An organic chemist evaluates the relative reactivities of the following four halomethanes with a standard strong nucleophile: (I) , (II) , (III) , (IV) . If these specific compounds are rigidly arrang

Question

An organic chemist evaluates the relative $S_N2$ reactivities of the following four halomethanes with a standard strong nucleophile: (I) $CH_3I$, (II) $CH_3Br$, (III) $CH_3Cl$, (IV) $CH_3F$. If these specific compounds are rigidly arranged in strictly descending order of their reaction rates, what is the precise numerical position (1, 2, 3, or 4) occupied by $CH_3Cl$?

Prashant Kotian · Accepted Answer

Correct answer: 3. In an reaction on identical carbon skeletons, the rate is exclusively determined by the leaving group ability of the halogen. A good leaving group is the weak conjugate base of a strong acid. The acidity order is . Therefore, the leaving group ability follows the sequence: . The reactivity of the halomethanes decreases in the exact same sequence: . Thus, occupies the 3rd position in this descending series.

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