Oxidation!

Stearic hindrance and stability of carbonium ion, less stearic hindrance and less stability of carbonium ion favors the SN2 reaction.

An SN2 reaction is a one step bimolecular substitution mechanism which is 2nd order in kinetics.

An electron rich species (called a nucleophile) attacks an electrophile (electron deficient species) while a leaving group (LG) leaves. Typically a good nucleophile for an SN2 reaction are halides and moderate to strong bases. Good leaving groups are species that are stable on their own like halides, water, tosylate, and protonated ethers.

Conditions for an SN2 reaction are similar to the conditions necessary for an E2 elimination reaction; the two are in constant competition.

This follows a Sn2 reaction.

Wurtz reaction is a SN2 reaction and thus requires an aprotic solvent as it medium. dry ether proves to be a very good, non-polar, aprotic solvent and is thus used in Wurtz reaction.