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

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

2019 p1 q20 question

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.

2019 p1 q20 deduction

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.

2019 p1 q20 stability of tertiary carbocation

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

2019 p1 q20 option A

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

Option B - (CH3CH2)3CCl

2019 p1 q20 option B

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

Option C - (CH3)2CHCH2C(CH3)2Cl

2019 p1 q20 option C

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

Option D - (CH3)2CHC(CH3)(CH2CH3)Cl

2019 p1 q20 option D

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

Back to list of questions for 2019 A Level H2 Chemistry Paper 1

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