What is S_N2 reaction? Is the S_N2 reaction exothermic or endothermic reaction? Draw an enthalpy diagram for an exothermic S_N2.

OR

Give reason:
(a) Why is PhO- a weaker nucleophile than RO– ?
(b) Chloride is a better nucleophile than bromide in a polar-aprotic solvent such as dimethylsulf oxide
(c) The SN2 reaction of optically active 2-iodobutane with I- ion leads to racemization rather than inversion.

S_N2 reaction is bimolecular nucleophilic substitution reactions, where two molecules takes part in rate determining step.
The reaction is thermo chemically favourable only when a stronger base displaces a weaker base. Hence, successful displacements are exothermic.
This is a one-step reaction. The enthalpy diagram is :


OR

(a) In Ph O– the nucleophilic pairs of electrons on oxygen are involved in resonance. This resonance interaction decreases their availability to participate in nucleophilic processes

No such resonance is possible in the case of the alkoxide ion. 
(b) C – Cl bond is stronger than. C – Br bond.
(c) This reaction do proceed by inversion. But, the product of each displacement is the enantiomer of the substance. This produce together with another substrate molecule, constitutes a racemic pair.