Dichlorocarbene

 ( Carbenes ) 

Dichlorocarbene is the reactive intermediate with chemical formula CCl₂. Although this chemical species has not been isolated, it is a common intermediate in organic chemistry, being generated from chloroform. This bent diamagnetic molecule rapidly inserts into other bonds.

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Preparation Dichlorocarbene is most commonly generated by reaction of chloroform and a base such as potassium tert-butoxide or aqueous sodium hydroxide. A phase transfer catalyst, for instance benzyltriethylammonium bromide, facilitates the migration of the hydroxide in the organic phase.

HCCl₃ + NaOH → CCl₂ + NaCl + H₂O

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Other reagents and routes Another precursor to dichlorocarbene is ethyl trichloroacetate. Upon treatment with sodium methoxide it releases CCl₂.

Phenyl(trichloromethyl)mercury decomposes thermally to release CCl₂.

PhHgCCl₃ → CCl₂ + PhHgCl

Dichlorodiazirine, which is stable in the dark, decomposes into dichlorocarbene and nitrogen via photolysis. (b) hydroxylamine (c) mesyl chloride (d) sodium hypochlorite (e) nitronium tetrafluoroborate (f) caesium chloride and tetrabutylammonium chlorides in an ionic liquid]]

Dichlorocarbene can also be obtained by dechlorination of carbon tetrachloride with magnesium with ultrasound chemistry. A Facile Procedure for the Generation of Dichlorocarbene from the Reaction of Carbon Tetrachloride and Magnesium using Ultrasonic Irradiation Haixia Lin, Mingfa Yang, Peigang Huang and Weiguo Cao Molecules 2003, 8, 608-613 Online Article This method is tolerant to and carbonyl compounds because it does not involve strong base.

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Reactions

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With alkenes Dichlorocarbene reacts with in a formal 1+2cycloaddition to form geminal cyclopropane. These can be reduced to or hydrolysed to give by a geminal halide hydrolysis. Dichlorocyclopropanes may also be converted to in the Skattebøl rearrangement.

Dichlorocarbene reacts with tetrachloroethylene to give hexachlorocyclopropane. A similar reaction occurs with trichloroethylene, yielding pentachlorocyclopropane.Strain and Its Implications in Organic Chemistry: Organic Stress and Reactivity. (2012). Page 384

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With phenols In the Reimer–Tiemann reaction dichlorocarbene reacts with phenols to give the ortho-formylated product. e.g. phenol to salicylaldehyde.

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With amines Dichlorocarbene is an intermediate in the carbylamine reaction. In this conversion, a dichloromethane solution of a primary amine is treated with chloroform and aqueous sodium hydroxide in the presence of catalytic amount of the phase-transfer catalyst. Illustrative is the synthesis of tert-butyl isocyanide:

Me₃CNH₂ + CHCl₃ + 3 NaOH → Me₃CNC + 3 NaCl + 3 H₂O

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History In 1835, the French chemist Auguste Laurent recognised chloroform as • (then written as •) in his paper on analysing some organohalides. Laurent also predicted a compound seemingly consisting of 2 parts dichlorocarbene which he named Chlorétherose (possibly Tetrachloroethylene, which was not produced until 1839).Auguste Laurent, Note sur les Chlorure, Bromure et Iodure d'Aldehydène (1835), Annales de Chimie et de Physique, p. 327

Dichlorocarbene as a reactive intermediate was first proposed by Anton Geuther in 1862 who viewed chloroform as CCl₂^.HCl Ueber die Zersetzung des Chloroforms durch alkoholische Kalilösung Annalen der Chemie und Pharmacie Volume 123, Issue 1, Date: 1862, Pages: 121-122 A. Geuther Its generation was reinvestigated by Jack Hine in 1950. Carbon Dichloride as an Intermediate in the Basic Hydrolysis of Chloroform. A Mechanism for Substitution Reactions at a Saturated Carbon Atom Jack Hine J. Am. Chem. Soc., 1950, 72 (6), pp 2438–2445 The preparation of dichlorocarbene from chloroform and its utility in synthesis was reported by William von Eggers Doering in 1954. The Addition of Dichlorocarbene to Olefins W. von E. Doering and A. Kentaro Hoffmann J. Am. Chem. Soc.; 1954; 76(23) pp 6162 - 6165;

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Related reactions The Doering–LaFlamme allene synthesis entails the conversion of alkenes to (a chain extension) with magnesium or sodium metal through initial reaction of the alkene with dichlorocarbene. The same sequence is incorporated in the Skattebøl rearrangement to cyclopentadienes.

Closely related is the more reactive dibromocarbene CBr₂.

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Chlorocarbene The related chlorocarbene (ClHC) can be generated from methyllithium and dichloromethane. It has been used in the synthesis of spiropentadiene.

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See also

• Tetrachloroethylene (CCl₂)₂
• Difluorocarbene

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Explanatory notes

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External links

• Addition of dichlorocarbene to 2-methyl-1-buten-3-yne, laboratory procedure
• [5] English translation of 1969 Polish patent on preparation of dichloropropane derivatives

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