Benzene polarisability

In the case of symmetrical molecules such as carbon tetrachloride, benzene, polyethylene and polyisobutylene the only polarisation effect is electronic and such materials have low dielectric constants. Since electronic polarisation may be assumed to be instantaneous, the influence of frequency and temperature will be very small. Furthermore, since the charge displacement is able to remain in phase with the alternating field there are negligible power losses. 

Yamase and Goto406 determined first- and second-order rate coefficients for the aluminium chloride-catalysed reaction of halide derivatives of benzoic acid (lO5 = F, 1.73 Cl, 4.49 Br, 4.35 I, 0.81) and phenylacetic acid (105fc2 = F, 12 Cl, 21 Br, 9 I, 6) with benzene. The maxima in the rates for the acid chloride are best accommodated by the assumption that a highly (but not completely) polarised complex takes part in the transition state. Polarisation of such a complex would be aided by electron supply, and consistently, the acetyl halides are about a hundred times as reactive as the benzoyl compounds (see p. 180, also Tables 105 and 108). 

Brown and Jensen395 suggested that the rate equation (194) for the reaction of benzene with excess benzoyl chloride could be interpreted according to the mechanisms given by the reactions (201) and (202), (203) and (204) and (205) and (206) which refer to nucleophilic attack of the aromatic upon the polarised acyl halide-catalyst complex, upon the free acylium ion, and upon an ion pair derived from the acyl halide-catalyst complex, viz. 

In exceptional circumstances the acylium ion (or the polarised complex) can decompose to give an alkyl cation so that alkylation accompanies acylation. This occurs in the aluminium chloride-catalysed reaction of pivaloyl chloride which gives acylation with reactive aromatics such as anisole, but with less reactive aromatics such as benzene, the acylium ion has time to decompose, viz. 

It seems likely that benzene forms a n complex (12) with, for example, Br2 (cf. p. 131), and that the Lewis acid then interacts with this. The catalyst probably polarises Br—Br, assists in the formation of a a bond between the bromine molecule s now electrophilic end and a ring carbon atom, and finally helps to remove the incipient bromide ion so as to form a [Pg.138]

These groups, and other such as S03H, CN, COR, etc., all have in common a positively charged, or positively polarised, atom adjacent to a carbon atom of the benzene ring ... 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

Enough mutual polarisation can apparently result, in (8), for (9) to form, but polarisation of the bromine molecule may be greatly increased by the addition of Lewis acids, e.g. AlBr3 (cf. bromination of benzene, p. 138), with consequent rise in the rate of reaction. Formation of (9) usually appears to be the rate-limiting step of the reaction. 

Reaction type (d) also complicates the Friedel-Crafts alkylation of benzene (a type db reaction, p. 141) by 1-bromopropane, MeCH2-CH2Br, in the presence of gallium bromide, GaBr3, as Lewis acid catalyst. The attacking electrophile is here a highly polarised complex,... 

The usual halogenation of benzene takes place in the presence of a Lewis acid, such as FeBrs (Unit 10, Class XII), which polarises the halogen molecule. In case of phenol, the polarisation of bromine molecule takes place even in the absence of Lewis acid. It is due to the highly activating effect of -OH group attached to the benzene ring. 

However, if we look at the LUMO, we find that it has the form 4.65, namely that of ift4 of benzene, but polarised by the nitrogen atom. This polarisation has reduced the coefficient at C-3, and the coefficient at C-4 is larger than that at C-2, as can be seen from the simple Hiickel calculation for pyridine itself in Fig. 4.11, which gives LUMO coefficients of 0.454 and —0.383, respectively, and an energy of 0.56/3 (compare benzene with 1/3 for this orbital). Thus, soft nucleophiles should attack at C-4, where the frontier orbital term is largest. Again this is the case cyanide ion, bisulfite, enolate ions, and hydride delivered from the carbon atom of the Hantsch ester 4.67 react faster at this site. 

Optical detection of intermediates produced in the reactions of triplet carbonyl compounds with electron donors has some obvious limitations. However, the technique of CIDNP is proving particularly effective at elucidating the reaction pathways in these systems. The outstanding work of Hendriks et al. (1979) illustrates the power of the technique. Not only was the role of radical ions in the reactions of alkyl aryl ketones with aromatic amines defined but the rate constants for many of the processes determined. The technique has been used to show that trifluoracetyl benzene reacts with electron donors such as 1,4-diazabicyclo[2.2.2]octane and 1,4-dimethoxy-benzene by an electron-transfer process (Thomas et al., 1977 Schilling et al., 1977). Chemically induced dynamic electron polarisation (CIDEP) has been... 

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