2002 AIPMT Haloalkanes and HaloarenesElimination Reactions Easy Question 3 of 3 in Elimination Reactions

Reactivity order of halides for dehydrohalogenation is : -

A R–F > R–Cl > R–Br > R–I
B R–I > R–Br > R–Cl > R–F
C R–I > R–Cl > R–Br > R–F
D R–F > R–I > R–Br > R–Cl

The answer is B.

Solution

In dehydrohalogenation (elimination with alcoholic KOH), the C–X bond is broken.
The C–X bond strength decreases in the order C–F $\gt$ C–Cl $\gt$ C–Br $\gt$ C–I, because the halogen atom becomes larger and the overlap weaker.
The weaker the bond, the easier it breaks; also $I^-$ is the best leaving group.
So the reactivity order is R–I $\gt$ R–Br $\gt$ R–Cl $\gt$ R–F.

Concept C-X bond strength and reactivity

Elimination Reactions in past papers

3 questions from this chapter have appeared across 3 exam years.