Alkylation

Alkylation using the example of a Friedel-Crafts alkylation with aluminum bromide as a catalyst. The alkyl group R of the bromoalkane R – Br is transferred to the benzene, resulting in an (alkylated) benzene derivative.

In organic chemistry, alkylation is the term used to describe the transfer of alkyl groups from one molecule to another during a chemical reaction . Here to carbon atoms of the parent compound, but also hetero atoms are alkylated. The opposite reactions (i.e. reactions in which alkyl groups are split off from organic molecules) are referred to as dealkylation. The compounds used for alkylation are referred to as alkylating agents .

Systematics

Classification according to the type of alkyl group introduced

Methylation , introduction of a methyl group (–CH ₃ )
Ethylation , introduction of an ethyl group (–C ₂ H ₅ )
Propylation, introduction of a propyl group (–C ₃ H ₇ )
Butylation, introduction of a butyl group (–C ₄ H ₉ )

etc.

Classification according to the binding site of the introduced alkyl radical

Depending on which type of atom is alkylated in the educt, a distinction is made:

C 8 alkylation, e.g. B. Conversion of benzene to alkylbenzenes by Friedel-Crafts alkylation
O alkylation, e.g. B. Conversion of alcohols into ethers
S alkylation, e.g. B. Conversion of thioalcohols into thioethers
N -alkylation, e.g. B. Conversion of primary or secondary amines into tertiary amines

Occurrence and use

There are a number of alkylations that are of great industrial importance. Friedel-Crafts alkylation, for example, with alkenes as the alkylating agent, is of great importance in the synthesis of ethylbenzene , cumene , alkylphenols and butylnaphthalene .

Acid-catalyzed additions to alkenes play in the petrochemical z. B. in the Alkar process a role. Alkylations are technically important in the production of fuels ( alkylates ), ethylbenzene and other alkylbenzenes, plastics, dyes and pharmaceuticals.

There are a variety of reactions that belong to the alkylations or can be used for the alkylation. These include the Williamson ether synthesis , the Kolbe nitrile synthesis , Gabriel synthesis of primary amines , the Arbusow reaction , the Michaelis-Becker reaction or the Finkelstein reaction .

Organometallic compounds with carbon-metal bonds can be alkylated to alkanes using haloalkanes. This takes place, for example, in the Wurtz synthesis .

The alkylation, alkenylation and arylation of alkenes with haloalkanes , haloalkenes and haloaromatics succeed in the presence of palladium (0) complexes as catalysts, which is referred to as the Heck reaction .

In biochemical processes, alkylations, especially methylations, and dealkylations occur in various processes, for example in the formation of creatine from guanidinoacetic acid with the participation of the cofactor S-adenosylmethionine . In addition, methylations play an important role in gene regulation and differentiation through the modification of cytosine residues to 5-methylcytosine on DNA . Non-specific alkylations of DNA by external influences can influence gene expression , which is why alkylating agents often act as carcinogens and mutagens .

Reaction mechanisms

Friedel-Crafts alkylation is an electrophilic aromatic substitution (S _E Ar for short ). Other alkylations such as the Williamson ether synthesis or the Kolbe nitrile synthesis belong to the S _N 2 reactions . The SN ₁ mechanism is also used in tert-alkylation of malonic esters (malonic ester synthesis) . Finally, organometallic reactions such as the Grignard reaction can also be used for alkylation, in which alkyl radicals are nucleophilically added to an electrophilic center such as a carbonyl group or the double bond of a Michael system ( Michael addition ).

There are also different reaction mechanisms in biochemistry. Enzymatic methylations are carried out by transferring C ₁ fragments with the help of transferases .

Individual evidence

↑ ^a ^b ^c ^d ^e Entry on alkylation. In: Römpp Online . Georg Thieme Verlag, accessed on June 15, 2014.
↑ HP Latscha, HA Klein: Organic Chemistry Chemistry - Basic Knowledge II . Springer-Verlag, 2013, ISBN 978-3-662-09143-2 ( limited preview in Google book search).
↑ Lexicon of Chemistry: Friedel-Crafts Reactions - Lexicon of Chemistry , accessed on August 23, 2018
↑ ^a ^b Reinhard Brückner: reaction mechanisms, organic reactions, stereochemistry, modern synthesis methods . Springer-Verlag, 2014, ISBN 978-3-662-45684-2 , pp. 96 ( limited preview in Google Book search).
↑ ^a ^b Eberhard Breitmaier, Günther Jung: Organic Chemistry, 7th complete revision. u. exp. Edition 2012 Basics, classes of compounds, reactions, concepts, molecular structure, natural substances, synthesis planning, sustainability . Georg Thieme Verlag, 2014, ISBN 3-13-179387-2 , p. 35 ( limited preview in Google Book search).

Web links

Commons : Alkylation reactions - collection of images, videos and audio files

[Roempp-1] Entry on alkylation. In: Römpp Online . Georg Thieme Verlag, accessed on June 15, 2014.

[H.P._Latscha,_H.A._Klein-2] HP Latscha, HA Klein: Organic Chemistry Chemistry - Basic Knowledge II . Springer-Verlag, 2013, ISBN 978-3-662-09143-2 ( limited preview in Google book search).

[spektrum.de-3] Lexicon of Chemistry: Friedel-Crafts Reactions - Lexicon of Chemistry , accessed on August 23, 2018

[Reinhard_Brückner-4] Reinhard Brückner: reaction mechanisms, organic reactions, stereochemistry, modern synthesis methods . Springer-Verlag, 2014, ISBN 978-3-662-45684-2 , pp. 96 ( limited preview in Google Book search).

[Eberhard_Breitmaier,_Günther_Jung-5] Eberhard Breitmaier, Günther Jung: Organic Chemistry, 7th complete revision. u. exp. Edition 2012 Basics, classes of compounds, reactions, concepts, molecular structure, natural substances, synthesis planning, sustainability . Georg Thieme Verlag, 2014, ISBN 3-13-179387-2 , p. 35 ( limited preview in Google Book search).