Chloroacetic acids

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

• Chloroacetic acid (chloroethanoic acid ), CH₂ClCOOH
• Dichloroacetic acid (dichloroethanoic acid), CHCl₂COOH
• Trichloroacetic acid (trichloroethanoic 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: 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

• Chloroacetic acid is mainly made by hydrolysing trichloroethylene in the presence of sulfuric acid:

CCl₂= CHCl + 2 H₂O → CH₂ClCOOH + 2 HCl

• Dichloroacetic acid is manufactured in small quantities by reducing trichloroacetic acid.
• Trichloroacetic acid is made by directly reacting chlorine with acetic acid using a suitable catalyst.

Uses

• Chloroacetic acid is chemical intermediate for production of various pharmaceuticals and insecticides.
• Trichloroacetic acid is used for various analytic tests in biochemistry. In sufficient concentration it will cause protein, DNA and RNA to precipitate out of solution.
• The sodium salt of trichloroacetic acid 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 chloroacetate anion is the most toxic, with a rat, oral LD₅₀ of about 0.5 g/kg.

References

hu:Triklórecetsav zh:氯代乙酸