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

Solution

Chemistry ma’am · 3 steps

All three are isomers, so the extra heat comes from strain, and every cis double bond adds some.

1. Same formula. All three compounds are hexa-2,4-diene, C₆H₁₀. Each burns to the same products, 6CO₂ + 5H₂O. So any difference in heat of combustion comes only from how much energy the molecule held to begin with.
2. Cis costs energy. In a cis (Z) double bond, the two groups sit on the same side and crowd each other. This steric strain makes the molecule less stable, so it releases more heat when it burns. A trans (E) bond has no such crowding.

   Compound	Geometry	Cis bonds	Heat of combustion
   (a)	2E, 4E	0	lowest
   (b)	2E, 4Z	1	middle
   (c)	2Z, 4Z	2	highest

3. So. Stability runs (a) > (b) > (c), so heat of combustion runs the other way: (a) < (b) < (c). That is option (c).

The idea to keep. For isomers, heat of combustion runs opposite to stability. The less stable isomer, here the one with more cis crowding, releases more heat when it burns.

Answer from the NTA final answer key. Solution by PrepWiser.