Question

Does CH3CH2CH2CH2CH2CH3 or (CH3)3CCH2CH3 have stronger intermolecular forces?

I. `CH_3CH_2CH_2CH_2CH_2CH_3`
II. `(CH_3)_3C CH_2CH_3`

My book says that choice I has a stronger intermolecular force. However both compounds have the same number of carbons and hydrogens. Why would `CH_3CH_2CH_2CH_2CH_2CH_3` have stronger intermolecular forces than `(CH_3)_3C CH_2CH_3`?

Answer 1

See Below

Explanation:

These london dispersion forces are a bit weird. I think of it in terms of "stacking together". Now that is not exactly correct, but it is an ok visualization.

In the long straight chain, the molecules can lay on one another more efficiently and have more surface area with which to interact.

The one with the `(CH_3)_3` group has a long chain, but the methyl groups fan out and sort of disrupt the ability of the molecule to lay on itself.

now in terms of surface area, the 2nd one has more...but this isn't the thing that decides intermolecular forces. The main thing is that those projections (methyl groups) prevent it from interacting well with itself there, and so the straight chain version is more stable (stable with respect to having more intermolecular forces)