Let's say there are 3 organic compounds A, B, C and you know their structures.

How exactly could you go about deducing the order of their heat of combustions?

For an example, ABC

  • 2
    $\begingroup$ Knowing the structures is of secondary importance since the compounds have different molecular formulas (C6H10, C5H8, and C4H6). $\endgroup$
    – Loong
    Feb 7 at 19:14

1 Answer 1


This problem has been thoroughly studied by Sydney W. Benson at the end of the $20$th century. He has developed a Theory of Increments for calculating the formation enthalpy of compounds of C, H and O. It states for example, that the increments are

-$42.2$ kJ/mol for $\ce{-CH3}$

-$392.1$ kJ/mol for $\ce{-COOH}$

-$312.6$ kJ/mol for $\ce{-COO-}$

-$131.8$ kJ/mol for ketones $\ce{-CO-}$.

The formation enthalpy of a given substance is the sum of its increments. The combustion enthalpies can then be calculated from the formation enthalpies with the usual method.

For acetic acid for example, its formation enthalpy is $-42.2 + (-392.1) = - 434.3$ kJ/mol.

For methyl acetate, the formation enthalpy is $-42.2 + (-312.6) +(-42.2)$ = $-397.0$ kJ/mol.

These numerical values are taken from my copy of the original documents. Other values are given in the original publications, and also adding strains for cyclic compounds . Unfortunately, I don't have their values for rings like cyclopropane, and I have no more access to the original publications. If you can read these old documents, you will find them by checking Google, under the reference Sydney W. Benson Increments.


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