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Which of the following gives the correct order of rate of haloform reaction with different halogens?

1. rCl2 > rBr2 > rI2
2. rI2 > rBr2 > rCl2
3. rCl2 = rBr2 = rI2
4. rBr2 > rCl2 > rI2

Correct answer: rI2 > rBr2 > rCl2

Solution

In the haloform reaction, iodine reacts fastest under alkaline conditions because: (1) the electron-withdrawing power of iodine promotes nucleophilic attack, and (2) the iodoform (CHI3) step is essentially irreversible, driving the reaction forward. Hence the rate order is I2 > Br2 > Cl2.

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