Predict the relative boiling points of the following pairs of compounds and arrange the two compounds of each pair in order of increasing boiling point.
When looking at general boiling point trends for organic compounds, remember that a higher molecular weight usually corresponds to an increase in boiling point. Also, more polar compounds tend to have higher boiling points than non-polar compounds of similar mass (due to increased intermolecular forces like hydrogen bonding that require more energy to disrupt).
The first pair of compounds are diethyl ether and 3-pentanone (diethyl ketone). Both have similar molecular weights. The carbonyl group of a ketone is more polar than the oxygen in an ether, so diethyl ketone will have a higher boiling point than diethyl ether (in fact, the actual numbers are 101 degrees C versus 35 degrees C).
The second pair of compounds are acetaldehyde and acetic acid. Again, both have similar molecular weights. But the carboxylic acid group of acetic acid is much more polar than the aldehyde group in acetaldehyde, so acetic acid will have a higher boiling point between the two (119 degrees C versus 20 degrees C).
The final pair of compounds are ethanol and pentanol. Both are primary alcohols, so they have similar polarity. But ethanol has a smaller molecular weight than pentanol, so pentanol will have the higher boiling point (78 degrees C versus 138 degrees C).
So arranging each pair in order of increasing boiling point:
1) diethyl ether, diethyl ketone
2) acetaldehyde, acetic acid
3) ethanol, pentanol