All the following compounds are isomers of $C_{5} H_{8}$ . Their heats of combustion are arranged in the order :
(I) (II) (III) (IV)

Question

All the following compounds are isomers of C 5 ​ H 8 ​ . Their heats of combustion are arranged in the order : (I) (II) (III) (IV)

TestHub · Accepted Answer

For isomers, the less stable the compound (the higher its enthalpy of formation), the more heat it gives on combustion.

(I) Penta-1,2-diene has cumulated double bonds (allene): the least stable, $Δ H_{f}$ about 140 kJ/mol.

(II) Penta-1,4-diene has isolated double bonds, about 106 kJ/mol.

(III) trans-Penta-1,3-diene is conjugated, so it is stabilised by delocalisation, about 76 kJ/mol.

(IV) Cyclopentene has only one double bond, about 34 kJ/mol: the most stable isomer.

Heat of combustion: I $>$ II $>$ III $>$ IV.

Answer: (C).

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