Question

How does the solubility of aldehydes and ketones change in water as the number of carbon atoms changes?

Answer 1

The solubilities of both aldehydes and ketones in water decrease as the number of carbon atoms increases.

Explanation:

However, alkyl groups are electron-donating groups, so ketones are more polar than aldehydes.

Thus, ketones are slightly more soluble than aldehydes with the same number of carbon atoms.

Here's a table of the solubilities `S` (in g/100 mL) for some aldehydes and ketones:

`bbul("Aldehyde"color(white)(....)Scolor(white)(.....)"Ketone"color(white)(mmm)S)`
`"propanal"color(white)(.......)20color(white)(... ,.)"propanone"color(white)(mm)∞`
`"butanal"color(white)(...,....)7.6color(white)(.,,)"butanone"color(white)(....,.)26`
`"pentanal"color(white)(.......)1.2color(white)(,.,)"pentan-2-one"color(white)(...)6`
`"hexanal"color(white)(........)0.6color(white)(,.,)"hexan-2-one"color(white)(.,)1.4`

The dividing line for "soluble" is about four carbon atoms for aldehydes and five carbon atoms for ketones.