Carbocations arguments against

In the argument against the ECbECN concept, much, perhaps too much, store has been placed by critics on observations which seem to undermine the zipper mechanism in the apparent belief that this mechanism is axiomatic with the ECbECN as a whole. However, as has been shown, changing physico-chemical properties of progressively fluorinated products inevitably result in different adsorptive forces for these compounds, with possible desorption from the electrode as a consequence this situation could apply equally well with products formed via a carbocation mechanism as with radically generated compounds, even though, as originally stated, the implication is otherwise. 

One of the strongest arguments against the formation of 1° carbocations in the deamination of 1° amines comes from stereochemical studies. Reaction of nitrous acid with optically active l-[ H]-l-butanamine (36) produced the diazonium ion 37, which then formed 1-butanol (38) with essentially complete inversion of configuration (equation 10.69). This result suggested that the formation of 38 does not occur by capture of the 1° butyl cation by water but is, instead, the product of an Sn2 reaction on 3 ... 

Since the 1950 s, an enormous amount of work has been carried out on other nucleophilic substitutions of norbornane derivatives and related compounds. This was particularly due to the alternative mechanistic explanation proposed by H. C. Brown in 1962 (see also Brown, 1966, 1976, 1977). He postulated that instead of the nonclassical intermediate 7.97, the expected classical carbocation 7.100 is formed in a rapid equilibrium with the ion 7.101. In his opinion, 7.97 is not an intermediate, but corresponds to the transition state in the equilibrium 7.100 7.101 (Scheme 7-33). Arguments for and against Winstein s and Brown s proposals by other researchers can be found first of all in Bartlett s anthology (1965) and in the literature until the present day (see some 30 references in the period 1977-1988 in March s book, 1992, p. 321) ... 

The arguments discussed to this point, both for and against the nonclassical structure, rest on indirect evidence derived from interpretation of the kinetic and stereochemical characteristics of the substitution reactions. When techniques for direct observation of carbocations became available, the norbomyl cation was subjected to intense study from this perspective. 

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