[noam]

changing the position of the OH Functional group changes the enthalpy of combustion. so primary secondary and tertiary have different enthalpies of combustion. However, I don't understand why!
primary alcohols have a higher enthalpy of combustion, then secondary then tertiary.
for example
n-butanol (butan-1-ol): -2670 kj/mol

sec-butanol (butan-2-ol): -2660.6 kj/mol

tert-butanol (2-methylpropan-2-ol): -2644 Kj/mol

I experimented with these alcohols myself and got the exact same trend but with a large percentage error between the literature values and my experimental values. this trend is the same in other alcohols so I'm sure something is up here.
I know that primary alcohols only have one alkyl group secondary has 2 and tertiary 3. Tertiary is also protected from oxidization as its carbon has no hydrogen to donate, so it's more stable in that sense. Also, less branching means molecules can pack closer together and experience stronger hydrogen bonds, so primary have stronger hydrogen bond forces than secondary and secondary more than tertiary, increasing boiling point and enthalpy of vaporization.
However, the enthalpy of combustion is unaffected by intermolecular forces, right? it has to do with intramolecular forces within the molecule. So why, why is there a change in enthalpy of combustion, when the formula is the same but the structure is different.
Is it to do with the branching and bond angles that cause weaker bonds and therefore lower enthalpy of combustion? why? do intermolecular forces actually affect the enthalpy of combustion? surely not when combusted as a gas like in a combustion chamber. Is it to do with the number of alkyl groups (tert has 3, sec has 2, primary only 1) and their electron-donating ability increasing stability? why?
PLZ HELP anything would be appreciated.

[rjb]

I should probably start by saying that I know very little about this subject so the following is just an 'educated' guess until somebody with more knowledge responds.

If what you say about the substances being combusted as gases is the case, then intermolecular forces are not responsible for the differences in enthalpy of combustion. Instead I would think more about the bonds that exist within the molecule itself, after all, the combustion process involves these being broken and the formation of new bonds within the product molecules.

Although each of the molecules that you are considering have the same molecular formula, I would imagine that the bonds within those molecules are not necessarily equivalent and that their bond energies would differ somewhat due to the differing chemical environments that exist within the molecules. This coupled with steric effects (which would be greater in the tert-butanol) would most likely give rise to differing bond energies and might well be the reason for the differences in enthalpy of combustion.
 

[Corribus]

You may also look at angle strain. The oxygen atom exerts a very strong electric push against nearby nuclei by virtue of its lone pairs. Any deviation of angles in the hydrocarbon skeleton from ideal tetrahedral shape will be converted within the molecule to a small amount of potential energy that would be released when the molecule is combusted.

Your differences in combustion enthalpies are on the order of 1%, so we're not talking major differences in energies here.

Also I point out that while combustion happens in the gas phase, it is not true to say that there are zero intermolecular forces at play. It's not a bad approximation by any means - but especially for gasses that behave very non-ideally (among which I would include alcohols), I wouldn't necessarily discount the possibility that weak intermolecular forces could slightly influence combustion enthalpies. Still, in this case I still think small deviations from ideal tetrahedral geometries or slight differences in bond energies is probably the best explanation.  Given the small differences in combustion enthalpies you're dealing with, and the possibility that experimental noise probably complicates the picture, it will probably be difficult to pin down an exact explanation.