Question

Define Racemisation.

Hint:

Racemisation = 1 pure enantiomer $\rightarrow$ 50:50 mixture of two enantiomers (zero net optical rotation).

Answer 1

Step 1: Concept
Stereochemistry and optical activity.

Step 2: Meaning
Enantiomers are chiral molecules that rotate plane-polarized light in opposite directions (dextro and laevo).

Step 3: Analysis
When a pure enantiomer undergoes a reaction such as an $S_N1$ reaction that produces equal amounts of both enantiomers, the resulting mixture becomes optically inactive because the optical rotations produced by the two enantiomers exactly cancel each other.

• Enantiomers are non-superimposable mirror images of one another.

• A pure sample containing only one enantiomer rotates plane-polarized light and is therefore optically active.

• In an $S_N1$ reaction, the leaving group first departs to form a planar carbocation intermediate.

• Since the carbocation is trigonal planar, the nucleophile can attack from either side with nearly equal probability.

• Consequently, both enantiomers are formed in approximately equal amounts.

• A mixture containing equal amounts of the two enantiomers is called a: \[ \boxed{\text{racemic mixture (racemate)}}. \]

• One enantiomer rotates plane-polarized light clockwise, whereas the other rotates it by exactly the same magnitude in the opposite direction.

• Therefore, the two optical rotations cancel each other completely.

• Hence, a racemic mixture is optically inactive despite containing optically active molecules.

Step 4: Conclusion
This process of forming a racemic mixture from a pure enantiomer is termed racemisation.

Final Answer: Racemisation is the process of conversion of an optically active enantiomer into an optically inactive racemic mixture (an equimolar 50:50 mixture of dextro and laevo enantiomers).