Benzene aromatic stabilisation

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

Even though there is still extensive delocalisation of the positive charge that has been introduced by the electrophile, it is limited to that which would have been available to a linear, non-cyclic system. The extra delocalising possibilities that were available to the benzene system, because it was cyclic, are no longer possible. This means that the cyclohexadienyl cation is significantly less stable than the initial benzene ring, because it has lost the aromatic stabilisation of the cyclic sextet of electrons. 

Taillefer et al. have reported a one-pot method for the preparation of a, 3-unsaturated organophosphorus compounds through the reaction of lithium diphenylphosphonium diylides with phosphorus electrophiles and aldehydes. In the first step, treatment of diylides (91) with chlorodiphenylphosphine results in the formation of mono-ylide intermediates (92) and (93). Subsequent addition of aldehyde (94) produces either alkenes (95) or phosphines (96) (Scheme 22). The product obtained is critically dependent upon the nature of the ylide substituents and the aldehyde employed. For example, non-stabilised ylide (91a) reacts with chlorodiphenylphosphine and aromatic, heteroaromatic or enolisable aldehydes (94a-f) producing the corresponding phosphines (96), predominantly as the Z isomer. However, with 4-phenylcyclohexanone the only product obtained from (91a) is the alkene, (4-methylenecyclohex-l-yl)benzene. Non-stabilised ylide (91b) reacts with chlorodiphenylphosphine and benzaldehyde (94a) to give primarily alkene product whereas para-nitrobenzaldehyde (94c) yields only the phosphine product. Semi-stabilised ylide (91c), and stabilised ylide (91d), react... 

The aromaticity of porphyrins is also indicated by measurements of their heats of combustion (because of aromatic stabilisation, benzene gives out less heat when it is burnt than if it consisted of three alternating double bonds, as cyclohexatriene). Also, X-ray crystallography of many porphyrins... 

The difference between AH for (9) and 4 x AH for (11) is thus minus 26 kJ ( — 6 kcal) mol -1 cyclooctatetraene, unlike benzene, exhibits no characteristic stabilisation when compared with the relevant hypothetical cyclic polyene (it is in fact slightly destabilised), i.e. it is not aromatic. This lack of aromatic character is, on reflection, not really... 

Many, but not all, endothermic compounds have been involved in violent decompositions, reactions or explosions, and in general, compounds with significantly positive values of standard heat of formation may be considered suspect on stability grounds. Notable exceptions are benzene and toluene (AH°f +82.2, 50.0 kJ/mol 1.04, 0.54 kJ/g, respectively), where there is the resonance stabilising effect of aromaticity. Values of thermodynamic constants for elements and compounds are tabulated conveniently [1], but it should be noted that endothermicity may change to exothermicity with increase in temperature [2], There is a more extended account of the implications of endothermic compounds and energy release in the context of fire and explosion hazards [3], Many examples of endothermic compounds will be found in the groups ... 

There are many other aromatic hydrocarbons, i.e. compounds like benzene, which contain rings of six carbon atoms stabilised by electron delocalisation. For example, if one of the hydrogen atoms in benzene is replaced by a methyl group, then a hydrocarbon called methylbenzene (or toluene) is formed. It has the structural formulae shown. Methylbenzene can be regarded as a substituted alkane. One of the hydrogen atoms in methane has been substituted by a or —group, which is known as a phenyl group. So an alternative name for methylbenzene is phenylmethane. Other examples of aromatic hydrocarbons include naphthalene and anthracene. 

The dehydrodimerization reaction involving aromatic radical-cations is fast only when electron donating substituents are present in the benzene ring. These substituents stabilise the a-intermediate. Benzene, naphthalene and anthracene radical-cations form a a-sandwich complex with the substrate but lack the ability to stabilise the a-intermediate so that radical-cation substrate reactions are not observed. The energy level diagram of Scheme 6.4 illustrates the influence of electron donating substituents in stabilising the Wheland type a-intermediate. 

Whether fluorine can activate or deactivate an aromatic ring relative to hydrogen depends on the nature of the attacking electrophile. When the reagent is less reactive (late transition state) such as in molecular chlorinations and brominations [42, 43] (Table 4.6), resonance stabilisation of the Wheland-like transition state becomes far more important and so fluorine activates the system. On the other hand, nitration of fluoro-benzene is slower than the corresponding reaction with benzene. 

In nearly all cases where there is an alkyl group that is joined to an aromatic ring, the abstracted hydrogen will be on the a-carbon of the alkyl side chain, because such a bond is relatively weak and the resultant radical is stabilised by delocalisation of the unpaired electron around the aromatic ring. The aromatic ring is very rarely attacked to form an aryl radical. Thus, benzene reacts much more slowly to form the C6H- radical. Suggest why this is so. 

The lone pair of electrons on the nitrogen atom of aminobenzene (or aniline) can be stabilised by the delocalisation of the electrons onto the 2-, 4- and 6-positions of the benzene ring. Aromatic amines are therefore less basic than aliphatic amines. 

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