Question

Define Racemisation.

Hint:

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

Answer 1

Step 1: Chirality and optical activity.
A chiral molecule (one with an asymmetric carbon) exists as two non-superimposable mirror images called enantiomers. A pure sample of one enantiomer rotates plane-polarised light and is said to be optically active.
Step 2: What happens during racemisation.
When an optically pure enantiomer undergoes a reaction that generates both mirror-image forms in equal amounts, the mixture becomes optically inactive because the equal and opposite rotations cancel each other.
Step 3: Mechanism example.
In an $S_N1$ reaction, the chiral centre forms a planar carbocation intermediate. The nucleophile attacks from either face with equal probability, producing a 50:50 mixture of both enantiomers.
Step 4: Definition.
A 50:50 mixture of two enantiomers is called a racemic mixture (racemate), and the process of its formation from a pure enantiomer is called racemisation. Racemisation is the conversion of an optically active compound into an optically inactive racemic mixture containing equal amounts of both enantiomers.