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The most obvious one in ##"hydrogen bonding"##

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The most obvious one in ##"hydrogen bonding"##.

One of the best indicators of intermolecular interaction is the normal boiling point. Molecules with significant intermolecular interaction tend to have higher boiling points. Water, a small molecule, has an exceptionally high boiling point because of intermolecular hydrogen , which persists BETWEEN molecules:

##H-O^(delta-)-H^(delta+)cdotsO^(delta-)-H^(delta+)""_2##

Now ##"1-propanol"## has a normal boiling point of ##97-98## ##""^@C##. And we compare this to that of ##"isopropanol"##, ##82.6## ##""^@C##, and ##"ethanol"##, ##78.0## ##""^@C.##

Given these data, there is another contributor to intermolecular interaction, and here it is the non-polar interaction between hydrocarbyl chains. The longer the chain, the greater the chain-chain interaction, and in long chain alcohols (and alkanes), boiling points will be elevated. Compare the boiling point of ##"n-butanol,"## ##117.7## ##""^@C##, and ##"n-pentanol,"## ##137-139## ##""^@C##.