Chloroacetic acids

For the specific acid, see Chloroacetic acid.

In organic chemistry, the chloroacetic acids (systematic name chloroethanoic acids) are three related chlorocarbon carboxylic acids (and thus haloacetic acids):

• Chloroacetic acid, CH₂ClCO₂H
• Dichloroacetic acid, CHCl₂CO₂H
• Trichloroacetic acid, CCl₃CO₂H

Properties

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: its density, boiling point and acidity all increase.

Acid	Melting point (°C)	Boiling point (°C)	Density (g/cm3)	pKa
Acetic acid	16.5	118.1	1.05	4.76
Chloroacetic acid	61–63	189	1.58	2.87
Dichloroacetic acid	9.5	194	1.57	1.25
Trichloroacetic acid	57	196	1.63	0.77

Production

Many routes have been devised to these compounds. Common approaches include hydrolysis of polychloroethanes or polychloroethylenes and chlorination of acetic acid or its anhydride. For example, chloroacetic acid can be made by hydrolysis of trichloroethylene in the presence of sulfuric acid:

CCl₂=CHCl + 2 H₂O → CH₂ClCO₂H + 2 HCl

Dichloroacetic acid is manufactured by hydrolysis of dichloroacetyl chloride:

CHCl₂COCl + H₂O → CHCl₂CO₂H + HCl

Trichloroacetic acid is made by chlorination of acetic acid using a suitable catalyst:

CH₃CO₂H + 3 Cl₂ → CCl₃CO₂H + 3 HCl

Uses

The chloroacetic acids are valuable building blocks in organic synthesis, often for pharmaceuticals and insecticides. For example dichloroacetic acid serves as a masked version of glycolic acid. In addition to being a precursor to herbicides, trichloroacetic acid is used in biochemistry to precipitate proteins.

Safety

For organic acids, these chloroacetic acids are relatively strong. They are, however, far less acidic than traditional mineral acids such as hydrochloric acid. They are skin irritants but are not particularly toxic: the LD₅₀ is 0.5 g/kg for trichloroacetate (rat, oral).