Question

Which attacking species shown below would be the best choice to enforce an E2 pathway over an SN2 pathway for the reaction below? Attacking Species? BI CH; H -¢-o CH,MgB HC-0-0 CH; (a) H (b) (c) NH OH (d) SH (1) e

Answer 1

If the attacking species is bulky, then instead of acting as a nucleophile, it abstracts the proton and act as a base and hence elimination is preferred

Out of the species given, a i.e, tert-butoxide will do the elimination (E2 pathway) over nucleophilic attack (SN2 pathway).

The best choice would be a. Option-a is the correct answer.

The S_N2 reaction is favored by polar aprotic solvents – these are solvents such as acetone, DMSO, acetonitrile, or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile. They don't stabilize the nucleophile otherwise the activation energy for the reaction would be high.

Hence, solvent I i.e, DMSO and solvent III i.e, DMF would be satisfactory for SN2 reactions as they are polar aprotic solvents.

Option-e i.e, I and III is the correct answer.

For the SN2 reaction, there is no carbocation intermediate as the intermediate is very less stable. Thus, for the alkyl halide b i.e, methyl bromide, there should be no carbocation intermediate after loss of Br^- . There would be no stabilization of carbocation intermediate through resonance , inductive effect or hyperconjugation.

Hence, alkyl halide b i.e, CH3Br would react fastest under SN2 conditions.

Option-b i.e, CH3Br is the correct answer.