2019 A Level H2 Chemistry Paper 1 Question 20 - Deduce Chloroalkane Forming Racemic Mixture via SN1 Mechanism

Here's 2019 A Level H2 Chemistry Paper 1 Question 20.

Given chloroalkane X undergoes nucleophilic substitution to form a racemic mixture.

A racemic mixture is an equimolar mixture of optical isomers.

This means the alcohol product must contain a chiral carbon, and chloroalkane X must also have a chiral carbon.

Also, we know that there are 2 pathways that nucleophilic substitution can occur - SN1 or SN2 mechanism.

Only SN1 mechanism can form a racemic mixture, so the mechanism for this question has to be via SN1.

For a detailed discussion of this mechanism, check out this previous video that I've done on nucleophilic substitution mechanism of halogenoalkanes.

Primary halides will favour SN2 mechanism, while tertiary halides will favour SN1 mechanism.

SN1 mechanism is a two-step mechanism where the C-X bond is broken to form carbocation and halide X- in the first step.

Tertiary halides form tertiary carbocations which are bonded to 3 electron-donating alkyl groups.

This will stabilise the carbocation significantly, which favours its formation and in turn favours SN1 mechanism.

Since we know that the mechanism for this question is via SN1, we would expect the chloroalkane X to be a tertiary chloroalkane.

Finally we can run through the options keeping in mind X must have the following criteria:

- chiral carbon
- tertiary chloroalkane

Option A - CH₃(CH₂)₆Cl

This compound has no chiral carbon so will not be the answer.

Option B - (CH₃CH₂)₃CCl

This compound has no chiral carbon so will not be the answer.

Option C - (CH₃)₂CHCH₂C(CH₃)₂Cl

This compound has no chiral carbon so will not be the answer.

Option D - (CH₃)₂CHC(CH₃)(CH₂CH₃)Cl

Finally we found our chiral carbon which is also a tertiary chloroalkane.

So the answer to this question will have to be option D.

Topic: Halogenoalkane, Organic Chemistry, A Level Chemistry, Singapore

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