Trichloromethyl group

The trichloromethyl group is a functional group that has the formula −CCl₃. The naming of this group is derived from the methyl group (which has the formula −CH₃), by replacing each hydrogen atom by a chlorine atom. Compounds with this group are a subclass of the organochlorines.

Structure of the trichloromethyl group

Trichloromethyl is an electron withdrawing group owing to the electronegativity of the three chlorides.^[1]

Group Electronegativity

Methyl group	Formula	Electronegativity
Methyl	CH3	2.472
Phenyl	C6H5	2.717
Chloromethyl	CH2Cl	2.538
Dichloromethyl	CHCl2	2.602
Trichloromethyl	CCl3	2.666
Trifluoromethyl	CF3	2.985

Trichloromethyl alkanes and derivatives

Some simple trichloromethyl compounds include trichloromethane, also known as chloroform (HCCl₃), 1,1,1-trichloroethane (H₃CCCl₃), and hexachloroethane (Cl₃CCCl₃). Trichloromethanol is however unstable, reflecting the lability associated with the RO-C-Cl center.^[2] Indeed, triphosgene (bis(trichloromethyl) carbonate, OC(OCCl₃)₂) fragments upon heating by scission of a C-Cl bond. It is used as a substitute for phosgene.^[3]

Because the trichloromethyl group is relatively electronegative, the behavior of trichloromethyl-substituted compounds can differ sharply vs the behavior of the methyl parents. The acidity constant (pK_a) of trichloroacetic acid CCl₃CO₂H is 0.77, whereas that of acetic acid is 4.76. In other words, trichloroacetic acid is 1000x the stronger acid. In a related manner, the trichloromethyl aldehyde CCl₃CHO (chloral) tends to hydrate to give chloral hydrate CCl₃CH(OH)₂.

Trichloromethyl arenes

Many trichloromethylarenes can be prepared by chlorination of the corresponding methyl arenes. For example toluene and the xylene isomers as well as their substituted derivatives can often be converted to the corresponding trichloromethyl derivatives simply by treatment with chlorine. The conversion involves a free radical reaction:^[4]

C₆H₅CH₃ + 3 Cl₂ → C₆H₅CCl₃ + 3 HCl

C₆H₄(CH₃)₂ + 6 Cl₂ → C₆H₄(CCl₃)₂ + 6 HCl

These trichloromethyl compounds are produced on an industrial scale as precursors to other useful compounds. Partial hydrolysis of benzotrichloride provides a route to benzoyl chloride:^[4]

C₆H₅CCl₃ + H₂O → C₆H₅COCl + 2 HCl

By reaction with hydrogen fluoride, trichloromethylated arenes convert to trifluoromethyl derivatives and hydrogen chloride, which can be recycled. In this way, benzotrifluoride (C₆H₅CF₃)and bis(trifluoromethyl)benzene are produced commercially:^[5]

C₆H₅CCl₃ + 3 HF → C₆H₅CF₃ + 3 HCl

C₆H₄(CCl₃)₂ + 6 HF → C₆H₄(CF₃)₂ + 6 HCl

Bioactive derivatives

Folpet, a trichloromethyl-containing insecticide.

A trichloromethyl-containing agricultural chemical is nitrapyrin, which inhibits nitrification, i.e. it enhances the efficiency of fertilizers.

Trichloromethyl-containing insecticides include metrifonate and Folpet.

The list of trichloromethyl-containing pharmaceuticals includes the anorectic drug Amfecloral and the sedative triclofos.

Trichloromethyl radical

The trichloromethyl radical (Cl₃C·) is a well known but transient intermediate in reactions.^[6] It is the agent responsible for the toxicity of carbon tetrachloride.^[7]