Nucleophilic Substitution

Halogenoalkanes are classified as primary, secondary or tertiary...

depending upon the number of carbon atoms bonded to the carbon atom to which the halogen is attached, as shown below:

Nucleophilic Substitution reactions

Their reactions are characterised by nucleophilic substitution of the halogen atom, owing to polarity of the carbon¾halogen bond in which the electron-deficient carbon is susceptible to attack by an electron-rich species, namely a nucleophile.

For example, consider the hydrolysis of halogenoalkanes.

R-X + H2O ® R-OH + H+ + X-

With primary and secondary halogenoalkanes, the reaction is very slow at ordinary temperatures, but rapid with tertiary halogenoalkanes. Alkaline hydrolysis is much faster.

R-X + OH- ® R-OH + X-

The rate expression for the hydrolysis of primary halogenalkanes is

Rate = k[RX][OH-]

A one-step mechanism is proposed, in which both reactants are involved in the rate-determining step. (The curly arrows show the direction of movement of a pair of electrons.)

As the OH- ion approaches the electron-deficient carbon atom donating a pair of electrons, the halide ion moves away taking with it a pair of electrons. A transition state is involved, in which the hydroxide and halide ions are both partially bonded to the same carbon.

The rate expression for the hydrolysis of tertiary halogenalkanes is:

Rate = k[RX]

Therefore, the mechanism must involve at least two steps, in which the halogenoalkane takes part in the slow rate-determining step. The following mechanism is proposed:

The tertiary halogenoalkane first ionises in the slow step, followed by the rapid addition of the nucleophile to the intermediate carbocation.

  1. (CH3)3C-Br    ®    (CH3)3C+ + Br-

  2. (CH3)3C+ + OH-    ®    (CH3)3C-OH

It is the extra stability associated with a tertiary carbocation compared with a primary carbocation, owing to electron-donating alkyl groups, which favours this two-step mechanism.

Secondary halogenoalkanes show a mixture of both of the above mechanisms, with the one-step mechanism predominating.