S_N1 and S_N2 Reactions

 S_N1 Reaction (Substitution Nucleophilic Unimolecular )

v  This is nucleophilic substitution reaction which follows first order kinetics.

v  The S_N1 Mechanism is facilitated by protic solvent such as water, alcohol etc.

v  The S_N1 reaction occurs in two steps.

v  v Step 1: Formation of Carbocation. The polarized C – halogen bond (C-Br) undergoes slow cleavage to form a carbocation and a halide ion.

 

v  Step 2: Nucleophilic attack on carbocation. The carbocation is very reactive therefore, it readily reacts with a nucleophile, OH^- to form the product.

 

 v  First step is the slowest step which is the rate determining step of the reaction. In first step there is only one reactant therefore, this mechanism follows first order mechanism.

v  v The increasing order of reactivity is

1° halide < 2° halide < 3° halide

Reason − Greater the stability of carbocation, more easily the alkyl halide is formed and hence, faster is the reaction rate.

      For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the mechanisms R–I> R–Br>R–Cl>>R–F.

v  v The increasing order of stability of carbocation is 1° < 2° < 3°. Since 1° halide forms 1° carbocation, 2° halide forms 2° carbocation, and 3° halide forms 3° carbocation. Therefore, the increasing order of reactivity is 1° halide < 2° halide < 3° halide.

v v Allylic and benzylic halides are very reactive towards S_N1 reaction because of stabilisation of their carbocations through resonance.

v  v In case of optically active alkyl halide, the product obtained is a Racemic mixture. Because the intermediate carbocation formed in the slow step has planar structure. Therefore the attack of nucleophile can take place with equal ease from both the side (front and rear) forming a mixture equimolar enantiomers.

     v This means in this reaction mechanism Retention of configuration and Inversion of configuration both takes place.

 

S_N2 Reaction (Substitution Nucleophilic Unimolecular )

v  v This is Nucleophilic substitution reaction which follows Second order kinetics.

v  v The S_N2 reaction occurs in single step through the formation of transition state. In this mechanism, simultaneously the Incoming nucleophile (such as OH^-) attacks the partially positive charged carbon from the direction 180⁰ away from the halogen atom i.e., from the backside. This leads to the formation of Transition state with a partial formed C----OH bond and partially broken C-----Br bond.

 

 v This reaction occurs in single step which has two reagents therefore, it follows second order kinetics.

v  v The increasing order of reactivity is

3° halide < 2° halide < 1° halide
Reason − Due to the presence of bulky substituents in 3° and 2° halides which exerted steric       hinderance, nucleophile cannot approach.

 

v  v As the Incoming nucleophile attacks from rear side, it inverts the configuration of the compound in much the same way as an umbrella is turned inside out in a strong wind. This process is called the Inversion of configuration.

 

    For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both  the mechanisms R–I> R–Br>R–Cl>>R–F.

 

By:

Mr. S. K. Nigam

PGT Chemistry