Order of Boiling Point

Question

Arrange the following in decreasing order of Boiling point

1. 3-Pentanol

2. n-Pentane

3. 2,2- Dimethylpropanol

4. n-Pentanol

Answer 1

Amongst the 4 choices, you have 1 alkane (n-pentane) and 3 alcohols. Alkanes will have significantly lower boiling points than alcohols because the former can form only very weak dispersion intermolecular forces whereas alcohols form much stronger hydrogen bonding interactions. As a result, n-pentane will have the lowest bp.

Between the 3 alcohols, 2 of them are pentanols while 1 is propanol. In general, the longer the carbon chain for alcohols, the higher the boiling point. As a result, 2,2-dimethylpropanol will have a lower bp compared to the 2 pentanols.

In general, for alcohols with the same # of carbons, straight chain alcohols will have a higher boiling point than alcohols that exhibit branching in its molecular structure. For this reason, n-pentanol will have a higher bp than 3-pentanol.

In summary, the decreasing order of bp is: n-pentanol > 3-pentanol > 2,2-dimethylpropanol > n-pentane