Ques. 1AIPMT 2014
Which of the following compounds will undergo racemisation when solution of KOH hydrolyses? [figure]
- (i) and (ii)
- (ii) and (iv)
- (iv)
- (i) and (iv)
Answer
(3) (iv)
Solution
Racemisation is possible only when the substrate is optically active, i.e. the carbon bearing Cl is a chiral (asymmetric) carbon, and the reaction proceeds through a planar carbocation (SN1).
(i) C6H5CH2Cl, (ii) CH3CH2CH2Cl and (iii) (CH3)2CHCH2Cl: the carbon bearing Cl carries two H atoms ⇒ achiral ⇒ no racemisation.
(iv) 2-Chlorobutane: the carbon bearing Cl has four different groups (H, Cl, CH3, C2H5) ⇒ chiral.
As per NCERT, in SN1 hydrolysis of an optically active halide the planar carbocation is attacked from both faces ⇒ racemic mixture.
∴ Only (iv) undergoes racemisation.
Answer: (3)