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Nucleophilic Substitution of Halogenoalkanes

Updated: May 7

A guide to the two pathways for nucleophilic substitution.


SN1 Mechanism

The mechanism involves two-steps.


With a tertiary halide:

In the first step, which is the slow step, the C-Cl bond will break and both electrons will go to Cl. A carbocation intermediate and Cl- form.

The second (fast) step sees the hydroxide, OH-, attacking the carbocation and forming the alcohol.

In the slow step, there is only one halogenoalkane, hence the rate is first order with respect to halogenoalkane and zero order with respect to hydroxide.


The rate equation:

rate = k[RX]


The overall order is one, hence SN1. Tertiary halides favour the SN1 mechanism - there are more electron-donating alkyl groups to stabilise the carbocation intermediate (carbocation is more likely to form).

SN2 Mechanism

The SN2 mechanism is a one-step reaction.

With a primary halide: