H2 Chemistry 11 - Organic Chemistry: Nucleophilic Substitution of Halogenoalkanes

Step 1 - State the problem

Explain why two-bromo-two-methylpropane undergoes nucleophilic substitution by an S N one mechanism, while one-bromobutane follows an S N two mechanism.

Step 1 - State the problem

Then describe the mechanism of each reaction with aqueous sodium hydroxide.

Step 1 - State the problem

This is a classic A-level comparison question on halogenoalkane reactivity.

Step 2 - SN1 mechanism for the tertiary halogenoalkane

For the tertiary substrate, the mechanism is S N one: substitution, nucleophilic, unimolecular.

Step 2 - SN1 mechanism for the tertiary halogenoalkane

Step one is the slow, rate-determining step: the carbon-bromine bond breaks heterolytically to form a tertiary carbocation and a bromide ion.

Step 2 - SN1 mechanism for the tertiary halogenoalkane

Step two is fast: the hydroxide ion, acting as a nucleophile, attacks the positively charged carbon to form two-methyl-propan-two-ol.

Step 2 - SN1 mechanism for the tertiary halogenoalkane

The tertiary carbocation is stabilised by the inductive effect of three methyl groups donating electron density.

Step 3 - SN2 mechanism for the primary halogenoalkane

For the primary substrate, the mechanism is S N two: substitution, nucleophilic, bimolecular.

Step 3 - SN2 mechanism for the primary halogenoalkane

This is a one-step process. The hydroxide ion attacks the carbon bonded to bromine from the back side, while the bromine leaves simultaneously.

Step 3 - SN2 mechanism for the primary halogenoalkane

The transition state has five groups around the central carbon: three hydrogens from one side, the incoming hydroxide, and the departing bromide.

Step 3 - SN2 mechanism for the primary halogenoalkane

This back-side attack causes inversion of configuration at the carbon centre.

Step 4 - Why tertiary favours SN1 and primary favours SN2

A tertiary carbon is too sterically hindered for the nucleophile to attack directly in a back-side approach.

Step 4 - Why tertiary favours SN1 and primary favours SN2

However, the tertiary carbocation intermediate is highly stabilised, so the S N one pathway is energetically favourable.

Step 4 - Why tertiary favours SN1 and primary favours SN2

A primary carbon has low steric hindrance, making back-side attack easy.

Step 4 - Why tertiary favours SN1 and primary favours SN2

But a primary carbocation would be very unstable, so the S N one pathway is not favoured for primary substrates.

Step 5 - Summarise and note common pitfalls

To summarise: tertiary halogenoalkanes undergo S N one via a carbocation intermediate, while primary halogenoalkanes undergo S N two via direct back-side attack.

Step 5 - Summarise and note common pitfalls

Common mistake one: drawing S N two for a tertiary substrate. The steric hindrance prevents this.

Step 5 - Summarise and note common pitfalls

Common mistake two: forgetting to show the curly arrows from the nucleophile's lone pair to the electrophilic carbon.

Step 5 - Summarise and note common pitfalls

Always draw the full mechanism with curly arrows, and state whether each step is slow or fast.