Question

Arrange the following alchohols and alkanes in increasing order of boiling point: 3-pentanol, n-pentane, 2,2-dimethylpropanol, n-pentanol.

I understand that n-pentane would have the lowest boiling point because alkanes have a lesser BP than alcohols for the same number of carbons.
2,2-dimethylpropanol would be next because it has the most amount of branching and branching decreases BP as it makes the compound more compact (giving it the shape of a sphere).
But I am confused between n-pentanol and 3-pentanol, from intuition, 3-pentanol should have a lesser BP because it seems it will have more H-Bonding compared to n-pentanol and that is the right answer as well. But is that the actual reason? If not then what is the reason?

Answer 1

Alcohols have higher boiling points than alkanes due to the presence of hydrogen bonding. 3-pentanol has a lower boiling point than n-pentanol because it can form more hydrogen bonds through intramolecular hydrogen bonding.

Step-by-step explanation:

Alcohols have higher boiling points than alkanes of the same molecular weight due to the presence of hydrogen bonding. In the case of 3-pentanol and n-pentanol, 3-pentanol has a lower boiling point compared to n-pentanol because it can form more hydrogen bonds. 3-pentanol has a hydroxyl group (OH) located on the third carbon atom, which allows it to form intramolecular hydrogen bonding. On the other hand, n-pentanol has the hydroxyl group located on the first carbon atom, which facilitates intermolecular hydrogen bonding, making it easier to break and therefore achieving a higher boiling point compared to 3-pentanol.