Question

Which of the following alkyl bromides will undergo the $S_{N}2$ reaction the fastest?

• A.
• B.
• C.
• D.

Answer 1

The correct option is A

The $S_{N}2$ reaction involves displacement of a leaving group (usually a halide or a tosylate), by a nucleophile. This reaction works the best with methyl and primary halides (a and b) because bulky alkyl groups block the backside attack of the nucleophile, but the reaction does work with secondary halides (c), and will not react at all with tertiary halides (d).
In the following example, the hydroxide ion is acting as the nucleophile and bromine is leaving group.
Now both a and b are primary halides but b is more substitiuted than a and thus b will provide hinderance to incoming nucleophile and slow down the reaction as compared to a.
Hence, overall reactivity order will be a>b>c>d