Rate of SN¹ reaction = K [substrate]¹
Rate of SN² reaction = K [substrate]¹[nucleophile]¹
The rate of an SN¹ reaction is unaffected by either the concentration or the nature of nucleophile, because the nucleophile does not participate in the rate determining step of an SN¹ reaction.
The rate of an SN² reaction depends on both the concentration and the nature of the attacking nucleophile.
The relative strength of a nucleophile (nucleophilicity) is measured in terms of the relative rate of SN² reaction with a given substrate.
A good nucleophile is one that reacts rapidly in an SN² reaction with a given substrate. A poor nucleophile is one that reacts slowly in an SN² reaction with the same substrate under comparable reaction conditions.
For example, methoxide anion is a good nucleophile for a substitution reaction with idomethane. It reacts rapidly by an SN² mechanism to form dimethyl ether:
Methanol, on the other hand, is a poor nucleophile for the reaction with idomethane. Under comparable conditions it reacts very slowly. It is not a sufficiently powerful Lewis base(i.e., nucleophile) to cause displacement of the iodide leaving group at a significant rate:
The relative strengths of nucleophiles can be correlated with three structural features:
- A negatively charged nucleophile is always a more reactive nucleophile than its conjugate acid. Thus HO‾ is a better nucleophile than H₂O and RO‾ is better than ROH.
- In a group of nucleophiles in which the nucleophilic atoms is the same, nucleophilicities parallel basicities. Oxygen compounds, for example, show the following order of reactivity:
- This is also their order of basicity. An alkoxide ion (RO‾) is a slightly stronger base than a hydroxide ion (HO‾), a hydroxide ion is a much stronger base than a carboxylate ion (RCO₂‾), and so on.
- When the nucleophilic atoms are different, nucleophilicities may not parallel basicities. For example, in protic solvents HS‾, N≡C‾ and I‾ are all weaker bases than HO‾, yet they are stronger nucleophiles than HO‾ .
