Nucleophilic Substitution Reactions II

Factors affecting the rate of reaction:

Four factors affect the rate of the reaction are as follows

1)  The substrate

2)  The nucleophie

3)  The leaving group

4)  The solvent

Nature of the substrate:

          The substrate plays the most important part in determining the rate of the reaction. Methyl and 1° alkyl halides undergo SN2 reactions with ease.  2° Alkyl halides react more slowly. 3° Alkyl halides do not undergo SN2 reactions. This order of reactivity can be explained by steric effects. Steric hindrance caused by bulky R groups makes nucleophilic attack from the backside more difficult, slowing the reaction rate. Therefore, to maximise the rate of the SN2 reaction, the back of the substrate must be as unhindered as possible.

          Moreover, compounds like '1-chloro-1-ethene' too do not undergo nucleophillic substitution easily because the carbon to chlorine bond is said to be of partial double bond character, thus is harder to break. Another factor leading to an SN2 reaction due to substrate involves the stability and ease by which the carbocation is formed after removing the leaving group. This means the more stable the carbocation is after removing the leaving group, the more likely it is an SN1 reaction will occur instead of an SN2. Among the stabilization methods to be considered are: resonance stabilization, hyper-conjugative stabilization, inductive effect stabilization, or the formation of an aromatic ring molecule (as in the case of 7-chloro cyclohept-1, 3, 5-triene, as it will form a tropolium carbocation which is aromatic).