Ch_322a_4.014

Ch_322a_4.014 - Heats of Combustion of the Cycloalkanes: A...

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Heats of Combustion of the Cycloalkanes: A Measure of their Relative Stabilities The general reaction for the combustion of a cycloalkane is: (CH 2 ) n + 1.5n O 2 nCO 2 + nH 2 O + heat As n increases, more heat is evolved. In order to use the heats of combustion to determine the relative stabilities of the cycloalkane structures, the amount of heat evolved must be adjusted for the number of CH 2 groups. The table that follows provides this information. The cycloalkanes form a homologous series (CH 2 ) n with n > 3.
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Heats of Combustion of Cycloalkanes cycloalkane (CH 2 ) n n ! H comb (kcal/mol) heat evolved per CH 2 group (kcal/mol) cyclopropane 3 499.8 166.6 cyclobutane 4 655.9 164.0 cyclopentane 5 793.5 158.7 cyclohexane 6 944.5 157.4 cycloheptane 7 1108.2 158.3 cyclooctane 8 1269.2 158.6 cyclononane 9 1429.5 158.8 cyclodecane 10 1586.0 158.6 unbranched alkanes (157.4) Note: The total amount of heat evolved increases with the size of the cycloalkane, as expected. However, the amount of heat evolved per CH 2 group is highest for the smallest cycloalkanes , and is lowest for cyclohexane, where the amount is consistent with that evolved in the combustion of unbranched alkanes.
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Ring Strain in Cycloalkanes Because the amount of heat evolved in the combustion of cyclohexane is consistent with the value expected from the combustion of unbranched (and unstrained) alkanes, it is assumed that cyclohexane is free of any "strain energy." The greater amounts of heat evolved per CH 2 group in the other cycloalkanes are assumed to be due to elements of "ring strain" that lead to higher energies.
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_4.014 - Heats of Combustion of the Cycloalkanes: A...

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