Why do the boiling and melting points of alcohols increase with molecular mass?

[doggbAT]

Homework Statement

The boiling points (ºC) of methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol are 64.7, 78.5, 97.2, 117 to 118, 137.5, respectively. The melting points are -97.8, -114.1, -1277.0, -90.0, and -79.0, respectively. Explain these trends with reference to molecular structure.

Homework Equations

N/A

The Attempt at a Solution

The bonds in alcohols are hydrogen-carbon bonds and the carbon-hydroxyl bonds. By looking at the H-C bond, one can understand that the H end is partially positive. In the hydroxyl group, the O is partially negative, and has two pairs of electrons, making it quite negative.

 

[Borek]

What you wrote doesn't explain observable trends. Besides, C-H bond is almost not polarised (especially when compared with O-H bond).

Think - what forces keep these molecules together in solids and liquids?

 

[doggbAT]

Okay, so it's the hydrogen bonding, and as the molecules grow larger, they can fit together less tightly. Now I'm going to take a guess and say that for solids, the ideal size carbon chain comes with 1-propanol; that is, the molecules can fit together in a way that aligns hydroxyl groups best. For liquids this doesn't matter because they have some movement, and the smaller the molecule, the more chance there is of hydoxyl contact.


Is this correct?

 

[Kushal]

what can be observed is that when the molecular mass increases, ie you are adding more -CH2 groups, the boiling point is increasing. you can use a knowledge of temporary dipole forces of attractions.

probably for the solids, the shape the alcohols take matters more. methanol, ethanol and propan-1-ol are linear. but butan-1-ol and pentan-1-ol may be branched. so the values of melting points for the two last alcohols may be averaged values.

the branched alcohols would normally have lower melting points because of less contact compared. this gives a lower average value.

but i wouldn't be able to explain why this is not the case for the boiling points!