[ideaessence]

Hi.

1-hexanol has a boiling point of about 158 degrees.
1-pentanol has a boiling point of about 137 degrees.
1-propanol has a boiling point of about 97.1 degrees.
Methanol has a boiling point of about 64.7 degrees.

This reveals that the intermolecular forces are stronger as the a chain of hydrogen and carbon gets longer.

If hydrogen bonds are strong dipole-dipole bonds, dipole-dipole are stronger than London Dispersion forces, and the OH group in an alcohol is responsible for the hydrogen bonding, then why wouldn't a jar full of 1-propanol have stronger intermolecular forces than a jar full of 1-hexanol? Wouldn't there be more hydrogen bonds in a jar full of 1-propanol?

I realize that dispersion forces increase as the electron cloud gets larger, are therefore usually more mass means stronger dispersion forces. But aren't dispersion forces significantly weaker than hydrogen bonds?

Thank you for any replies.

[macman104]

Individual dispersion forces are weaker than a strong dipole-dipole interaction like the OH hydrogen bonding.

Your trend can be explained like this, the hydrogen bonding interaction is present in all of the molecules.  However, as the chain length increases the amount of available sites for LDF interaction to occur increases.  So, hexanol has the same hydrogen bonding interaction that the others do PLUS the increased amount of LDF interaction.  The fact that the molecule exists in a straight chain, allows them to line up nicely near eachother, and increase more the amount of LDF interaction.

Can you apply this idea now to explaining why neopentyl alcohol has such a lower boiling point than 1-pentanol?  http://en.wikipedia.org/wiki/Neopentyl_alcohol