Polarised complexes

Brominarion of the aromatic nucleus is now regarded as replacement of a hydrogen atom of the intact nucleus as a result of an attack by a polarised complex with a positive end. Iron acts as a carrier by forming FcBrj, which as a Lewis acid forms a polarised complex with one mol. of Bri ... 

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). 

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. 

In the presence of strong acid, however, HO—Hal becomes a very powerful halogenating agent due to the formation of a highly polarised complex (15) ... 

In other cases it seems more likely that the attacking electrophile is a polarised complex (19), the degree of polarisation in a particular case depending on R in R—Hal and the Lewis acid employed ... 

The actual proportions of products obtained in many cases are not necessarily found to reflect the relative stabilities of the incipient carbocations, unrearranged and rearranged, however. This follows from the fact that their relative rates of reaction with the aromatic species almost certainly do not follow the order of their relative stabilities, and may well be diametrically opposed to it. Attack on the aromatic species by the first formed polarised complex may be faster than its rearrangement. The study of these rearrangements is also complicated by the fact that Lewis acids are found to be capable of rearranging both the original halides, and the final, alkylated end-products, e.g. ... 

There is also the similar general dilemma of whether the effective electrophile is the acyl cation (26) as a constituent of an ion pair, or a polarised complex (27) ... 

Acyl cations have been detected in a number of solid complexes, in the liquid complex between MeCOCl and A1C13 (by i.r. spectroscopy), in solution in polar solvents, and in a number of cases where R is very bulky. In less polar solvents, and under a number of other circumstances, acyl cations are not detectable, however, and it must be the polarised complex that acts as the electrophile. 

The cationic polymerisation of olefins by metal halides has been interpreted in two ways. The first theory, proposed by Hunter and Yohe [1], ascribed the catalysis to the formation of a polarised complex between the metal halide and the olefin ... 

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,... 

Plane trigonal hybridisation, 5 Polarisability, 24, 96 Polarisation, 22, 29 Polarised complexes, 108,141, 144 Polar non-protic solvents, 81 Polyenes... 

Note that the polarised complex formed in tiris interaction is not active for initiation, but (-)-1-phenylethyl chloride is racemised by SnCl4 in CCl4 ... 

The reaction of benzene and alkenes like an isopropyldiene (Me2C==C) moiety produces an non-cyclic "ene-type of reaction product with effective p-substituent. The reaction is a non-concerted process and is considered to proceed either via a biradical or from an equivalent polarised complex. The reaction involves charge transfer by donation of an electron from alkene to excited benzene. 

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