Nucleophilic substitution

In chemistry, nucleophilic subsitution is a type of chemical reaction in which one nucleophile (electron donor) replaces another.

An example is the hydrolysis of halogenoalkanes by alkalis like hydroxide ions, OH^-.

The two main mechaisms are called SN1 and SN2. S stands for chemical substitution, N stands for nucleophilic, and the number represents the order of the reaction.

SN1

The SN1 reaction takes place in two steps.

1. C(CH₃)₃Br --> C(CH₃)₃⁺ + Br^-
2. C(CH₃)₃⁺ + OH^- --> C(CH₃)₃OH

This gives the overall reaction

C(CH₃)₃Br + OH^- --> C(CH₃)₃OH + Br^-

SN1 occurs where the central carbon atom is surrounded by bulky groups. The rate depends only on the concentration of the substrate

rate = k[RX]

Because the intermediate cation, R⁺, is planar, an optically active substrate gives a racemic mixture because attack can occur from either side. In the SN2 reaction, the addition of OH^- and the elimination of Br^- take place simultaneously.

SN2

SN2 occurs where the central carbon atom is easily accessible to the nucleophile. The rate is second order, depending on the nucleophile concentration as well as the substrate.

rate = k[RX][OH^-]

The nucleophile enters on the opposite side of the carbon to the leaving group, so an optically active substrate has its structure inverted.