Reactant A converts to product D through the given mechanism (with the net evolution of heat): A B slow; H = +ve B C fast; H = -ve C D fast; H = -ve Which of the following represents the above reaction mechanism?
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Answer
(4) [figure]
Solution
Read the energy profile from the data:
(i) A B is endothermic intermediate B lies above A.
(ii) A B is the slow (rate-determining) step it has the highest transition state (the tallest barrier).
(iii) B C and C D are fast and exothermic small barriers, and C lies below B, D below C.
(iv) Net evolution of heat product D lies below reactant A.
Graph (1): D is above A (net endothermic). Graph (2): the last barrier is the highest. Graph (3): C is above B and the later barriers are higher.
Only graph (4) satisfies all four conditions.
Answer: (4)