Question:

Reactant A converts to product D through the given mechanism (with the net evolution of heat):
A → B slow;   ΔH = +ve
B → C   fast;   ΔH = -ve
C → D   fast;   ΔH = -ve
Which of the following represents the above reaction mechanism?

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The slow step has the highest barrier; the overall reaction is exothermic so D lies below A.
Updated On: Oct 26, 2026
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The Correct Option is D

Solution and Explanation

A reaction profile for this mechanism must show: (i) the first step ($A \to B$) is slow, so it has the highest activation barrier; (ii) it is endothermic, so B lies above A; (iii) $B \to C$ and $C \to D$ are fast (small barriers) and exothermic (C below B, D below C); (iv) the net reaction evolves heat, so D lies below A.
Only profile (4) satisfies all of these: option (4).
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