Toluene, hyperconjugation

Phenyl norhornane is benzoylated faster than isopropylbenzene or toluene despite the bulkiness of the norbomyl group probably because of hyperconjugation (87). Hyperconjugation of the C—C bond is at least as or more important as that of the C—H bond since 1-phenylnorhornane has no a-hydrogen atom. 

In this work the relative reactivities of the alkylbenzenes (PhR) varied as follows (R = ) Me, 100 Et, 76 i-Pr, 44 f-Bu, 23. This relates to 85 % aqueous acetic acid at 24 °C and with 88 % acid at 25 °C the same relative rate of toluene to /-butylbenzene was obtained261, showing the hyperconjugative order to prevail for this reaction. 

Kinetic studies of mercuration have also been used as a test for hyperconjugation. Toluene and toluene-aaa-mercuric acetate (0.5 M) in acetic acid containing water (0.25 M) and perchloric acid (0.050 M) and an isotope effect, ArH/ArD = 1.00 0.03, obtained. This insignificant effect was considered as evidence against the participation of hyperconjugation in electron supply by a methyl group449. 

This came to be called the Baker-Nathan effect and has since been found in many processes. Baker and Nathan ° explained it by considering that hyperconjugative forms contribute to the actual structure of toluene ... 

The interaction between a and -it electrons of single and multiple bonds (or the -tt electrons in the aromatic ring) is usually interpreted (Baker, 1952) as a, -conjugation (hyperconjugation). In this process the hydrocarbons become stronger protolytes. For example, the acidic and basic behaviour of propene and toluene are more pronounced than the corresponding properties of ethylene and benzene. 

The ease of these reactions can be explained in terms of the hyperconjugation of toluene accentuated by the influence of the nitro groups. 

The methyl group in trinitrotoluene becomes strongly activated by the nitro groups present. This could be ascribed to the hyperconjugation of toluene (p. 200)... 

In toluene (LXX), like in a number of analogs, the preferred conformation shows eclipsing between a hydrogen atom and the w-system, as represented in diagram LXX. This conformation minimizes a- and w-electron loss to the 7r-system (hyperconjugation) and therefore allows maximal one-electron interaction [80]. Toluene is characterized by an extremely low rotation barrier (a few cal/mol) due to the fact that this barrier is sixfold, and the latter is always found to be extremely small. Indeed, an equivalent conformation is found after every 60° rotation, and no real relief of conformational strain is obtained after a rotation of only 30°. 

This picture is somewhat generalized, since there are some exceptions. For instance, the chlorination of toluene proceeds faster than that of rcrr-butylbenzene. Hyperconjugation (Scheme 2.16) is at a maximum for a methyl group and has been offered as an explanation for these anomalies. 

The striking examples are o-dinitrobenzene, 2,3-dinitrotoluene and 3,4-dinitrotoluene which show jt — jr peaks at < 210, and 219 nm respectively, whereas all other dinitro compounds examined show peaks between 242 and 283.5 nm. Derivatives of toluene show peaks at a higher wave length probably due to the hyperconjugation of the methyl group. 

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