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The Appel reaction is an 
Drawbacks to the reaction are the use of toxic halogenating agents and the coproduction of organophosphorus product that must be separated from the organic product. The phosphorus reagent can be used in 
Illustrative use of the Appel reaction is the chlorination of
organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical rea ...
that converts an alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
into an alkyl chloride
An organochloride, organochlorine compound, chlorocarbon, or chlorinated hydrocarbon is an organic compound containing at least one covalently bonded atom of chlorine. The chloroalkane class (alkanes with one or more hydrogens substituted by chlor ...
using triphenylphosphine
Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists ...
and carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...
. The use of carbon tetrabromide or bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simil ...
as a halide source will yield alkyl bromides, whereas using carbon tetraiodide, methyl iodide
Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one ...
or iodine
Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
gives alkyl iodides. The reaction is credited to and named after Rolf Appel, it had however been described earlier. The use of this reaction is becoming less common, due to carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...
being restricted under the Montreal protocol
The Montreal Protocol is an international treaty
A treaty is a formal, legally binding written agreement between actors in international law. It is usually made by and between sovereign states, but can include international organization ...
.
Drawbacks to the reaction are the use of toxic halogenating agents and the coproduction of organophosphorus product that must be separated from the organic product. The phosphorus reagent can be used in catalytic
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycl ...
quantities. The corresponding alkyl bromide can also be synthesised by addition of lithium bromide as a source of bromide ions. A greener, more sustainable catalytic Appel reaction, free from chlorinated solvents, has also been reported.
Mechanism
The Appel reaction begins with the formation of thephosphonium
In polyatomic cations with the chemical formula (where R is a hydrogen or an alkyl, aryl, or halide group). These cations have tetrahedral structures. The salts are generally colorless or take the color of the anions.
Types of phosphonium c ...
salt 3, which is thought to exist as a tight ion pair with 4 and therefore is unable to undergo an alpha-elimination to give dichlorocarbene. Deprotonation
Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju. ...
of the alcohol, forming chloroform
Chloroform, or trichloromethane, is an organic compound with formula C H Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to PTFE. It is also a precursor to various re ...
, yields an alkoxide 5. The nucleophilic displacement of the chloride by the alkoxide yields intermediate 7. With primary and secondary alcohols, the halide reacts in a SN2 process forming the alkyl halide 8 and triphenylphosphine oxide. Tertiary alcohols form the products 6 and 7 via a SN1 mechanism.
The driving force behind this and similar reactions is the formation of the strong PO double bond. The reaction is somewhat similar to the Mitsunobu reaction, where the combination of an organophosphine Organophosphines are organophosphorus compounds with the formula PR''n''H3−''n'', where R is an organic substituent. These compounds can be classified according to the value of ''n'': primary phosphines (''n'' = 1), secondary phosphines ...
as an oxide acceptor, an azo compound
Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups).
IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...
as a hydrogen acceptor reagent, and a nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...
are used to convert alcohols to esters and other applications like this.
Illustrative use of the Appel reaction is the chlorination of geraniol
Geraniol is a monoterpenoid and an alcohol. It is the primary component of citronella oil and is a primary component of rose oil, palmarosa oil. It is a colorless oil, although commercial samples can appear yellow. It has low solubility in w ...
to geranyl chloride.
Modifications
The Appel reaction is also effective oncarboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyl ...
s; this has been used to convert them to oxazolines, oxazines and thiazoline
Thiazolines (or dihydrothiazoles) are a group of isomeric 5-membered heterocyclic compounds containing both sulfur and nitrogen in the ring. Although unsubstituted thiazolines are rarely encountered themselves, their derivatives are more common ...
s.
::
See also
*Atherton–Todd reaction
The Atherton-Todd reaction is a name reaction in organic chemistry, which goes back to the British chemists F. R. Atherton, H. T. Openshaw and A. R. Todd. These described the reaction for the first time in 1945 as a method of converting Dialkyl pho ...
* Corey–Fuchs reaction
* Mitsunobu reaction
References
{{Organic reactions Substitution reactions Name reactions