Why does glycerol have a higher normal boiling than triacetin, #HC{OC(=O)CH_3}_3#, even though the latter molecule has a higher molecular mass?

1 Answer
Nov 24, 2016

Answer:

What intermolecular force exists in the triol, #"glycerol"#, that does not exist in the ester?

Explanation:

The answer is #"hydrogen bonding"#. #"Glycerol"#, #H_2C(OH)CH(OH)CH_2(OH)# is extremely viscous, and is quite involatile, #"b.p = 290"^@C#. Clearly, the degree of intermolecular force, i.e. the force BETWEEN molecules, is substantial. And that intermolecular force is likely hydrogen-bonding, which is also repsonsible for the elevated boiling points of #OH_2#, #NH_3#, and #HF#, much smaller molecules, but also where hydrogen-bonding operates decisively.

On the other hand, its triester with 3 equiv acetic acid (which I think is commonly called #"triacetin"#), removes this intermolecular force by eliminating the alcoholic function. The result?

The normal boiling point of triacetin is #"b.p = 259 "^@C#, substantially lower than glycerol, even though glycerol has LESS dispersion forces in that it is a smaller molecule. Hydrogen bonding WINS as an intermolecular force.