JEE Main · 2020 · 09 Jan 2020 · Shift 1 · Chemical Thermodynamics

The correct order of heat of combustion for following alkadienes is:

(A)(a) $<$ (b) $<$ (c)✓ Correct
(B)(a) $<$ (c) $<$ (b)
(C)(c) $<$ (b) $<$ (a)
(D)(b) $<$ (c) $<$ (a)

Answer: (A)

Solution

Heat of combustion runs opposite to stability: the more stable a diene, the less energy it gives up on burning. The conjugated diene (a) is stabilised by resonance and is the most stable, the isolated diene (b) is intermediate, and the cumulated diene (c), lacking any such stabilisation, is the least stable. Their stabilities therefore order as (a) more than (b) more than (c), so the heats of combustion run the other way, (a) less than (b) less than (c).

JEE Main · 2020 · 09 Jan 2020 · Shift 1 · Single correct option

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