Answer to Question #297543 in Organic Chemistry for chem

Question

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

(butan-1-ol): -2670 kj/mol

(butan-2-ol): -2660.6

(2-methylpropan-2-ol): -2644.0

I know that: primary alcohols only have 1 alkyl group secondary has 2, tertiary 3. the alkyl groups provide stability due to I effect. Also tertiary is protected from oxidization as its carbon has no hydrogen to donate. Also, less branching means molecules can pack closer together and experience stronger hydrogen bonds.

However, the enthalpy of combustion is unaffected by intermolecular forces, right? it has to do with intramolecular forces within the molecule.

Is it to do with the branching? do intermolecular forces actually affect the enthalpy of combustion? surely not when combusted as a gas like in a combustion chamber.

Answer 1

The heat of combustion doesn't relate to intermolecular forces in particular. The dominant forces are the intramolecular forces. In particular, covalent bonding is typically involved.