Lactic Acid : Preparation, Properties, Reaction and Uses

Lactic Acid.

Lactic Acid Structure.

Lactic acid is the main constituent of milk that has gone sour and hence its name (L. Lactis=milk).
Preparation. (1) By the hydrolysis of acetaldehyde cyanohydrin.

Lactic Acid Preparation By the hydrolysis of acetaldehyde cyanohydrin.
From Molasses. (2) Lactic acid is manufactured by fermentation of molasses (or milk whey containing lactose) by the microorganism Bacillus acidi lactiti (BAL).

Lactic acid preparation from molasses.

A dilute solution of molasses (or whey) is treated with BAL (sour milk). The fermentation is carried at 35-40°C in the presence of CaCO3, As the lactic acid is produced, it reacts with CaCO3, to form calcium lactate. The calcium lactate is filtered off and decomposed with calculated quantity of dilute H2SO4. The insoluble Calcium sulphate is removed and the lactic acid set free in the solution is recovered by distillation in vacuo. The product is D-lactic acid.

Physical Properties.

Lactic acid is a colourless, crystalline solid, mp 53°C, and has a sour taste. The acid obtained from molasses is D-isomer. The synthetic product is racemic lactic acid, mp 18°C, specific rotation [α] = +3.82, Lactic acid is soluble in water, alcohol, and ether.

Chemical Properties.

Lactic acid molecule contains a secondary alcohol group (>CHOH) and a carboxyl group (COOH), and gives reactions of both.

Reaction Involving COOH Group.

(1) Formtion of salt. It reacts with alkalis to form salts.

Lactic acid reaction involving COOH Group

Reaction Involving OH Group.

(2) Reaction with CH3COCl. The alcoholic OH group is acetylated on reaction with acetyl chloride in the presence of a base (pyridine).

Lactic Acid Reaction involving OH Group
(3) Reduction. When heated with Hl, it is reduced to propionic acid.

Lactic Acid Reaction involving Reduction
(4) Oxidation. On oxidation with Fenton’s reagent (H2O2/Fe3+) it is converted to pyruvic acid. With KMnO4 it yields acetic acid.

Reaction Involving Both OH And COOH.

(5) Reaction with PCl5. Both the OH and COOH groups are attacked by phosphorus pentachloride to form lactyl chloride.

Lactic Acid Reaction involving Both OH and COOH
(6). Action of Heat. When heated alone, it forms an intermolecular cyclic diester called a Lactide.

Lactic acid Reaction involving Action of Heat.
(7) Reaction With H2SO4. When heated with dilute sulphuric acid at 130°C, lactic acid is decomposed to yield acetaldehyde and formic acid.

Lactic acid Reaction with H2SO4.
With concentrated H2SO4, formic acid is further decomposed to CO and H2O. (test)


Lactic acid is used.
  1. in dairy products.
  2. as acidulant in beverages and candies.
  3. for deliming of hides.
  4. as a mordant.
  5. Ethyl and butyl lactates are used as plasticisers.
  6. Calcium and iron lactates are used in medicine. 

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