Question

Rank the following compounds in terms by increasing boiling point, starting with the lowest boiling point first. I. 1-pentanol II. n-pentane III. 2,2-dimethylpropane IV. (R)-4-hydroxypentanoic acid.

Answer 1

Answer:

III < II < I < IV

Explanation:

Boiling point is determined by the strength of the intermolecular forces in a substance.

n-pentane and 2,2-dimethylpropane are isomers of each other (same molecular formula) and have only single bonds. Both molecules have only London dispersion forces (LDFs), which are the weakest intermolecular forces. The difference in their boiling point is due to the connectivity of the carbon skeleton. n-pentane is a linear hydrocarbon while 2,2-dimethylpropane is a branched hydrocarbon. The linear structure of n-pentane produces a larger surface area between molecules, so the amount of LDFs between the molecules is increased. Thus, n-pentane has a higher boiling point.

Both 1-pentanol and (R)-4-hydroxypentanoic acid have a higher boiling point than the hydrocarbons because they have hydrogen bonding. 1-pentanol has a single alcohol group while (R)-4-hydroxypentanoic acid has both an alcohol group and an acid group, which can also hydrogen bond. Thus, (R)-4-hydroxypentanoic acid has more hydrogen bonding than 1-pentanol and therefore a higher boiling point.