The Reaction of Alcohols with Hydrogen Halides

Alcohols react with hydrogen halides (HCl, HBr, and HI) to give alkyl halides (chlorides, bromides, and iodides). 

This substitution reaction provides a useful general route to alkyl halides. Because halide ions are good nucleophiles, we obtain mainly substitution products instead of dehydration. Once again, the reaction rate and mechanism depend on the class of alcohol (tertiary, secondary, or primary). 

Tertiary alcohols react the fastest. For example, we can convert t-butyl alcohol to f-butyl chloride simply by shaking it for a few minutes at room temperature (rt) with concentrated hydrochloric acid. 

The reaction occurs by an S l mechanism and involves a carbocation intermediate. The first two steps in the mechanism are identical to those shown in eqs. 7.18 and 7.19. The final step involves capture of the t-butyl carbocation by chloride ion. 

On the other hand, 1-butanol, a primary alcohol, reacts slowly and must be heated for several hours with a mixture of concentrated hydrochloric acid and a Lewis acid catalyst such as zinc chloride to accomplish the same type of reaction. 

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Mechanism of the Reaction of Alcohols with Hydrogen Halides... 

MECHANISM OF THE REACTION OF ALCOHOLS WITH HYDROGEN HALIDES... 

The dehydration of alcohols resembles the reaction of alcohols with hydrogen halides (Section 4 7) m two important ways... 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m 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 tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

The principles developed in this chapter can be applied to a more detailed examination of the reaction of alcohols with hydrogen halides than was possible when this reaction was first introduced m Chapter 4... 

The reactions of alcohols with hydrogen halides to give alkyl halides (Chapter 4) are nucleophilic substitution reactions of alkyloxonium ions m which water is the leaving group Primary alcohols react by an 8 2 like displacement of water from the alkyloxonium ion by halide Sec ondary and tertiary alcohols give alkyloxonium ions which form carbo cations m an S l like process Rearrangements are possible with secondary alcohols and substitution takes place with predominant but not complete inversion of configuration... 

We first encountered nucleophilic substitution in Chapter 4, in the reaction of alcohols with hydrogen halides to fonn alkyl halides. Now we ll see how alkyl halides can themselves be converted to other classes of organic compounds by nucleophilic substitution. 

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