Trihalogen Derivatives

Trihalogen Derivatives

When three hydrogen atoms of alkane is replaced by three halogen atoms, then the compound obtained is called as trihalogen derivative of alkanes. Their general formula is C_nH_2n-1X₃.

The most important trihalogen derivatives are those of methane, and they are usually known by their trivial names: chloroform, CHCl₃, Bromoform, CHBr₃ and iodoform, CHI₃.

Chloroform may be prepared in the laboratory or industrially by heating ethanol or acetone with bleaching powder, or with chlorine and alkali (yield about 40 per cent). The reaction is extremely complicated, and the mechanism is obscure.

The equations usually given for the action of bleaching powder on ethanol are: (i) oxidation of ethanol to acetaldehyde; (ii) chlorination of acetaldehyde to trichloroacetaldehyde; (iii) decomposition of trichloroacetaldehyde (choral) by free calcium hydroxide (present in the bleaching powder) into chloroform and formic acid:

(i)                CH₃CH₂OH   +Cl₂    →    CH₃CHO   +   2HCl

(ii)              CH₃CHO   +   3Cl₂    →    CCl₃CHO   +   3HCl

(iii)            2CCl₃CHO   +   Ca(OH)₂   →    2CHCl₃   +   (HCO₂)₂Ca

When acetone is used, the first product given is trichloroacetone, which is then decomposed by the calcium hydroxide into chloroform and acetic acid:

(i)                CH₃COCH₃   +   3Cl₂   →   CCl₃COCH₃   +   3HCl

(ii)              2CCl₃COCH₃   +   Ca(OH)₂    →   2CHCl₃   +   (CH₃CO₂)₂Ca

Chloroform is also prepared industrially by the chlorination of methane. Chloroform is a sickly, sweet-smelling, colorless liquid, b.p.61^oC. it is sparingly soluble in water but readily soluble in ethanol and ether. It does not burn in air under usual conditions, but its vapour may be ignited, when it burns with a green-edged flame. According to Hine, chloroform undergoes alkaline hydrolysis to produce the formate ion and carbon monoxide by what Hine calls the alpha-elimination mechanism: this involves the removal of hydrogen and chloride ions from the same carbon atom.

The mechanism proposed is as follows, involving the intermediate formation of dichloromethylene:

The removal of a proton is facilitated by the –I effect of the chlorine atoms. However, this cannot be the complete story since, although fluorine has a much stronger –I effect than chlorine, CHCIF₂ behaves differently. Hine et al. (1954-57) have shown that CHCl₃, CHBr₃, CHBr₂F etc., readily undergo deuterium exchange in alkaline solution:

CHCIF₂ was found to react more slowly. To account for this, it has been suggested that the CCl₃^- ion is stabilized by resonance. Cl can expand its shell to a decet because it has d-orbitals, but F has no d-orbitals and so this expansion cannot take place:

This type of resonance, which requires expansion of an octet, is known as d orbital resonance. When chloroform is treated with zinc and hydrogen chloride in ethanolic solution, methylene dichloride (q.v.) is obtained; when treated with zinc and water, methane is obtained:

When chloroform is warmed with silver powder, acetylene is obtained:

2CHCl₃   +   6Ag    →   C₂H₂   +   6AgCl

When treated with concentrated nitric acid, chloroform forms chloropicrin:

CHCl₃   +   HNO₃   →    CCl₃NO₂   +   H₂O

Chloropicrin or nitrochloroform (liquid, b.p. 112^oC) is used as an insecticide, and has been used as a war-gas. Chloroform adds onto the carbonyl group of ketones in the presence of potassium hydroxide, e.g., with acetone it forms chloretone (colorless needless, m.p.97^oC), which is used as a drug. The mechanism is possibly:

Chloroform has been used in surgery as an anesthetic, and for this purpose it has to be pure. In the presence of air and light, chloroform slowly forms carbonyl chloride, which is extremely poisonous:

Chlorine, water and carbon dioxide are also produced. Anesthetic chloroform was therefore kept in well-Stoppard dark-brown bottles. Ethanol was also added but its function is not quite clear. According to some authors, it retards the decomposition of the chloroform. This is supported by the fact that infra-red measurements of such mixtures show that absence of the carbonyl frequency.

A delicate test for chloroform is the ‘isocyanide test’. This is carried out by heating chloroform with ethanolic potassium hydroxide and aniline, whereby phenyl isocyanide is formed, and is readily detected by its nauseating odor.

CHCl₃   +   3KOH   +   C₆H₅NH₂   →    C₆H₅NC   +   3KCl   +   3H₂O

Chloroform is widely used in industry as a solvent for fats, waxes, resins, rubber, etc.