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Alkane Heats of Combustion

The combustion of carbon compounds, especially hydrocarbons, has been the most important source of heat energy for human civilizations throughout recorded history. The practical importance of this reaction cannot be denied, but the massive and uncontrolled chemical changes that take place in combustion make it difficult to deduce mechanistic paths. Using the combustion of propane as an example, we see from the following equation that every covalent bond in the reactants has been broken and an entirely new set of covalent bonds have formed in the products. No other common reaction involves such a profound and pervasive change, and the mechanism of combustion is so complex that chemists are just beginning to explore and understand some of its elementary features.

\[\ce{CH3CH2CH3}  + 5 \ce{O2} \rightarrow 3 \ce{CO2} + 4\ce{H2O}  +   \text{heat} \label{1}\]

Two points concerning this reaction are important:

  1. Since all the covalent bonds in the reactant molecules are broken, the quantity of heat evolved in this reaction is related to the strength of these bonds (and, of course, the strength of the bonds formed in the products). Precise heats of combustion measurements can provide useful information about the structure of molecules.
  2. The stoichiometry of the reactants is important. If insufficient oxygen is supplied some of the products will consist of the less oxidized carbon monoxide \(\ce{CO}\) gas.

\[\ce{CH3CH2CH3}  + 4 \ce{O2} \rightarrow \ce{CO2} + 2 \ce{CO} + 4\ce{H2O}  +   \text{heat} \label{2}\]

Heat of Combustion

From the previous discussion, we might expect isomers to have identical heats of combustion. However, a few simple measurements will disabuse this belief. Thus, the heat of combustion of pentane is –782 kcal/mole, but that of its 2,2-dimethylpropane (neopentane) isomer is –777 kcal/mole. Differences such as this reflect subtle structural variations, including the greater bond energy of 1º-C–H versus 2º-C–H bonds and steric crowding of neighboring groups. In small-ring cyclic compounds ring strain can be a major contributor to thermodynamic stability and chemical reactivity. The following table lists heat of combustion data for some simple cycloalkanes and compares these with the increase per CH2 unit for long chain alkanes.

Table \(\PageIndex{1}\): Heats of combustion of select hydrocarbons
Cycloalkane
(CH2)n
CH2 Units
n
ΔH25º
kcal/mole
ΔH25º
per CH2 Unit
Ring Strain
kcal/mole
Cyclopropane n = 3 468.7 156.2 27.6
Cyclobutane n = 4 614.3 153.6 26.4
Cyclopentane n = 5 741.5 148.3 6.5
Cyclohexane n = 6 882.1 147.0 0.0
Cycloheptane n = 7 1035.4 147.9 6.3
Cyclooctane n = 8 1186.0 148.2 9.6
Cyclononane n = 9 1335.0 148.3 11.7
Cyclodecane n = 10 1481 148.1 11.0
CH3(CH2)mCH3 m = large 147.0 0.0

The chief source of ring strain in smaller rings is angle strain and eclipsing strain. As noted elsewhere, cyclopropane and cyclobutane have large contributions of both strains, with angle strain being especially severe. Changes in chemical reactivity as a consequence of angle strain are dramatic in the case of cyclopropane, and are also evident for cyclobutane. Some examples are shown in the following diagram. The cyclopropane reactions are additions, many of which are initiated by electrophilic attack. The pyrolytic conversion of β-pinene to myrcene probably takes place by an initial rupture of the 1:6 bond, giving an allylic 3º-diradical, followed immediately by breaking of the 5:7 bond.

Figure \(\PageIndex{1}\): Changes in chemical reactivity as a consequence of ring strain

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