Whcich of these alcohols is likely to be least soluble in water?

Question

a) 1,2-ethanediol b) 2-butanol c) 3-pentanol d) 2,3-butanediol e) ethanol

Answer 1

To answer this question, I would start by looking at the structure for each.

a) HO-CH2-CH2-OH

b) CH3CH(OH)CH2CH3

c)CH3CH2CH(OH)CH2CH3

d)CH3CH(OH)CH(OH)CH3

e)CH3CH2OH

In order for water to dissolve a substance, it must be silimar to water in terms of intermolecular forces and molecular size.

Water, as we know, is a polar solvent with very low molar mass and can do hydrogen bonding. So, you are looking for the alcohol that would have the highest molar mass with the least opportunities for hydrogen bonding.

Given that, I would pick 3-pentanol since it is a 5 carbon long chain with only one site for H-bonding. It has relatively high molar mass compared to water with relatively large nonpolar sections to the molecule as well.

Answer 2

The alcohol with the longest the hydrocarbon chain is the least soluble one in water. That must be 3-pentanol.