Viehe substitution mechanism

In equation (256), haloacetylene and nucleophile could be eliminated and these could form the substitution products by addition-elimination steps, the IIT mechanism or the Arens process. By using LiNRJ as a base in equation (255) and looking for two ynamines, or by adding ArSH to reaction (256) and looking for two thioether products, one can seek evidence of the elimination process. Until such tests are performed, the applicability of the Viehe substitution mechanism remains uncertain. 

The third carbanion mechanism for process (2) was proposed by Viehe . While it may well be the most interesting, it is the least established by precedent and example. Here, Cp is the principal site of nucleophilic attack in RC CX (see Table 6 for examples), except when X = F or when attack on X is facile. In this mechanism, the substitution product forms via carbanion 42 in which Nu slides from Cp over to C with ejection of X, i.e. steps (e) and (f) of Scheme 6. Viehe labels this an onium rearrangement through species 43 and 44 in equation (253). As precedent for this... 

The mechanism of nucleophilic addition of water to co-halogenoaceto-phenones, giving either dehalogenation to the parent acetophenone or the corresponding substituted acetic acid, remains an unsettled problem. Various, rather confusing possibilities have been proposed (Delavarenne and Viehe, 1969 Miller and Dickstein, 1986b). 

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