Nucleophilic Substitution Mechanisms

In the previous lesson, you learned that halogenoalkanes undergo nucleophilic substitution. Now we examine the two different mechanisms by which this can occur: SN2 and SN1. The distinction between these pathways is one of the most important concepts in A-Level organic chemistry.

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SN2: Bimolecular Nucleophilic Substitution

The Mechanism

SN2 stands for Substitution, Nucleophilic, 2nd order (bimolecular). It is a one-step mechanism:

The nucleophile (e.g., OH⁻) attacks the δ+ carbon from the opposite side to the halogen leaving group. This is called backside attack.

In the mechanism diagram:

1. A curly arrow is drawn from the lone pair on OH⁻ to the δ+ carbon bonded to the halogen.
2. Simultaneously, a curly arrow is drawn from the C-X bond to the halogen, showing the bond breaking and X⁻ departing.

Both bond-making (C-OH) and bond-breaking (C-X) happen at the same time in a single concerted step. There is no intermediate — only a transition state in which the carbon is partially bonded to both the incoming nucleophile and the departing halogen.

Key Features of SN2