Optical Isomers Example: lactic acid

Optical Isomers of lactic acid


In lactic acid CH₃ - CHOH - COOH, second carbon is chiral.

There are two optically active isomers of Lactic acid: d-lactic acid and l-lactic acid. In addition to these optically active varieties there is an optically inactive form which results when dextro and laevo (levo) varieties are present in equal quantities. It is called racemic mixture or (+-) lactic acid.

The racemic mixture is 50:50 mixture of d and l -isomers and hence have zero optical rotation as the rotation due to one enantiomer cancels the rotation due to the other. That is racemic mixture is optically inactive due to external compensation. The process of conversion of an enantiomer in to a racemic mixture is known as racemisation. Racemisation can be brought about by the action of heat, light and chemical reagent.

Dextro rotatory lactic acid may be obtained from meat extract and is known as sarcolactic acid. With muscular activity glycogen present in muscles break down to sarcolactic acid. During rest sarcolactic acid is converted back to glycogen.

Leavo rotatory lactic acid may be obtained by the fermentation of sucroseby Bacillus Acidi laevolactiti. Ordinary lactic acid in sour milk or manufactured by fermentation or by synthetic method is racemic mixture.