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[1] |
| 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.