Stereochemistry of Markovnikov Addition to Alkenes

A proton adds to the double bond of 1-butene to give an intermediate secondary carbocation. It is achiral because it has a plane of symmetry. Bromide ion can attack with equal probability from the top or the bottom to give a racemic mixture. 

Biochemical processes are catalyzed by enzymes that have multiple stereogenic centers and are therefore chiral. Enzymes provide a chiral environment in which to form stereogenic centers. As a consequence, only one enantiomer forms firom an enzyme-catalyzed reaction, even if the reactant is achiral. For example, fumaric acid reacts with water in an addition reaction catalyzed by the enzyme fumarase in the citric acid cycle to give only (i)-malic acid. We show the carboxyhc acids as their conjugate bases because they are ionized at pH 7. These ionic compounds are called fumarate and malate. This reaction converts fumarate to (i)-malate. 

Only one enantiomer forms in the reaction, and only the trans geometric isomer reacts in the presence of fumarase. The cis unsaturated isomer is not converted to a hydrated product by fumarase. In fact, it does not bind to the enzyme at all. 

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