Question

Which one of the following compounds most readily undergoes substitution by S_N2 mechanism?

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The correct option is B

The explanation for the correct option

(B)

SN2 reaction

• SN2 stands for Substitution nucleophilic bimolecular reaction.
• In this reaction, the rate of reaction depends upon the concentration of both reactants.
• It follows second-order kinetics.
• This reaction occurs in one step.
• In this reaction, the formation of the bond and cleavage of the bond occurs simultaneously.

Order of SN2 reaction

• The increasing order of the nucleophilic substitution by SN2:

tertiary alkyl halide < secondary alkyl halide < primary alkyl halide < methyl

• Structure compound B:

• Compound B is the primary alkyl halide.
• IUPAC name of Compound B is $1-\mathrm{chloro}\mathrm{ethane}$ and molecular formula is ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl}$.

The explanation for incorrect options

(A)

• Structure of compound A:

• Compound A is also a primary alkyl halide but the carbon atom attached to it is more hindered.
• IUPAC name of Compound A is $1-\mathrm{chloro}-2-\mathrm{methyl}\mathrm{propane}$ and molecular formula is ${\mathrm{C}}_4{\mathrm{H}}_9\mathrm{Cl}$.

(C)

• Structure of compound C:

• Compound C is also a secondary alkyl halide.
• IUPAC name of Compound C is $2-\mathrm{chloro}\mathrm{pentane}$ and molecular formula is ${\mathrm{C}}_5{\mathrm{H}}_{11}\mathrm{Cl}$.

(D)

• Structure of compound D:

• Compound D is also a tertiary alkyl halide.
• IUPAC name of Compound D is $2-\mathrm{chloro}-2-\mathrm{methyl}\mathrm{propane}$ and molecular formula is ${\mathrm{C}}_4{\mathrm{H}}_9\mathrm{Cl}$.

Therefore, the correct option is (B).