Arrange the following compounds in the decreasing order of reactivity towards $S_N1$ reaction
(A) $(Ph)_3C-I$
(B) $(Ph)_3C-Br$
(C) $(Ph)_3C-Cl$
(D) $(Ph)_3C-F$
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B), (C), (D)
For an SN1 reaction, the rate-determining step is leaving group departure, so better leaving group ⇒ higher reactivity.
Leaving group ability:
$\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-$
All compounds form the same triphenylmethyl (trityl) carbocation, so stability is the same; only the leaving group matters.
Decreasing order of SN1 reactivity:
$(Ph)_3C\!-\!I > (Ph)_3C\!-\!Br > (Ph)_3C\!-\!Cl > (Ph)_3C\!-\!F$