Four substrates CH3-X are treated with Br- under identical S_N2 conditions. The fastest reaction is shown by

JEE Main levelSingle correct optionDifficulty 2 of 3o1012

Four substrates $CH X_{3} - X$ are treated with $Br X^{-}$ under identical $S_{N} 2$ conditions. The fastest reaction is shown by

The leaving groups are

H X_{2} O

OH X^{-}

CH X_{3} O X^{-}

and

NH X_{2} X^{-}

respectively. Neutral water, the conjugate base of

H X_{3} O X^{+}

(pKa about -1.7), is by far the best leaving group. The other three leave as strongly basic anions with conjugate acid pKa values of 15 or more.