Racemic Mixture and Racemisation Preparation and Properties

Racemic Mixture and Racemisation Preparation and Properties

→ When equal amounts of dextro rotatory and laevo rotatory isomers are present, the resulting mixture becomes optically inactîve because the optical activities of each isomer cancel each other. Such a mixture is called a racemic or dl mixture or racemic modification.

→ Racemic mixtures can be symbolized by a — or (±) prefix in front of the substance’s name. Since eriantiomers have equal and opposite specific rotations, a racemic mixture exhibits no optical activity.

Racemic Mixture and Racemisation Preparation and Properties

→ For example,if racemic mixture of lactic acid is taken, it is written as dl-lactic acid. Thus, it is optically inactive and cannot rotate plane of polarised light in any direction, Infact, the amount of light rotated by half part i.e., d-lactic acid is neutralised by another half part i.e., i-lactic acid. Thus, both the parts combine together to form a racemic mixture. This is known as external compensation.
For example, following reactions give racemic mixtures,

Racemic Mixture and Racemisation Preparation and Properties 1

→ The process by which a racemic mixture is formed from chiral materials is called racemisation. Thus, above reactions are racemisation reactions.

Racemic Mixture and Racemisation Preparation and Properties

→ Such reactions which occur by unimolecular nucleophilic substitution always form racemic mixture. In these reactions, a highly reactive carbocation is formed on which nucleophile can attack by both sides in the next fast step. Thus, mixture of 50% d and 50% ¡ isomer is formed. Example, 2-Chloropropanoic acid on alkaline hydrolysis gives dl racemic mixture. The reaction can be represented as follows:

Racemic Mixture and Racemisation Preparation and Properties 2

Racemisation can be affected by following reasons:

  • By heating at high temperature
  • By heating with water
  • By adding base
  • By adding sulphuric acid
  • By auto racemisation

Chemistry Notes