Question

Out of neo-pentyl alcohol and n-pentyl alcohol, which has higher boiling point? Why?​

Answer 1

N pentyl.

• Because as branching increases the boiling point decreases

Answer 2

If one considers boiling points (in °C) of primary alcohols, one finds the following:

methanol: 65

ethanol: 79

1-propanol: 97

1-butanol: 117

1-pentanol: 138

This trend is due to Van der Waals forces increasing with molecular weight.

Now if one focuses on melting points, I would expect the same trend, but these are the experimental values (in °C):

methanol: −98

ethanol: −117

1-propanol: −127

1-butanol: −90

1-pentanol: −79

the lowest value being the melting point of 1-propanol.

The general idea stating that with all else being equal, more massive molecules have higher boiling and melting points, does not fully apply for this set of alcohols.