We mentioned previously that the hydration reaction of alkenes requires a strong acid as a catalyst, because water itself is too weak an acid to initiate the proton-transfer step. However, if a small amount of a strong acid such as sulfuric acid is present, hydronium ions, H3O⊕, are formed in sufficient amount to protonate reasonably reactive alkenes, although by no means as effectively as does concentrated sulfuric acid. The carbocation formed then is attacked rapidly by a nucleophilic water molecule to give the alcohol as its conjugate acid,2 which regenerates hydronium ion by transferring a proton to water. The hydration reactions sequence follows for 2-methylpropene:
In this sequence, the acid acts as a catalyst because the hydronium ion used in the proton addition step is regenerated in the final step.
Sulfuric acid (or phosphoric acid) is preferred as an acid catalyst for addition of water to alkenes because the conjugate base, HSO4⊖ (or H2PO4⊖), is a poor nucleophile and does not interfere in the reaction . However, if the water concentration is kept low by using concentrated acid, addition occurs to give sulfate (or phosphate) esters. The esters formed with sulfuric acid are either alkyl acid sulfates R—OSO3H or dialkyl sulfates (RO)2SO2. In fact, this is one of the major routes used in the commercial production of ethanol and 2-propanol. Ethene and sulfuric acid give ethyl hydrogen sulfate, which reacts readily with water in a second step to give ethanol:
A Biological Hydration Reaction
The conversion of fumaric acid to malic acid is an important biological hydration reaction. It is one of a cycle of reactions (Krebs citric acid cycle) involved in the metabolic combustion of fuels (amino acids and carbohydrates) to CO2 and H2O in a living cell.
The reaction is remarkable for a number of reasons. It is readily reversible and is catalyzed by an enzyme (fumarase) at nearly neutral conditions (pH s 7). Without the enzyme, no hydration occurs under these conditions. Also, the enzymatic hydration is a completely stereospecific antarafacial addition and creates L-malic acid. The enzyme operates on fumaric acid in such a way that the proton adds on one side and the hydroxyl group adds on the other side of the double bond of fumaric acid. This biological hydration reaction happens as one of the metabolic reactions that happens in the body of a living organism that has been well studied and is shown schematically in Figure 10-9.
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