Question

What is the difference between aldehyde and ketone in terms of boiling point?

Answer 1

Ketones have high melting point.
The melting and boiling points of some short straight-chained aldehydes, in ${}^{\circ }$C, are as follows:
Propanal or propionaldehyde: -81, 46-50
Butanal or butyraldehyde: -99, 75
Pentanal or valeraldehyde: -60, 102-03

And for ketones:
Acetone: -95 to -93, 56-67
Butanone: -86, 80
2-pentanone: -78, 101-105
3-pentanone: -39, 100-102

These data do not support the conclusion whether aldehydes or ketone a have higher melting or boiling point. Another interesting conclusion you can draw is that melting points are far more unpredictable than boiling points. Notice, for example, how butanal has a lower melting point than propanal despite its greater molecular mass; this discrepancy is not present in the boiling points. Also note that while pentanal, 2-pentanone, and 3-pentanone have very similar boiling points, their melting points are significantly different.

In fact, it is symmetric compounds, and not asymmetric compounds, that tend to have higher melting points because they tend to form more tightly packed lattices and hence more stable solids. (A dramatic example is benzene, m.p. 5.5 ${}^{\circ }$C, and toluene, m.p. -95 ${}^{\circ }$C.) This is probably why 3-pentanone melts nearly 40 degrees higher than 2-pentanone. However, it is very difficult to predict just what effect any particular feature will have on melting point, which is why it is more often the boiling point used to illustrate differences in intermolecular forces.