Nucleophilic substitution reactions

• In the C-X bond there is a partial +ve charge on the carbon atom and -ve on the halogen atom. Thus nucleophilies attack the electron deficient carbon resulting in the displacement of the weaker nucleophile, the halide ion.

The halide ions are substituted only if the attacking nucleophile is stronger. As the halide ion itself is a very weak nucleophile, the attacking nucleophile should be stronger than it. The order of reactivity of various alkyl halides towards nucleophilic substitution is:RI > RBr > RCl > RF

SN1 reaction (substitution

nucleophilic, first order).

The first step is slow and is the rate-determining step. As the nucleophile (Z-) is not involved in the rate-determining step, the reaction depends only upon the concentration of alkyl halide (RX) and is, therefore, a first order reaction.Rate = k [RX]

The order of reactivity depends upon the stability of carbonium ion formed in the first step. Since the 3° carbonium ion is most stable, the ionization of tertiary alkyl halide is favored. The order of reactivity for SN

1 reaction is, tertiary > secondary > primary

SN2 reaction (substitution

nucleophilic, second order). This type of reaction occurs in one step through the

formation of transition state as:

Here, the rate of reaction depends upon the concentration of both the alkyl halide and the nucleophile.The transition state from tertiary alkyl halide is less stable due to steric hindrance i.e., crowding of bulky groups. The order of reactivity is: primary > secondary > tertiary.

Nucleophilic reactions of haloalkanes

2.Replacement by alkoxy group(Formation of ethers) - Williamson Synthesis

3.Substitution by cyano group

4.Substitution by isocyanide group

5.Substitution by nitrite group

6.Substitution by nitro group

7.Substitution by amino group (formation of amines)

8.Substitution by carboxyl group (formation of esters)

9.Substitution by hydrosulphide group

10.Substitution by mercaptide group

11.Substitution by alkynyl group (formation of alkynes)

Haloarenes are lesser reactive towards nucleophilic reactions than haloalkanes due to following reasons :

1.Resonance effect