Chlorobenzene, reaction with potassium amide

A solution to the question of the mechanism of these reactions was provided by John D Roberts m 1953 on the basis of an imaginative experiment Roberts prepared a sample of chlorobenzene m which one of the carbons the one bearing the chlorine was the radioactive mass 14 isotope of carbon Reaction with potassium amide m liquid... 

An early ease in whieh the existenee of benzyne as a reaetion intermediate was established was in the reaction of chlorobenzene with potassium amide. Carbon-14 label in the starting material was found to be distributed in the aniline as expeeted for a benzyne intermediate. ... 

Evidence for symmetrical intermediates such as benzyne cannot be established by quantitative analysis of the reaction mixture unless a labelled starting substance is used. By applying labeling techniques, Roberts and his collaborators obtained results which indicated that benzyne (13) occurs as an intermediate in the amination of chlorobenzene with potassium amide in liquid ammonia. From chlorobenzene-1-C (12) about equal amounts of anUine-l-C (14) and aniline-2-C (15) were formed. More or less probable alternative... 

It is important to learn if the same intermediate forms in several reactions, for if so its identity will very likely be manifest by the common features that can be observed. Here is an example cited by Bunnett.6 It was suspected that the reaction of chlorobenzene with potassium amide in liquid ammonia occurs by way of benzyne. Experiments were done with a I4C label (blackened) at the 1-position.14 The reaction gives equal amounts of aniline-1-14C and aniline-2-l4C,... 

It is believed that such reactions proceed through aryne intermediates. These aryne intermediates have been confirmed by reactions of an isotopically labeled chlorobenzene with potassium amide in liquid ammonia1011. Additionally, aryne intermediates have been observed in flash-photolysis experiments and in mass spectrometry12 and trapped as a stable nickel complex (Figure 1), which was characterized by 111 NMR spectroscopy13. 

Roberts et al. s [50] classic demonstration of a symmetric intermediate in the reaction of chlorobenzene with potassium amide in liquid ammonia is an example of a double-labelling experiment where the second label is a heteroatom rather than an isotope. Direct attack will yield product labelled only on the carbon bearing the nitrogen whereas the route through the symmetrical intermediate (the benzyne) gives 50% of each of the possible products (Eqn. 69). 

The C-labeling experiments of Roberts in 1953 put the existence of ortho-benzyne as an intermediate beyond doubt when it was found that treatment of 1-i C-chlorobenzene with potassium amide in liquid ammonia gave a 1 1 mixture of 1- and 2- C labeled aniline. The reaction had clearly proceeded through a symmetrical intermediate ortho-benzyne. The overall process whereby a nucleophile apparently enters ortho to the leaving group is referred to as cine substitution (Scheme 7.10). 

The use of sodium amide or potassium amide in liquid ammonia with bromo- or chlorobenzene leads inevitably to the capture of benzyne by its reaction with ammonia. However, the utility of bromo- or chlorobenzene as a benzyne precursor is extended to ethereal solvent systems by employing the conjugate base of a hindered secondary amine (diisopropylamine, 2,2,6,6-tetramethylpiperidine) which can be formed in situ from the amine and alkyllithium. Alternatively, butyllithium itself is used with the halogeno-benzene, and pentafluorobenzene and butyllithium are the usual source of tetrafluorobenzyne. In all of these reactions the aryne is generated by decomposition of an o-halogenoaryl anion at temperatures below 0°C. 

Propose an explanation for the observation that potassium amide enhances the rate of formation of tetraphenylmethane in the reaction of chlorobenzene with potassium triphenylmethide in liquid ammonia. 

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