Crotonic acid

Crotonic acid ((2E)-but-2-enoic acid) is a short-chain unsaturated carboxylic acid described by the formula CH3CH=CHCO2H. The name crotonic acid was given because it was erroneously thought to be a saponification product of croton oil. It crystallizes as colorless needles from hot water. With a cis-alkene, Isocrotonic acid is an isomer of crotonic acid. Crotonic acid is soluble in water and many organic solvents. Its odor is similar to that of butyric acid.

Production
Crotonic acid produced industrially by oxidation of crotonaldehyde:
 * CH3CH\dCHCHO + 1/2 O2 ->  CH3CH\dCHCO2H

A number of other methods exist, including the Knoevenagel condensation of acetaldehyde with malonic acid in pyridine:


 * Synthesis Crotonic acid A.svg

The alkaline hydrolysis of allyl cyanide followed by the intramolecular rearrangement of the double bond:


 * Hydrolysis Allyl cyanide.svg

Furthermore, it is formed during the distillation of 3-hydroxybutyric acid:


 * Synthesis Crotonic acid D.svg

Properties
Crotonic acid crystallizes in the monoclinic crystal system in the space group P21/a (space group 14, position 3) with the lattice parameters, , and. The unit cell contains four formula units.

Reactions
Crotonic acid converts into butyric acid by hydrogenation or by reduction with zinc and sulfuric acid.
 * Hydration Crotonic acid.svg

Upon treatment with chlorine or bromine, crotonic acid converts to 2,3-dihalobutyric acids:
 * Chlorination Crotonic acid.svg

Crotonic acid adds hydrogen bromide to form 3-bromobutyric acid.


 * Hydrobromination Crotonic acid.svg

The reaction with alkaline potassium permanganate solution affords 2,3-dihydroxybutyric acid.
 * Oxidation Crotonic acid.svg

Upon heating with acetic anhydride, crotonic acid converts to the acid anhydride:

Esterification of crotonic acid using sulfuric acid as a catalyst provides the corresponding crotonate esters:


 * Synthesis Ethyl crotonate.svg

Crotonic acid reacts with hypochlorous acid to 2-chloro-3-hydroxybutyric acid. This can either be reduced with sodium amalgam to butyric acid, can form with sulfuric acid 2-chlorobutenoic acid, react with hydrogen chloride to 2,3-dichlorobutenoic acid or with potassium ethoxide to 3-methyloxirane-2-carboxylic acid.


 * Crotonic acid Reaction B.svg

Crotonic acid reacts with ammonia at the alpha position in the presence of mercury(II) acetate. This reaction provides -threonine.

Use
Crotonic acid is mainly used as a comonomer with vinyl acetate. The resulting copolymers are used in paints and adhesives.

Crotonyl chloride reacts with N-ethyl-2-methylaniline (N-ethyl-o-toluidine) to provide crotamiton, which is used as an agent against scabies.


 * Crotamiton synthesis V.svg

Safety
Its is 1 g/kg (oral, rats). It irritates eyes, skin, and respiratory system.