The Appel reaction is an

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, mechanistic organ ...

that converts an

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

into an

alkyl chloride Organochlorine chemistry is concerned with the properties of organochlorine compounds, or organochlorides, organic compounds that contain one or more carbon–chlorine bonds. The chloroalkane class (alkanes with one or more hydrogens substituted ...

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 versatile compound that is widely used as a reagent in organic synthesis and as a l ...

and

carbon tetrachloride Carbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC), is a chemical compound with the chemical formula CCl4. It is a n ...

. The use of carbon tetrabromide or

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

as a halide source will yield alkyl bromides, whereas using

carbon tetraiodide Carbon tetraiodide is a tetrahalomethane with the molecular formula . Being bright red, it is a relatively rare example of a highly colored methane derivative (chemistry), derivative. It is only 2.3% by weight carbon, although other methane derivat ...

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 h ...

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

gives

alkyl iodide Organoiodine chemistry is the study of the synthesis and properties of organoiodine compounds, or organoiodides, organic compounds that contain one or more carbon–iodine bonds. They occur widely in organic chemistry, but are relatively rare ...

s. 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

being restricted under the

Montreal protocol The Montreal Protocol on Substances That Deplete the Ozone Layer is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion. It was agreed on 16 ...

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 increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...

quantities. The corresponding alkyl bromide can also be synthesised by addition of

lithium bromide Lithium bromide (LiBr) is a chemical compound of lithium and bromine. Its extreme hygroscopic character makes LiBr useful as a desiccant in certain air conditioning systems.Wietelmann, Ulrich and Bauer, Richard J. (2005) "Lithium and Lithium Compo ...

as a source of bromide ions. A more sustainable version of the Appel reaction has been reported, which uses a catalytic amount of phosphine that is regenerated with oxalyl chloride. Catalytic Appel Reaction Scheme

Mechanism

The Appel reaction begins with the formation of the

phosphonium In chemistry, the term phosphonium (more obscurely: phosphinium) describes polyatomic cations with the chemical formula (where R is a hydrogen or an alkyl, aryl, organyl or halogen group). These cations have tetrahedral structures. The ...

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 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 rapi ...

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.ed ...

of the alcohol, forming

chloroform Chloroform, or trichloromethane (often abbreviated as TCM), is an organochloride with the formula and a common solvent. It is a volatile, colorless, sweet-smelling, dense liquid produced on a large scale as a precursor to refrigerants and po ...

, yields an alkoxide 5. The

nucleophilic displacement In chemistry, a nucleophilic substitution (SN) is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). ...

of the chloride by the alkoxide yields intermediate 7. With primary and secondary alcohols, the halide reacts in a S_N2 process forming the alkyl halide 8 and

triphenylphosphine oxide Triphenylphosphine oxide (often abbreviated TPPO) is the organophosphorus compound with the formula , also written as or (Ph = ). It is one of the more common phosphine oxides. This colourless crystalline compound is a common but potentially u ...

. Tertiary alcohols form the products 6 and 7 via a S_N1 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 The Mitsunobu reaction is an organic reaction that converts an alcohol into a variety of functional groups, such as an ester, using triphenylphosphine and an azodicarboxylate such as diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxy ...

, 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 phosphin ...

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 are ...

are used to convert alcohols to esters and other applications like this. Appel-Reaktion_M-v3

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 and palmarosa oil. It is a colorless oil, although commercial samples can appear yellow. It has low solubility i ...

to geranyl chloride.

Modifications

The Appel reaction is also effective on

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

s; this has been used to convert them to

oxazoline Oxazoline is a five-membered heterocyclic organic compound with the formula . It is the parent of a family of compounds called oxazolines (emphasis on plural), which contain non-hydrogenic substituents on carbon and/or nitrogen. Oxazolines are the ...

oxazine Oxazines are heterocyclic compound, heterocyclic organic compounds containing one oxygen and one nitrogen atom in a cyclohexa-1,4-diene ring (a doubly unsaturated six-membered ring). Isomers exist depending on the relative position of the heteroa ...

s 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 derivative (chemistry), der ...

s. ::

References

{{Organic reactions Substitution reactions Name reactions

Mechanism

Modifications

See also

References