Chloroethanoic acids

In organic chemistry, the chloroethanoic acids (trivial name chloroacetic acids) are three related chlorocarbon carboxylic acids:

• chloroethanoic acid (chloroacetic acid), CH₂ClCOOH
• dichloroethanoic acid (dichloroacetic acid), CHCl₂COOH
• trichloroethanoic acid (trichloroacetic acid), CCl₃COOH

As the number of chlorine atoms increases, the electronegativity of that end of the molecule increases, and the molecule adopts a progressively more ionic character: it's density, boiling point and acidity all increase.

Acid	Melting point	Boiling point	Density	pKa
Ethanoic acid	16.5°C	118.1°C	1.05	4.76
Chloroethanoic	61-63°C	189°C	1.58	2.87
Dichloroethanoic	9.7°C	194°C	1.57	1.25
Trichloroethanoic	57°C	196°C	1.63	0.77

Production

• Chloroethanoic acid is mainly made by hydrolysing trichloroethene in the presence of sulphuric acid:

CCl₂=CHCl + 2H₂O → CH₂ClCOOH + 2HCl

• Dichloroethanoic acid is manufactured in small quantities by reducing trichloroethanoic acid.
• Trichloroethanoic acid is made by the direct action of chlorine on ethanoic acid in the presence of a suitable catalyst.

Uses

• Chloroethanoic acid is chemical intermediate for production of various pharmaceuticals and insecticides.
• Trichloroethanoic acid is used for various analytic tests in biochemistry. Its sodium salt is used as a weedkiller.

Safety

All of these acids are unusually strong for organic acids, and should generally be treated with similar care as for strong mineral acids like hydrochloric acid. Even neutral salts however, tend to be significantly toxic, because the ions interfere in biological processes (such as the citric acid cycle) that normally process plain acetate ions. Interestingly, the chloroethanoate ion is the most toxic, with a rat, oral LD₅₀ of about 0.5 g/kg.