Question

When pure (S)-lactic acid is esterified by racemic butan-2-ol, the product is 2-butyl lactate, with the following structure:

(a) Draw three-dimensional structures of the two stereoisomers formed, specifying the configuration at each asymmetric carbon atom. (Using your models may be helpful.)
(b) Determine the relationship between the two stereoisomers you have drawn

Accepted Answer

Step 1: Analyze the reaction. The esterification of (S)-lactic acid with racemic butan-2-ol produces 2-butyl lactate. The reaction involves the formation of an ester bond between the carboxylic acid group of lactic acid and the hydroxyl group of butan-2-ol. Step 2: Identify the stereocenters. (S)-lactic acid has one stereocenter at the carbon bearing the hydroxyl group. Racemic butan-2-ol has one stereocenter at the carbon bearing the hydroxyl group. The product, 2-butyl lactate, will have two stereocenters: one from lactic acid and one from butan-2-ol. Step 3: Draw the two stereoisomers of 2-butyl lactate. Since butan-2-ol is racemic, it contains both (R)- and (S)-enantiomers. When combined with (S)-lactic acid, the two possible products are (S,S)-2-butyl lactate and (S,R)-2-butyl lactate. Draw these structures in three dimensions, showing the configurations at each stereocenter. Step 4: Determine the relationship between the two stereoisomers. The two stereoisomers, (S,S)-2-butyl lactate and (S,R)-2-butyl lactate, are diastereomers. Diastereomers are stereoisomers that are not mirror images of each other and have different physical and chemical properties. Step 5: Verify the configurations. Use the Cahn-Ingold-Prelog priority rules to confirm the absolute configurations at each stereocenter in the two stereoisomers. This ensures that the structures are correctly assigned as (S,S) and (S,R).

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Concetti chiave

Stereochemistry

Esterification Reaction

Stereoisomer Relationship