Reactions of Haloalkanes - Substitution Nucleophilic Bimolecular

Description:

Bimolecular indicates− second order kinetics, i.e., the rate depends both on the concentration of R − X and Nu⁻.

It is a one-step reaction and no intermediates are formed.

In the transition state,

• Two reacting species are involved in the transition state (RDS), thus bimolecular.

• The new bond formation between HO − − − C and the bond breaking between C − − − Cl bond take place simultaneously.

• Inversion of configuration takes place: back-side attack.

• A complex with carbon containing 5 bonds makes it unstable.

Since Nu⁻ approaches the tetrahedral R − X, a bulky nucleophile or a bulky substrate is not favoured due to steric inhibition.

Thus, order of reactivity of R − X towards S_N2 reaction is −

1⁰ > 2⁰ > 3⁰

Strong nucleophile is required.

S_N2 is carried out in the presence of polar aprotic solvents like DMF, DMSO favour S_N2 reaction. Polar Protic solvents are not preferred for S_N2 reaction because it deactivates the strong (charged) nucleophile when proton of the polar protic solvents forms hydrogen bonds with the negatively charged nucleophile. Since the reactivity of the nucleophile is decreased, it does not undergo S_N2 reaction.

Reactivity of halides − R − I > R − Br > R − Cl > R − F

The steric hindrance caused by various haloalkanes can be visualized as given below: Greater the crowding around the C − X bond, more is the steric inhibition and lower is the reactivity towards S_N2 reaction.

Comparison between S_N1 and S_N2 reaction