Acid-Catalyzed Hydration and Related Addition Reactions

Acid-Catalyzed Hydration and Related Addition Reactions 

The formation of alcohols by acid-catalyzed addition of water to alkenes is a fundamental organic reaction. At the most rudimentary mechanistic level, it can be viewed as involving a carbocation intermediate. The alkene is protonated and the carbocation is then captured by water. 

This mechanism explains the observed formation of the more highly substituted alcohol from unsymmetrical alkenes (Markownikoff s rule). A number of other points must be considered in order to provide a more complete picture of the mechanism. Is the protonation step reversible Is there a discrete carbocation intermediate, or does the nucleophile become involved before proton transfer is complete Can other reactions of the carbocation, such as rearrangement, compete with capture by water  

The overall process is reversible, however, and some styrene remains in equilibrium with the alcohol, so exchange eventually occurs. 

SECTION 6.2. ACID-CATALYZED HYDRATION AND RELATED ADDITION REACTIONS 

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both the observation of general acid catalysis and the kinetic evidence of a solvent isotope effect are consistent with rate-limiting protonation with simple alkenes such as 2-metlQ lpropene and 2,3-dimethyl-2-butene. 

Much of the early mechanistic work on hydration reactions was done with conjugated alkenes, particularly styrenes. Owing to the stabilization provided by the phenyl group, this reaction involves a relatively stable carbocation. With styrenes, the rate of hydration is increased by ERG substituents and there is an excellent correlation 

Alkenes lacking the phenyl group are somewhat less convenient to study by kinetic methods, but such data as observation of general acid catalysis and solvent 

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Acid- Catalyzed Hydration and Related Addition Reactions... 

The mechanistic pattern of hydration and alcohol addition reactions of ketones and aldehydes is followed in reactions of carbonyl compounds with amines and related nitrogen nucleophiles. These reactions involve addition and elimination steps proceeding through tetrahedral intermediates. These steps can be either acid catalyzed or base catalyzed. The rates of the reactions are determined by the energy and reactivity of the tetrahedral intermediates. With primary amines, C=N bond formation ultimately occurs. These reactions are reversible and the position of the overall equilibrium depends on the nitrogen substiments and the structure of the carbonyl compound. 

In the second step of the reaction, water acts as a nucleophile and adds to the Lewis acid (the protonated carbonyl) to generate a protonated diol (Fig. 16.25). In the final step of this acid-catalyzed reaction, the protonated diol is deprotonated by a water molecule to generate the neutral diol and regenerate the add catalyst, H30. A diol with both OH groups on the same carbon is called a em-diol (gem stands for geminal) or a hydrate. Note how closely related are the acid-catalyzed additions of water to an alkene and to a carbonyl group. Each reaction is a sequence of three steps protonation, addition of water, and deprotonation. 

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