

The crystal structure of the potassium salt of 1,3,5,7-tetramethyl-cyclootatetraene dianion has been determined by X-ray diffraction. The eight-member ring is planar, with ‘aromatic’ C–C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. A dication derived from 1,3,5,7-tetramethyl-cyclooctatetraene is formed at −78°C in SO2C1 by reaction with SbF5. Both the proton and carbon NMR spectra indicate that the ion is a symmetrical, diamagnetic species, and the chemical shifts are consistent with an aromatic ring current. At about −20°C, this dication undergoes a chemical transformation to a more stable dication.

Reduction of the non-aromatic polyene [12]-annulene, either electrochemically or with lithium metal, generates a 14π-electron dianion. The NMR spectrum of the resulting dianion shows a diamagnetic ring current indicative of aromatic character, even though steric interactions among the internal hydrogens must prevent complete coplanarity. In contrast to the neutral [12]-annulene, which is thermally unstable above −50°C, the dianion remains stable at 30°C. The dianion of [16]-annulene has also been prepared and shows properties consistent with its being regarded as aromatic. It is consistent with the applicability of Huckel’s rule to charged, as well as neutral, conjugated planar cyclic structures.

 

Table 12.1 Hückel’s Rule Relationships for Charged Species

Compound π electrons
Aromatic species  
     Cyclopropenium cation   2
     Cyclopentadienide anion   6
     Cycloheptatrienyl cation   6
     Cyclooctatetraene dianion 10
     Cyclononatetraenide anion 10
     [12]-Annulene dianion 14
Anti-aromatic species  
     Cyclopropenide anion   4
     Cyclopentadienyl cation   4
Non-aromatic species  
     Cycloheptatrienyl anion   8
12.6 HOMOAROMATICITY

In 1959 Winstein introduced the term ‘homoaromatic’ to describe compounds that display aromaticity despite one or more saturated linkages interrupting the formal cyclic conjugation. Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The saturated unit is generally a CH2 group, but can be a larger alkyl residue or even a heteroatomic moiety. The resulting stabilization would, in general, be expected to be reduced because of poorer overlap of the orbitals. The properties of several such cationic species, however, suggest that substantial stabilization does exist.

Homoaromaticity is well established in cationic systems where delocalization of charge provides an additional driving force for homoaromaticity. The homocyclopropenium cation (I) is the simplest homoaromatic, and its derivative (II) was the first system proposed to be homoaromatic. These homoaromatic cations adopt a puckered geometry with a relatively short non-bonded 1,3- distance (~1.8 Å, consistent with homoaromaticity), a high barrier to inversion (Ia Ib, ~ 8 kcal/mol) through a planar non-aromatic transition state. The size of the barrier to inversion is taken as an indicator of the degree of homoaromaticity.

Roberts et. al. first proposed the enhanced stability of the bishomocyclopropenyl cation (III); numerous 3,5-bridged derivatives (IV) were prepared and fully characterized as homoaromatic, both by experiment and theory.

The homotropylium cation (V) and its numerous derivatives are the most extensively studied and well-established homoaromatics. V is the archetype no-bond homoaromatic species. It adopts a boat-shaped conformation with the seven-member ring not greatly deviating from planarity. There is no bond-critical point linking C1-C7, but there is a significant build up of electron density between these carbon atoms, which mediates effective (through-space) delocalization. The positive charge is evenly distributed over the seven-member ring, as evidenced by calculation, and 13C NMR chemical shifts for C1-C7. All estimates indicate that the homotropylium cation is more stable than the reference compound used and, therefore, support its homoaromaticity.

A significant feature of the NMR spectrum of this cation is that both C8 protons are shielded and exhibit sharply different chemical shifts; that is, the 8-endo proton resonates at higher field than tetramethylsilane. The NMR chemical shift difference between the 8-endo and 8-exo protons is large (5.86 ppm). Molecular orbital calculations and the effects of electron correlation indicate that the homoconjugated structure is a good description of the cation and find that there is a strong aromatic ring current.

The temperature-dependent NMR spectrum of the ion can be analyzed to show that there is a barrier (8.4 kcal/mol) for the ring flip that interchanges the two hydrogens of the methylene group. The 13C-NMR chemical shift is also compatible with the homoaromatic structure. Molecular orbital calculations are successful in reproducing the structural and spectroscopic characteristics of the cation and are consistent with a homoaromatic structure.

Childs, Cremer and Elia in 1995 considered that there may be examples of anionic homoaromatics. The bicyclo[3.2.1]octadienyl anion (VI) is the potential homoaromatic anion.

The existence of stabilizing homoconjugation in anions has been more difficult to establish. Much of the discussion has revolved about anion VI. This species was proposed to have aromatic character on the basis of the large upfield shift of the CH2 group that would lie in the shielding region generated by a diamagnetic ring current. The 13C NMR spectrum can also be interpreted in terms of homoaromaticity. Both gas-phase and solution measurements suggest that the parent hydrocarbon is more acidic than would be anticipated if there were no special stabilization of the anion. An X-ray crystal structure of the lithium salt has been done. The lithium is not symmetrically disposed toward the anion, but is closer to one carbon of the allyl system. There is no indication of flattening of the homoconjugated atoms, and the C(6)-C(7) bond distance is in the normal double-bond range (1.354 Å). Recently, many anionic compounds have been fully characterized as homoaromatic.

12.7 FUSED-RING SYSTEMS

Many completely conjugated hydrocarbons can be built up from the annulenes and related structural fragments. It is of interest to be able to predict the stability of such fused-ring compounds. Because Huckel’s rule applies only to monocyclic systems, it cannot be applied to the fused-ring compounds, and there have been many efforts to develop relationships that would predict their stability. The underlying concepts are the same as for monocyclic systems. Stabilization should result from a particularly stable arrangement of molecular orbitals, whereas instability would be associated with unpaired electrons or electrons in high-energy orbitals.

Figure 12.4 Some representative examples of polyenes

However, attempts to correlate stability with the Huckel delocalization energy, relative to isolated double bonds, give poor correlation with the observed chemical properties of the compounds. By choosing a polyene as the reference state, much better agreement between calculated stabilization energy and experimental chemical properties is achieved. Hess and Schaad have empirically established a series of energy terms corresponding to the structural units in the reference polyene. The difference between the energy of the conjugated hydrocarbon by HMO calculation and the sum of the energies of the appropriate structural units gives stabilization energy. For azulene, for example, the HMO calculation gives energy of 10α+13.36β. The energy for the polyene reference is obtained by summing contributions for the component bond types: 3(HC=CH) + 2(HC=C) + 3(HC–CH) + 2(HC–C) + 1(C–C) = 10α+13.13β. The difference, 0.23β, is the stabilization or resonance energy assigned to azulene by this method. For comparison of non-isomeric molecules, the Hess-Schaad treatment uses resonance energy per electron, which is obtained simply by dividing the calculated stabilization energy by the number of π electrons. Although the resulting stabilization energies are based on a rudimentary HMO calculation, they are in good qualitative agreement with observed chemical stability.

All these approaches agree that benzene and the structures that can be built up by fusing together benzenoid rings are strongly stabilized relative to the reference polyenes. The structures with more rings tend to have lower resonance energies per π electron compared to benzene. This feature is in agreement with experimental trends in reactivity. Because the structures with fewer rings are more stable, there is a tendency for species in which several rings are fused together to react by addition to an internal ring to give the smaller and more stable structures.

This trend is revealed, for example, by the rates of Diels-Alder addition reactions of anthracene, naphthacene and pentacene, in which three, four and five rings, respectively, are linearly fused. Benzene rings can also be fused in angular fashion, as in phenanthrene, chrysene and picene. These compounds, while reactive toward additions in the centre ring, retain most of the resonance energy per electron (REPE) stabilization of benzene, naphthalene and phenanthrene.

Azulene is one of the nonbenzenoid hydrocarbons that appear to have appreciable aromatic stabilization. Naphthalene is more stable than azulene by about 38.5 kcal/mol. The structure of azulene has been determined by both X-ray crystallography and electron-diffraction measurements. The bond shared by two rings is significantly longer, indicating that it has a predominantly single-bond character. The molecule has C2v symmetry indicating delocalization of π electrons. The molecule has dipolar structure as the fusion of a cyclopentadienide anion and cycloheptatrienyl cation. It has appreciable dipole moment (0.8D).

The possibility of extra stabilization in conjugated systems that have conjugated components exocyclic to the ring has also been examined. Some representative examples are as below:

12.8 AROMATIC HYDROCARBONS

There is evidence that aromatic segments can exist as part of larger conjugated units, resulting in an aromatic segment in conjugation with a ‘localized’ double bond. For example, in acenaphthylene, the double bond in the five-member ring is both structurally and chemically similar to a normal localized double bond. The resonance energy 0.57β is slightly less than that for naphthalene (0.59β). The additional double bond of acenaphthylene has only a small effect on the stability of the conjugated system. The molecular structure determined at 80 K by neutron diffraction shows bond lengths for the aromatic portion that are very similar to those of naphthalene. The double bond is somewhat longer than a normal double bond, but this may reflect the strain imposed by the naphthalene framework on the double bond.

The predictions of relative stability obtained by the various approaches diverge more widely when nonbenzenoid systems are considered. The simple Huckel method using total π delocalization energies relative to isolated double-bond reference energy (α + β) fails. This approach predicts stabilization of the same order of magnitude for such unstable systems as pentalene and fulvalene, as it does for much more stable aromatics. The HMO, RE and SCF-MO methods, which use polyene reference energies, do much better. All show drastically reduced stabilization for such systems and, in fact, indicate destabilization of systems such as butalene and pentalene.

It is of interest to consider at this point some of the specific molecules and compare their chemical properties with the calculated stabilization energies. Benzocyclobutadiene has been generated in a number of ways, including dehalogenation of dibromobenzocyclobutene. The compound is highly reactive, dimerizing or polymerizing readily. Benzocyclobutadiene is very reactive as a dienophile in the Diels-Alder reaction. Generation of benzocyclobutadiene by fluoride-induced elimination has permitted the NMR spectrum to be observed under flow conditions.

All the peaks are somewhat upfield of the aromatic region, suggesting polyene character. This structure would also be consistent with the observed reactivity since the polyene has a quinodimethane structure. The implication of a non-aromatic structure is that the combination of ring strain and the anti-aromaticity associated with the four-member ring results in a localized system.

Azulene is one of the few nonbenzenoid hydrocarbons that appear to have appreciable aromatic stabilization. There is some divergence on this point between the SCF-MO and HMO results. The latter estimates a resonance energy about half that for the isomeric naphthalene, whereas the SCF-MO method assigns a resonance energy that is only about one-seventh that of naphthalene. Naphthalene is more stable than azulene by about 38.5 kcal/mol. Molecular mechanics calculations attribute about 12.5 kcal/mol of this difference to strain and about 26 kcal/mol to greater resonance stabilization of naphthalene. Based on heats of hydrogenation, the stabilization energy of azulene is about 16 kcal/mol. The parent hydrocarbon and many of its derivatives are well-characterized compounds with considerable stability. The structure of azulene has been determined by both X-ray crystallography and electron diffraction measurements. The peripheral bond lengths are in the aromatic range and show no regular alternation. The bond shared by the two rings is significantly longer, indicating that it has predominantly single-bond character. Theoretical calculations indicate that the molecule has C2v symmetry, indicating delocalization of the π electrons.

An interesting structural question involves the contribution of a dipolar structure, which pictures the molecule as the fusion of a cyclopentadienide anion and a cycloheptatrienyl cation.

Azulene does have an appreciable dipole moment (0.8 D). The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several molecular orbital calculations have been applied to azulene. Calculations, which include electron correlation effects, give a delocalized π system as the minimum-energy structure. In contrast to the significant resonance stabilization of azulene, pentalene and heptalene are indicated to be destabilized relative to a reference polyene.

Preparation of pentalene is followed by immediate dimerization. Low-temperature photolysis produces a new species believed to be pentalene, but the compound reverts to dimer at 100°C. The matrix-isolated monomer has been characterized spectroscopically. The results are in accord with the predicted lack of stabilization.

Heptalene readily polymerizes and is sensitive to oxygen. The NMR spectrum does not indicate the presence of an aromatic ring current. The conjugate acid of heptalene, however, is very stable (even at pH 7 in aqueous solution), reflecting the stability of the cation, which is a substituted tropylium ion.

Another structure with a 10-π-electron conjugated system is bicyclo[6.2.0]deca-1,3,5,7,9-pentaene (I), which is neither aromatic nor anti-aromatic. The crystal structure of the 9,10-diphenyl derivative (II) shows the conjugated system to be nearly planar. There is significant bond alternation; however, the bond at the ring fusion is quite long (1.539 Å). A molecular mechanics calculation on this molecule that included an SCF-MO treatment of the planar-conjugated system found the molecule to be slightly destabilized (4kcal/mol) relative to a polyene reference.

The possibility of extra stabilization in conjugated systems that have conjugated components exocyclic to the ring has also been examined. The substituents complete conjugated rings but are not part of the cyclic system. Cyclopropenes and cyclopentadienes with exocyclic double bonds provide the possibility of dipolar resonance structures that suggest aromatic character in the cyclic structure.

For methylenecyclopropene, a microwave structure determination has established bond lengths, which show the strong alternation anticipated for a localized structure. The molecule does have a significant dipole moment (1.90 D), implying a contribution from the dipolar resonance structure. Fulvenes are cyclic cross-conjugated molecules with an odd number of carbon atoms in the ring. According to the size of the ring skeleton, they are named tri-, penta-, hepta- and nona-fulvenes. The molecular geometry of dimethylfulvene has been examined by electron diffraction methods. Strong bond-length alternation indicative of a localized structure is found. The fulvalene systems are not predicted to be aromatic by any of the theoretical estimates of stability. Even simple resonance considerations would suggest polyene behaviour, since only dipolar resonance structures can be drawn in addition to the single non-polar structure. Trifulvalene (cyclopropenylidenecyclopropene) has not been isolated. A substantial number of pentafulvalene derivatives have been prepared. The chemical properties of these molecules are those of reactive polyenes. The NMR spectrum of pentafulvalene is characteristic of a localized system. Heptafulvalene is a well-characterized compound with the properties expected for a polyene.

Because the five-member ring is a substituted cyclopentadienide anion in some dipolar resonance structures, it might be expected that exocyclic groups that could strongly stabilize a positive charge would lead to a larger contribution from dipolar structures and enhanced stability. The stability of such dipolar systems depends on the balance between the increase in energy required to separate unlike charges and the aromaticity associated with Huckel 4n + 2 systems. Phenyl-substituted analogs are known and the large measured dipole moments suggest considerable charge separation. Some alkyl derivatives have been prepared. Their chemical behaviour is that of highly reactive polyenes. One interesting property that is revealed by the NMR spectra is a reduced barrier to rotation about the double bond between the two rings. This property suggests that rotation about this bond takes place easily through a transition state in which the two charged aromatic rings are twisted out of conjugation.

The hydrocarbon phenalene is the precursor of both a highly stabilized anion and a highly stabilized cation. The single orbital at the nonbonding level is the LUMO in the cation and the HOMO in the anion. The stabilization energy calculated for both would be the same and is 0.41β by the HMO comparison. The pK for conversion of phenalene to its anion is 19. Several methods for generating the phenatenyl cation have been developed. Because the centre carbon is part of the conjugated system, the Huckel rule, which applies only to monocyclic conjugated systems, cannot be applied to just the peripheral conjugation.

In general, the HMO and SCF methods both appear able to make reasonably accurate predictions about the stabilization in conjugated molecules. The stabilization is general for benzenoid compounds but quite restricted in nonbenzenoid systems. Because the HMO method of estimating stability is based on the ideas of HMO theory, its general success vindicates the ability of this very simplified molecular orbital approach to provide insight into the structural nature of the annulenes and other conjugated polyenes.

12.9 HETEROCYCLIC RINGS

Certain structural units containing heteroatoms can be substituted into conjugated systems in such a way that the system remains conjugated and isoelectronic with the original hydrocarbon. The most common examples are -CH=N- and -N=N- double bonds and divalent sp2 -O-, -S- and -NR- units. Each of these structural fragments can replace a -CH=CH- unit in a conjugated system and contribute two π electrons. These compounds are called heteroaromatic to recognize both the heterocyclic structure and the relationship to benzene and other aromatic structures.

MO calculations on compounds in which a -CH=N- unit replaces -CH=CH indicate that the resonance stabilization is very similar to that of the original compound. For the -O-, -S- and -NR- fragments, the resonance stabilization is somewhat reduced but, nevertheless, high enough to consider the resulting compounds to be aromatic in character. Various approaches have been used to estimate the aromaticity of these compounds. Generally speaking, the various approaches suggest that the aromatic stabilization of pyridine is similar to that of benzene. This is in agreement with the non-chemical estimates of the pyridine stabilization energy. Typically, the five-member compounds are found to be somewhat less stabilized than benzene, with resonance energies in the range of one-half to three-quarters of that for benzene. Magnetic and polarizability criteria put the order of aromaticity as thiophene > pyrrole > furan.

Figure 12.5 Structures isoelectronic with benzene

Figure 12.6 Structures isoelectronic with naphthalene

Additional heteroaromatic structures can be built up by fusing benzene rings to the aromatic heterocyclic rings or by fusing together heterocyclic rings. When benzene rings are fused to the heterocyclic five-member rings, the structures from fusion at the 2,3-positions are much more stable than those from fusion at the 3,4-positions. The π-electron system in the 3,4-fused compounds is more similar to a peripheral 10-π-electron system than to the 10-electron system of naphthalene. As a result, these compounds have a strong tendency to undergo reactions that restore benzene conjugation in the carbocyclic ring. The isobenzofuran structure is known to be an exceptionally reactive diene.

12.10 SUMMARY

Benzene is an unusually stable molecule. This aromatic stabilization is associated with its 6π-electron system. Other molecules besides benzenoid compounds also show increased stability. Aromatic stability is found in planar, cyclic conjugated systems that satisfy the Huckel rule in possessing (4n + 2) π-electrons. (4n + 2) π-electrons correspond to a completely filled set of bonding molecular orbitals. Ionic species that comply with all the requirements for aromaticity are best regarded as resonance-stabilized ions. Protons in an aromatic environment usually resonate at δ 6.5–8.5 ppm. Cyclic, planar molecules with 4n π-electrons are particularly unstable and are said to be anti-aromatic. Substitution reactions, which are the most important type shown by aromatic compounds, conserve aromatic stability. Addition reactions, which are less common, destroy the aromatic system.

PROBLEMS
  1. (a) Write an equation to show the formation of the cycloheptatrienyl cation by addition of bromine to the hydrocarbon. Use curly arrows to illustrate the stabilization of the cation.

    (b) Cyclopropenones are described as having aromatic character. How would you account for this, given that the ring contains three π electrons?

    (c) How many monobromonaphthalenes are there?

  2. (a) The [10]- and [12]-annulenes have been synthesized and neither have been found to be aromatic. Explain.

    (b) Cyclooctatetraene is not aromatic whereas its dianion is aromatic. Explain.

  3. Why is a cyclic compound with an odd number of pairs of delocalized π electrons more stable than one with an even number of pairs of delocalized π electrons?
  4. Comment on the aromaticity of the following hydrocarbons.
  5. Explain the following terms with appropriate examples.
    1. Non-benzenoid aromatics
    2. Anti-aromaticity
    3. Homoaromaticity
    4. Aromatic annulenes
  6. Which of the following compounds are aromatic and which anti-aromatic?
  7. Phenanthrene has more resonance energy than anthracene; hence, it should be more aromatic. However, anthracene does not undergo electrophilic addition whereas phenanthrene readily undergoes electrophilic addition across the 9,10-bond. Explain.
  8. Ordinarily, the barrier to rotation about a carbon–carbon double bond is quite high, but compound I was observed by NMR to have a rotational barrier of only about 20kcal/mol. Explain this result.
  9. Draw the molecular orbital diagrams of the π-electron system for the following species:
    1. the cyclopropenyl radical
    2. the cyclopropenyl cation
    3. the cyclobutadienyl cation
    4. the cyclopentadienyl anion
    5. the tropylium cation
    6. the cyclooctatetraenyl dication
  10. Offer an explanation for the following observations:
    1. Hydrocarbon A (pK ~ 14) is much more acidic than B (pK ~ 22).
    2. The barrier for rotation about the marked bond in D is only about 14 kcal/mol.
    3. The hydrocarbon E is easily reduced to a dianion. The proton NMR spectrum of the dianion shows an average downfield shift, relative to the hydrocarbon. The centre carbon shows a very large upfield shift in the 13C NMR spectrum.
    4. Cyclopentadienone is a kinetically unstable molecule.
OBJECTIVE TYPE QUESTIONS
  1. Which species is not aromatic?
  2. Cyclopropenium cation is
    1. an antiaromatic compound
    2. a nonaromatic compound
    3. a homoaromatic compound
    4. an aromatic compound
  3. Aromatic character is possible only when
    1. six electrons are delocalized
    2. the molecule is cyclic and planar having delocalized (4n + 2)π electrons
    3. a molecule has at least two resonance structures
    4. all of the above
  4. Polycyclic benzenoid hydrocarbons are compounds that
    1. contain two or more fused benzene rings
    2. contain fused rings in a linear fashion only
    3. contain fused rings in a nonlinear fashion only
  5. Aromatic compounds undergo
    1. electrophilic substitution reactions
    2. electrophilic addition reactions
    3. nucleophilic addtion reactions
    4. electrophilic and nucleophilic substitution reactions
  6. Electrophilic addition to benzene is an
    1. endergonic reaction
    2. exergonic reaction
    3. none of the above
  7. Which of the following electrophilic aromatic substitution reaction is reversible?
    1. nitration
    2. halogenation
    3. sulphonation
    4. Friedel-Crafts alkylation
    5. none of the above
  8. Cyclic, planar molecules with 4n π-electrons are particularly unstable and are said to be
    1. antiaromatic
    2. aromatic
    3. homoaromatic
    4. nonaromatic
  9. The Huckel rule can be applied to
    1. only monocyclic systems
    2. neutral monocyclic as well as charged systems
    3. only fused ring systems
    4. all of the above
  10. Conjugated monocyclic polyenes (CnHn) in which n is greater than or equal to 10, are usually called
    1. annulenes
    2. azulenes
    3. fulvenes
    4. none of the above