Pyruvic acid

Pyruvic acid

Pyruvic acid (acetylformic acid, pyroracemic acid, a-ketopropionic acid, 2-oxopropanoic acid), b.p. 165^oC, is the simplest keto acid. It may be prepared:

1. By heating tartaric acid alone, or better, with potassium hydrogen sulphate at 210-220^oC.

The reaction is believed to take place via the formation of hydroxymaleic acid (shown in below figure 1), which rearranges to oxalacetic acid, (shown in Figure II)

This is the best method for preparing pyruvic acid, and it was this method which gave rise to the name pyroracemic acid.

2 Other methods of preparation are the oxidation of lactic (a-hydroxypropionic) acid, the hydro-lysis of a,a-dibromopropionic acid or acetyl cyanide, e.g.,

CH₃COCl    (KCN) ⟶     CH₃COCN    (H⁺)⟶   CH₃COCO₂H

A general method of preparing a-keto-acids uses the Claisen condensation between ethyl oxalate and monocarboxylic esters:

Pyruvic acid behaves as a ketone and as an acid; it forms an oxime, hydrazone, etc. it is reduced by sodium amalgam to lactic acid and dimethyltartaric acid (cf. glyoxylic acid, above):

Pyruvic acid reduces ammoniacal silver nitrate, itself being oxidized to acetic acid:

CH₃COCO₂H    +   [O]    ⟶    CH₃CO₂H   +   CO₂

It is oxidized by warm nitric acid to oxalic acid:

CH₃COCO₂H   +   [O]    ⟶    (CO₂H)₂   +   CO₂   +   H₂O

Pyruvic acid is easily decarboxylated with warm dilute sulphuric acid to give acetaldehyde:

CH₃COCO₂H    ⟶    CH₃CHO   +   CO₂

The mechanism of this reaction is uncertain, but since it occurs only with a-keto-acids, it suggests that the –I effect of the a-carbonyl group plays an important part (cf. trichloroacetic acid). On this basis, the following mechanism could be postulated, involving a carbine intermediate.

On the other hand, when warmed with concentrated sulphuric acid, pyruvic acid undergoes decarbonylation, i.e., eliminates a molecule of carbon monoxide, to form acetic acid:

Both of these reactions with sulphuric acid are characteristic of a-keto-acids. A-Keto-esters are also decarbonylated when heated, and calvin et al. (1940), using ethyl pyruvate labeled with 14C, showed that the carbon monoxide came from the carbethoxyl group. in this case, the results may be explained by the formation of an epoxide intermediate:

The methyl group is made reactive by the adjacent carbonyl group, and so pyruvic acid undergoes many condensation reactions characteristic of a compound containing an active methylene group; e.g., in the presence of dry hydrogen chloride, pyruvic acid forms a-keto-y-valerolactone-y-carboxylic acid:

Pyruvic acid is a very important substance biologically, since it is an intermediate product in the metabolism of carbohydrates and proteins.