I have a quick question about pyruvic acid and lactic acid, which I think I know the answer too but I'm unsure.

The melting point of lactic acid is said to be higher than that of pyruvic acid despite very similar molecular masses.

Is this because lactic acid has 2 $\ce{OH}$ groups, and pyruvate only has 1, thus there are twice as many hydrogen bonds between lactic acid molcules that must be broken for the substance to melt?

  • 1
    $\begingroup$ Do you men boil or melt? $\endgroup$
    – A.K.
    Jan 4 '17 at 21:01
  • $\begingroup$ @A.K. I know it sounds weird, and it seems way more intuitive to ask about the boiling point, but that literally is what the question asks. $\endgroup$ Jan 5 '17 at 0:47

Your analysis is practically correct but not quite — more due to technical reasons. You said:

Is this because lactic acid has 2 $\ce{OH}$ groups, and pyruvate only has 1 […]?

You probably know the structures of both; they both include a $\ce{COOH}$ group which you apparantly counted as a hydroxy group. You should not do that; the carboxy group ($\ce{COOH}$) has different properties than a lone hydroxy group ($\ce{CR2OH}; \ce{R} \ne \ce{OX}$). The most obvious is the $\mathrm{p}K_\mathrm{a}$ value which differs by about $10$ logarithmic units.

So you should say that lactic acid has higher melting and boiling points becuase it contains a (or: one) hydroxy group while pyruvic acid has a carbonyl group in the same position.

  • $\begingroup$ So basically the presence of the OH causes there to be hydrogen bonding between lactic acid molecules, which don't exist between pyruvate molecules due the the presence of a Carbonyl group instead. $\endgroup$ Jan 5 '17 at 0:53
  • $\begingroup$ @frostedcake Well, the carboxy group does hydrogen bond, too; but you can form more hydrogen bonds per molecule with lactic acid than with pyruvic acid. $\endgroup$
    – Jan
    Jan 5 '17 at 9:29

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