The best reaction sequence to convert 2-methyl-1-bromopropane into 4-methyl-2-bromopentane is

Correct Answer: (B)

Step (i): NaC≡CH in ether

This is nucleophilic substitution (SN2) where the bromide in 2-methyl-1-bromopropane is replaced by an acetylide ion (HC≡C⁻).

Reaction:

CH3-CH(Br)-CH3 + NaC≡CH → CH3-CH(C≡CH)-CH3

Result: 2-methylpropyl acetylene.

Step (ii): H3O⁺, HgSO4

This is acid-catalyzed hydration of alkyne (Markovnikov addition of water).

Terminal alkyne → ketone.

The triple bond reacts with water in the presence of Hg²⁺ to form:

CH3-CH(C≡CH)-CH3 → CH3-CO-CH2-CH3

Now we have a ketone at the proper position.

Step (iii): HBr, heat

This is halogenation of the ketone via enol intermediate.

The ketone tautomerizes to the enol, then bromine adds to the alpha carbon:

CH3-CO-CH2-CH3 → CH3-C(Br)-CH2-CH3

Result: 4-methyl-2-bromopentane.

We first extend the carbon chain using acetylide ion.

Then, we convert the alkyne into a ketone at the correct position.

Finally, bromination gives the desired bromopentane.

This sequence works smoothly because:

SN2 with acetylide extends chain.

HgSO4/H3O⁺ ensures Markovnikov hydration.

HBr adds bromine at the alpha position to ketone.