Optical Activity of Nucleophilic Addition Product

Let's consider the nucleophilic addition reaction of carbonyl compounds.

230320 nucleophilic addition reaction

Notice the cyanohydrin product has a chiral carbon which is bonded to 4 different groups.

Does this mean the product is optically active?

Check out this video lesson on optical isomerism and cis-trans isomerism.

Interestingly the product is optically inactive due to a racemic mixture being formed.

We need to use the mechanism to understand why this happens.

Here's the detailed explanation of nucleophilic addition mechanism of carbonyl compounds.

Since the carbonyl carbon is trigonal planar, both sides of the sp2 carbon are equally exposed.

Hence the nucleophile CN- can attack the carbonyl carbon from both sides to equal extent to form an equimolar mixture of optical isomers, ie racemic mixture which is optically inactive.

230320 racemic mixture

This outcome is also seen in SN1 mechanism of halogenoalkanes where the carbocation formed is trigonal planar.

Learn all about nucleophilic substitution mechanism of halogenoalkanes.

So when we are given that the product is optically inactive, the possible reasons are:

1. No chiral carbon
2. Presence of chiral carbon but racemic mixture due to sp2 hybridised carbon being attacked eg SN1 and NA

230320 product optically inactive

Topic: Carbonyl Compounds, Organic Chemistry, A Level Chemistry, Singapore

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