Ferf-Butyl cation hydrogen chloride

FIGURE 3.6 Deprotection of functional groups by acidolysis. Protonation followed by carbocation formation during the removal of ferf-butyl-based protectors by hydrogen chloride.8 One mechanism is involved in generating the ferf-butyl cation, which is the precursor of two other molecules. 

The major difference between the two mechanisms is the second step. The second step in the reaction of ferf-butyl alcohol with hydrogen chloride is the unimolecular dissociation of ferf-butyloxonium ion to ferf-butyl cation and water. Heptyloxonium ion, however, instead of dissociating to an unstable primary carbocation, reacts differently. It reacts with bromide ion, which acts as a nucleophile. We can represent the transition state of this displacement as ... 

These common features suggest that carbocations are key intermediates in alcohol dehydrations, just as they are in the reaction of alcohols with hydrogen halides. Figure 5.6 portrays a three-step mechanism for the acid-catalyzed dehydration of tert-butyl alcohol. Steps 1 and 2 describe the generation of terf-butyl cation by a process similar to that which led to its formation as an intermediate in the reaction of ferf-butyl alcohol with hydrogen chloride. 

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