Hydrogen halides with alcohols

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

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

As carbocations go CH3" is particularly unstable and its existence as an inter mediate m chemical reactions has never been demonstrated Primary carbocations although more stable than CH3" are still too unstable to be involved as intermediates m chemical reactions The threshold of stability is reached with secondary carbocations Many reactions including the reaction of secondary alcohols with hydrogen halides are believed to involve secondary carbocations The evidence m support of tertiary carbo cation intermediates is stronger yet... 

Reaction of Primary Alcohols with Hydrogen Halides The Sn2 Mechanism... 

The SnI mechanism is generally accepted to be correct for the reaction of tertiary and secondary alcohols with hydrogen halides It is almost certainly not correct for methyl alcohol and primary alcohols because methyl and primary carbocations are believed to be much too unstable and the activation energies for their formation much too high for them to be reasonably involved The next section describes how methyl and primary alcohols are converted to their corresponding halides by a mechanism related to but different from S l... 

REACTION OF PRIMARY ALCOHOLS WITH HYDROGEN HALIDES THE Sn2 MECHANISM... 

It IS important to note that although methyl and primary alcohols react with hydro gen halides by a mechanism that involves fewer steps than the corresponding reactions of secondary and tertiary alcohols fewer steps do not translate to faster reaction rates Remember the order of reactivity of alcohols with hydrogen halides is tertiary > sec ondary > primary > methyl Reaction rate is governed by the activation energy of the slowest step regardless of how many steps there are... 

Reactions of alcohols with hydrogen halides (Section... 

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... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

One important experimental fact is that the rate of reaction of alcohols with hydrogen halides increases in the order methyl < primary < secondary < tertiary. This reactivity order parallels the caibocation stability order and is readily accommodated by the mechanism we have outlined. 

Reactions of alcohols with hydrogen halides (Section 4.7) Alcohols react with hydrogen halides to yield alkyl halides. The reaction is useful as a synthesis of alkyl halides. The reactivity of hydrogen halides decreases in the order HI > HBr > HCI > HF. Alcohol reactivity decreases in the order tertiary > secondary > primary > methyl. 

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. 

Remarks on 1 and 2.—These two reactions are special cases of a general change, namely, the substitution of an alcoholic hydroxyl group by a halogen atom. This substitution can be carried out in two ways first, as in the preparation of ethyl bromide, 1, by acting on alcohols with hydrogen halides, e.g. 

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