Neil Keddie- Lecture 3 Nucleophilic Substitutions 2

Question 1

Why is SN1 substitution impossible at ring junctions?

Answer 1

The 120° bond angle is impossible so the carbocation is unstable.

Question 2

In unsymmetrical allylic captions in an SN1 reaction where does substitution occur?

Answer 2

The least sterically hindered end.

Question 3

What happens when an enantiomerically pure secondary substrate is used in an SN1 reaction?

Answer 3

A racemic mixture of products from attack above and below

Question 4

Do additional groups on the reaction centre favour or hinder SN2 reactions?

Answer 4

Hinder due to steric effects

Question 5

Explain the trends and exceptions in SN2 substitutions on secondary alkyl bromides

Answer 5

The rate increases as the ring increases in size from 3 to 6 carbons however the rate is highest in a five carbon ring as it is relatively close to the desired 120° bond angle and has no ring flip thatight require energy input.

Question 6

Is equatorial or axial positioning of the leaving group faster for SN2 reactions?

Answer 6

Axial as there is no steric hindrance for attack at 180°

Question 7

What happens when an SN2 reaction uses an enantiomerically pure secondary substrate?

Answer 7

Stereochemical inversion