Aromatic stabilization

Several quantities have been used to obtain a measure of aromatic stabilization. They include  

As noted above, the first definition of aromaticity was in terms of substitution rather than addition. This is certainly true for many benzene derivatives. However, it must be used with some care since thiophene is by most criteria about as aromatic as benzene, but when treated with chlorine or bromine it gives an addition product. The latter is, however, the kinetically controlled product, for when heated or treated with base it loses hydrogen halide and gives the 2-halothiophene.20 Compounds such as anthracene and phenanthrene, which are recognized as having considerable resonance stabilization, also undergo addition reactions. 

Thermochemical stabilization is probably the most generally applicable of the simple criteria for aromaticity . Pauling made use of heat of combustion of benzene and a set of average bond energies to derive a resonance energy of 37 kcal/mol.21 The most useful measure of this quantity is derived from the heats of hydrogenation (kcal/mol) obtained by Kistiakowsky and his coworkers 22 

The first step in the reduction is endothermic, accounting for the difficulty in hydrogenating benzene. A minimum value of the stabilization is given by the difference between the first and last steps in the above sequence, or 34 kcal/mol. It is believed that 1,3-cyclohexadiene is stabilized by about 2 kcal/mol, leading to the commonly stated 36 kcal/mol resonance energy of benzene. 

The resonance energies, or perhaps better, stabilization energies, are really not wholly satisfactory for the bond lengths in cyclohexene and benzene are not the same. There is an additional term, the compression energy required to make the bond lengths the same. 

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A wide variety of carbocations and carbodications, including those that are aromatically stabilized as well those as stabilized by heteroatoms, were reported in the nearly 200 publications on the topic during my Cleveland years. 

A planar monocyclic continuous system of p orbitals possesses aromatic stability... 

Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het erocyclic aromatic compounds Some representative heterocyclic aromatic compounds are pyridine pyrrole furan and thiophene The structures and the lUPAC numbering system used m naming their derivatives are shown In their stability and chemical behav lor all these compounds resemble benzene more than they resemble alkenes... 

The oxygen m furan has two unshared electron pairs (Figure 11 16c) One pair is like the pair m pyrrole occupying a p orbital and contributing two electrons to complete the SIX TT electron requirement for aromatic stabilization The other electron pair m furan IS an extra pair not needed to satisfy the 4n + 2 rule for aromaticity and occupies an sp hybridized orbital like the unshared pair m pyridine The bonding m thiophene is similar to that of furan... 

We will return to the aromatic stabilization of benzene in more detail in Chapter 9, but substituted benzenes provide excellent examples of how proper use of the resonance concept can be valuable in predicting reactivity. Many substituents can be readily classified... 

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 , Z,Z,Z,Z-isomer. MO (MP2/DZd) calculations suggest an aromatic stabilization of almost ISkcal for a conformation of the , Z,Z,Z,Z-isomer in which the irmer hydrogens are twisted out of the plane by about 20°, but other calculations point to a polyene structure. ... 

Aromatic stabilization in kcal/mol based on semlempirical AMI calculations. ... 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

Whereas the initial hydrogenation both breaks a % bond and destroys any aromatic stabilization , the second hydrogenation only breaks a % bond. The difference between the two then corresponds to any aromatic stabilization. Is this difference large as in benzene (see discussion at left) or is it neglible Is cyclooctatetraene aromatic ... 

In a formal sense, isoindole can be regarde,d as a IOtt- electron system and, as such, complies vith the Hiickel (4w- -2) rule for aromatic stabilization, with the usual implicit assumption that the crossing bond (8, 9 in 1) represents a relatively small perturbation of the monocyclic, conjugated system. The question in more explicit terms is whether isoindole possesses aromatic stabilization in excess of that exhibited by pyrrole. 

Assuming that aromatic stabilization of 24a and 24b is of the same magnitude, and this is also true for the lone-pair repulsion in the pairs 24a/25a and 24b/25b, the energy, A , of the isodesmic reaction (1) corresponds to the contribution from greater aromatic stabilization of 25a with respect to 25b (Scheme 26). At the MP2/6-31G approximation, AE = 10.5 kJ mol (94JOC2799). Similar arguments applied to the isodesmic reaction (2) allow estimation of the energy contribution due to the repulsion of adjacent lone pairs in 25a. In MP2/6-31G approximation, A = -25.9 kJ mol ... 

Naphthalene and other polycyclic aromatic hydrocarbons show many of the chemical properties associated with aromaticity. Thus, measurement of its heat of hydrogenation shows an aromatic stabilization energy of approximately 250 kj/mol (60 kcal/mol). Furthermore, naphthalene reacts slowly with electrophiles such as Br2 to give substitution products rather than double-bond addition products. 

Although [34]octaphyrin 80 fulfills Hiickel s rule, the II NMR spectrum indicates by the high-field shift of the methine protons that the system is nonaromatic. The X-ray structure analysis demonstrates clearly the reason for the lack of aromatic stabilization, namely the nonplanar loop conformation in which the whole macrocycle is twisted similarly to the [32]octaphyrin structure and which is also found for [36]octaphyrin and [40]decaphyrin structures (vide infra). 

According to Hiiekel s rule, turcasarin should not be aromatic, but even if the macrocycle should fulfill the (4n +2) rule for aromatic systems the lack of planarity due to the loop conformation would prevent aromatic stabilization. In fact, the existence of the loop conformation in which the whole macrocycle is twisted was demonstrated by X-ray structure analysis and NMR investigations. 

The pKa values for 3-phenyl- and 6-methyl-3-phenyl-2//-thiopyran-S,S-dioxides have been discussed in connection with the aromatic stabilization energies of their corresponding anions (91JOC4218). 

Based on experimental results and complementary calculations, an out-of-plane n-delocalization is suggested for thiirene dioxides39. As far as the thiirene oxide is concerned, theoretical calculations predict possible spiroconjugative-type53 interaction between the n c—c orbital of the ring and the jr-orbitals of the SO (which leads to aromatic stabilization and a n charge transfer backward from the SO to the C=C). There exists, however, a rather strong destabilization effect, due to the jr so(d )-orbital. 

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