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.

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 (pKa) 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

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]

References

  1. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 18, ISBN 978-0-471-72091-1
  2. ^ Wallington, T.J; Schneider, W.F; Barnes, I.; Becker, K.H; Sehested, J.; Nielsen, O.J (2000). "Stability and infrared spectra of mono-, di-, and trichloromethanol". Chemical Physics Letters. 322 (1–2): 97–102. Bibcode:2000CPL...322...97W. doi:10.1016/S0009-2614(00)00384-5.
  3. ^ Cotarca L, Geller T, Répási J (2017-09-15). "Bis(trichloromethyl)carbonate (BTC, Triphosgene): A Safer Alternative to Phosgene?". Organic Process Research & Development. 21 (9): 1439–1446. doi:10.1021/acs.oprd.7b00220.
  4. ^ a b Lipper, Karl-August; Löser, Eckhard; Brücher, Oliver (2017). "Benzyl Chloride and Other Side-Chain-Chlorinated Aromatic Hydrocarbons". Ullmann's Encyclopedia of Industrial Chemistry. pp. 1–22. doi:10.1002/14356007.o04_o01.pub2. ISBN 978-3-527-30673-2.
  5. ^ Siegemund, Günter; Schwertfeger, Werner; Feiring, Andrew; Smart, Bruce; Behr, Fred; Vogel, Herward; McKusick, Blaine (2000). "Fluorine Compounds, Organic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a11_349. ISBN 978-3-527-30385-4.
  6. ^ De Vleeschouwer, Freija; Van Speybroeck, Veronique; Waroquier, Michel; Geerlings, Paul; De Proft, Frank (2007). "Electrophilicity and Nucleophilicity Index for Radicals". Organic Letters. 9 (14): 2721–2724. doi:10.1021/ol071038k. PMID 17559221.
  7. ^ Unsal, Velid; Cicek, Mustafa; Sabancilar, İlhan (2021). "Toxicity of carbon tetrachloride, free radicals and role of antioxidants". Reviews on Environmental Health. 36 (2): 279–295. Bibcode:2021RvEH...36..279U. doi:10.1515/reveh-2020-0048. PMID 32970608.
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