Arrange the following in decreasing order of heat of combustion per mole.
(a)
(b)
(c)

Question

Arrange the following in decreasing order of heat of combustion per mole. (a) (b) (c)

TestHub · Accepted Answer

Heat of combustion on a molar basis grows with the number of carbons : $C_{5}$ $>$ $C_{4}$ $>$ $C_{3}$ . The ring strain changes the heat per $C H_{2}$ group, but the carbon count dominates the molar value, so c $>$ b $>$ a.

The molar values cannot be used to compare stability here, because the formulae differ. Stability is compared by the heat per $C H_{2}$ group (cyclopropane is the least stable).

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