Reaction Stereochemistry Addition of H2O to an Achiral Alkene

Most of the biochemical reactions that take place in the body, as well as many organic reactions in the laboratory, yield products with chirality centers. For example, acid-catalyzed addition of H2O to 1-butene in the laboratory yields 2-butanol, a chiral alcohol. What is the stereochemistry of this chiral product If a single enantiomer is formed, is it R or 5 If a mixture of enantiomers is formed, how much of each In fact, the 2-butanol produced is a racemic mixture of R and 5 enantiomers. Let s see why. 

Because they are energetically Identical, they are equally likely and lead to a racemic mixture of products. The dotted C-0 bond In the transition state Indicates partial bond formation. 

As a general mle, the formation of a new chirality center by reaction of achiral reactants always leads to a racemic mixture of enantiomeric products. Put another way, optical activity can t appear from nowhere an optically active 

In contrast to laboratory reactions, enzyme-catalyzed biological reactions often give a single enantiomer of a chiral product, even when the substrate is achiral. One step in the citric acid cycle of food metabolism, for instance, is the aconitase-catalyzed addition of water to (Z)-aconitate (usually called czs-aconitate) to give isocitrate. 

Even though czs-aconitate is achiral, only the (2R,3S) enantiomer of the product is formed. As discussed in Sections 5.11 and 5.12, czs-aconitate is a prochiral molecule, which is held in a chiral environment by the aconitase enzyme during the reaction. In that chiral environment, the two faces of the double bond are chemically distinct, and addition occurs on only the Re face at C2. 

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