Hydrocarbon length and enthalpy of formation

[September24]

Look at the following groups:

Group 1:
Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane

Group 2:
Propane
Butane
Pentane
Hexane

In each group, which one has the least enthalpy of formation and which one is least. I know cyclo hexane should have the least in group 1 since its very stable but does adding CH groups decrease delta H or increase it?

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[benjaminl1nus]

adding C-H groups should decrease delta H. If you look at any bond enthalpy table, the H-H bond enthalpy > C-H bond enthalpy (436>416kj/mol)

So to form a hydrocarbon from its elements, you must break an H-H bond in order to make a C-H bond, BUT for each H-H bond you break, you can make TWO C-H bonds. So,

Delta H = bonds broken - bonds formed is negative. the overall magnitude is proportional to the amount of C-H bonds made.

FYI : This is my speculation, open to criticism. Feel free to judge. If I'm wrong, tell me where I'm wrong, please.