The Student Room Group

A level chemistry optical isomerism MC questions

Can someone please explain why the answer is D for all of them (question 1-3) ? I really don’t understand how they got to that answer for them
(edited 9 months ago)
Original post by 1234kelly
Can someone please explain why the answer is D for all of them (question 1-3) ? I really don’t understand how they got to that answer for them


Q1: A Racemic mixture forms when a nucleophile attacks a trigonal planar carbonyl
from both sides. This carbonyl must be unsymmetrical. Hence, there is an equal chance of
either enantiomer forming so
a racemate forms. No
optical activity is seen.

It is D as only one product will form even with the attack being from above or below will form. This means the compound will not have a chiral centre. Centre carbon is attached to: OH, C-N (triple bond), and 2 methyl groups on either side.

Q2: Stereoisomerism occurs when molecules have the same molecular formula but different spatial arrangement of atoms. An enantiomer is a stereoisomer and so to have enantiomers, a molecule must have at least 1 chiral centre, which are carbon atoms bonded to four different substituents. In 1,3-dichloropropane, there are no carbon atoms with four different group of atoms/atom attached.

Q3: Upon reduction, alkenes will become alkanes and aldehyde and ketones will revert to primary alcohols and secondary alcohols respectively. It is D because when D is reduced to butan-2-ol a chiral centre is evident. The carbon is attached to the following: one OH group, one hydrogen, 1 methyl, and one ethyl. This makes it optically active and a stereoisomer.

Hope this helps!

Quick Reply