Chloral

Chloral

Chloral is also known as trichloroacetaldehyde. Chloral is a colorless, oily, pungent-smelling liquid, b.p. 98^oC, soluble in water, ethanol and ether.

Preparation of chloral

Chloral is prepared industrially by the chlorination of ethanol. Chlorine is passed into cooled ethanol, and then at 60^oC, until no further absorption of chlorine takes place. The final product is chloral alcoholate, CCl₃CH(OH)OC₂H₅, which separates out as a crystalline solid which, on distillation with concentrated sulphuric acid, gives chloral.

When heated with concentrated potassium hydroxide, it yields pure chloroform:

Chloral is oxidized by concentrated nitric acid to trichloroacetic acid and is reduced by aluminium ethoxide to trichloroethanol.

CCl₃CO₂H    (HNO₃) -    CCl₃CHO    Al(OC₂H₅)₃ -    CCl₃CH₂OH

Chloral undergoes the usual reactions of an aldehyde, but its behavior towards water and ethanol is unusual. When chloral is treated with water or ethanol, combination takes place with the evolution of heat, and a crystalline solid is formed, chloral hydrate, m.p. 57^oC, or chloral alcoholate, m.p. 46^oC, respectively. These compounds are stable, and the water or ethanol can only be removed by treatment with concentrated sulphuric acid. Thus, in chloral hydrate the water is present as water of constitution, i.e., the structure of chloral hydrate is CCl₃CH(OH)₂; similarly, the structure od  chloral alcoholate is CCl₃CH(OH)OC₂H₅.

Carbonyl compounds are hydrated by water, the reaction being catalysed by acids and bases. the mechanisms are believed to be:

Since the reactions are reversible, any factor that stabilizes the hydrate will shift the equilibrium to the right. In practice, aldehydes are more hydrated than ketones. One contributing factor is the steric effect. Another factor is the +I effect of alkyl groups. Both mechanisms involve the addition of a nucleophile in the r/d step and the greater the +I effect, the less easy is the addition of the nucleophile and the easier is its removal. Ketones, with two alkyl groups, will therefore exert a greater +I effect than will aldehydes (with one alkyl group), and consequently the equilibrium will be more to the left with ketones than with aldehydes.

The unusual stability of chloral hydrate has been attributed to the –I effect of chlorine and to the formation of intramolecular hydrogen bonds, the presence of which has been shown by Davies (1940) from infra-red studies. At the same time, the combination with water as water of constitution has also been shown, from i.r. and u.v. spectroscopy, by the presence of the carbonyl group in chloral itself and its absence in the hydrate.