Hydration and Other Acid-Catalyzed Additions

SECTION 4.2. HYDRATION AND OTHER ACID-CATALYZED ADDITIONS 

In addition to halide ions, various other nucleophilic species can be added to olefinic bonds under acidic conditions. A fundamental example is the hydration of olefins in aqueous systems  

Addition of a proton occurs to give the more substituted carbonium ion, so that addition occurs regioselectively and in accordance with Markownikoff s rule. A more detailed discussion of the reaction mechanism is given in Part A, Section 6.2. The reaction is occasionally applied to the synthesis of tertiary alcohols  

Because of the strongly acidic and rather vigorous reaction conditions required to effect hydration of most alkenes, the reaction is applicable only to molecules that have no other very sensitive functional groups. Also, because of the involvement of cationic intermediates, rearrangements can occur in systems where a more stable cation would result by aryl, alkyl, or hydrogen migration. A much milder procedure for alkene hydration is discussed in the next section. 

Additions of nucleophilic solvents such as alcohols and carboxylic acids can be effected by use of strong acid catalysts  

In addition to halide ions, a variety of nucleophilic species can be added to 

Trifiuoroacetic acid is a sufficiently strong acid to react with alkenes under relatively mild conditions. The addition is regiospecific in the direction predicted by Markownikoff s rule. 

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Hydration and Other Acid-Catalyzed Additions of Oxygen Nucleophiles... 

Addition of Water Alkenes don t react with pure water, but in the presence of a strong acid catalyst such as sulfuric acid, a hydration reaction takes place to yield an alcohol. An -H from water adds to one carbon, and an -OH adds to the other. For example, nearly 300 million gallons of ethyl alcohol (ethanol) are produced each year in the United States by the acid-catalyzed addition of water to ethylene ... 

Ethanol is prepared in large quantities by fermentation of sugars (see Chapter Opening) or by the phosphoric acid-catalyzed hydration of ethene (ethylene). The hydration (and other addition reactions) of alkenes are considered in detail in Chapter 12. 

The mechanistic pattern established by study of hydration and alcohol addition reactions of ketones and aldehydes is followed in a number of other reactions of carbonyl compounds. Reactions at carbonyl centers usually involve a series of addition and elimination steps proceeding through tetrahedral intermediates. These steps can be either acid-catalyzed or base-catalyzed. The rate and products of the reaction are determined by the reactivity of these tetrahedral intermediates. 

These brackets indicate that H+ is not consumed in the reaction. In other words, H+ is a catalyst, and therefore, we call this reaction an acid-catalyzed hydration. In order to understand why this reaction proceeds via a Markovnikov addition, we turn our attention to the mechanism. The proposed mechanism of an acid-catalyzed hydration... 

In addition to mineral acids, other acidic reagents may also catalyze hydration and related additions. Heteropoly acids are used mainly in industry.8 Ion-exchange resins applied primarily in industrial processes were studied in hydration9 and in the addition of alcohols to alkenes.10,11... 

Except for very reactive alkynes, acid-catalyzed hydrations are usually sluggish. This slow hydration can be overcome by the addition of catalytic amounts of mercury(II) salts. Such hydrations are generally mild and will tolerate the presence of other functional groups. Specific examples of mercury-catalyzed hydrations are discussed in the next section. 

In acid-catalyzed reactions, the distinction between single-species and complex catalysis is not always clear-cut. The actual catalyst is the solvated proton, H30+ in aqueous solution, and H20 (or a molecule of the nonaqueous solvent) may thus appear as a co-product in the first step and as a co-reactant in the step reconstituting the original solvated proton, possibly also in other additional steps, e.g., if the overall reaction is hydrolysis or hydration. Moreover, the acid added as catalyst may not be completely dissociated, and its dissociation equilibrium then affects the concentration of the solvated proton. At high concentrations this is true even for fairly strong acids such as sulfuric, particularly in solvents less polar than water. Such cases are better described as acid-base catalysis (see Section 8.2.1). 

The mechanistic picture established by study of hydration and alcohol addition reactions sets a pattern that will be evident in a number of other carbonyl reactions. The initial step is usually nucleophilic addition to the carbonyl group. This can be acid-catalyzed or base-catalyzed. Other reactions of carbonyl groups branch out mechanistically from this common nucleophilic addition step, and differ primarily in the fate of the addition intermediate. 

Ethanoi found in aicoholic beverages is generally produced via the fermentation of sugars by yeast. However, there are many other important industriai uses for ethanol (as a gasoline additive, as a solvent, for perfumes and paints, etc.), and therefore, an aiternative, more efficient process for producing pure ethanol is required. This can be accomplished by the acid-catalyzed hydration of ethyiene obtained from petroleum ... 

In cases where protonation of the alkene ultimately leads to carbocation rearrangements, acid-catalyzed hydration is an inefficient method for adding water across the alkene. Many other methods can achieve a Markovnikov addition of water across an alkene without carbocation rearrangements. One of the oldest and perhaps best known methods is called oxymercuration-demercuration ... 

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