Radical substitution

The radical substitution (abbreviated S _R ) is a reaction mechanism of organic chemistry , in which on a sp ³ - substituted carbon atom, a hydrogen atom is replaced, often by halogen - or oxygen -atoms (for example in the. Autoxidation ). The reaction takes place as a radical chain reaction over three reaction steps:

Start reaction (radical formation)
Chain propagation
Chain termination (through recombination)

The radical substitution only takes place if radicals can be formed. For this purpose, a homolytic cleavage of a covalent bond must take place, e.g. B. with bromine by UV light, or by heat with benzoyl peroxide or azobis (isobutyronitrile) (AIBN). The generated starting radicals either take part in the chain propagation themselves (bromine) or transfer their radical function to the reaction partners.

Description of the reaction steps using the example of halogenation

Start reaction (initiation)

During the initial reaction, the halogen molecule X _{2 is} split into two radicals (homolytic splitting):

{\ displaystyle \ mathrm {X_ {2} \ longrightarrow 2 \ X {\ cdot}}}

At room temperature, this homolytic cleavage for the halogen fluorine leads to a very vigorous and rapid course of the overall reaction, whereas the halogens chlorine or bromine only react when the reaction mixture is exposed to light ( photochlorination ). Splitting of iodine is not possible at room temperature.

Follow-up reaction (chain propagation, chain reaction, prolongation)

In the subsequent reaction, the halogen radical (X ^• ) reacts with the hydrocarbon (R – H) to form hydrogen halide (H − X), at the same time an alkyl radical (R ^• ) is formed:

{\ displaystyle \ mathrm {X {\ cdot} + R {-} H \ longrightarrow H {-} X + R {\ cdot}}}

The alkyl radical now attacks another halogen molecule and splits it homolytically . The alkyl radical binds a halogen atom via a carbon-halogen bond, a haloalkane and a halogen radical are formed:

{\ displaystyle \ mathrm {R {\ cdot} + X_ {2} \ longrightarrow R {-} X + X {\ cdot}}}

Termination reaction

If two radicals meet, they can recombine to form a covalent bond . In any case, this ends the chain reaction, and undesirable by-products can also arise:

{\ displaystyle \ mathrm {R {\ cdot} + X {\ cdot} \ longrightarrow R {-} X}}

{\ displaystyle \ mathrm {2 \ R {\ cdot} \ longrightarrow R {-} R}}

{\ displaystyle \ mathrm {2 \ X {\ cdot} \ longrightarrow X_ {2}}}

Examples

Radical substitution has the greatest practical importance in the mechanistic consideration of the combustion processes of alkanes , e.g. B. Methane . Mixtures of methane and atmospheric oxygen are kinetically stable, but highly reactive when free radicals R • are present. The latter react with oxygen (more precisely, the diradical • O – O •) and trigger a chain reaction known as combustion. The fuel (e.g. methane) is set in motion by an ignition reaction (match, electric spark, etc.). In this initial reaction, a free radical R • cleaves a C – H bond of the CH ₄ molecule to form a methyl radical (• CH ₃ ). The subsequent steps in the combustion reaction of methane are far more complex; in the end, the reaction products carbon dioxide and water are formed .

Reaction equation	Name and respondent
${\ displaystyle \ mathrm {R {-} H + Y {-} Y \ longrightarrow R {-} Y + H {-} Y}}$	Halogenation with molecular halogens Y = F, Cl, Br
${\ displaystyle \ mathrm {R {-} H + Cl {-} Z \ longrightarrow R {-} Cl + H {-} Z}}$	Chlorination with N -chloramines , N -chlorosuccinimide , sulfuryl chloride , phosphorus pentachloride , phosgene , tert-butyl hypochlorite , carbon tetrachloride
${\ displaystyle \ mathrm {R {-} H + Br {-} Z \ longrightarrow R {-} Br + H {-} Z}}$	Bromination with N -bromo succinimide , tert -butyl hypobromite , bromotrichloromethane
${\ displaystyle \ mathrm {R {-} H + \ cdot O {-} O \ cdot \ longrightarrow R {-} O {-} O {-} H}}$	Peroxygenation (and autoxidation ) with diradical oxygen
${\ displaystyle \ mathrm {R {-} H + SO_ {2} + Cl_ {2} \ longrightarrow R {-} SO_ {2} Cl + HCl}}$	Sulfochlorination
${\ displaystyle \ mathrm {R {-} H + 2 \ SO_ {2} + O_ {2} \ longrightarrow 2 \ R {-} SO_ {3} H}}$	Sulfoxidation
${\ displaystyle \ mathrm {R {-} H + NO_ {2} {-} Z \ longrightarrow R {-} NO_ {2} + H {-} Z}}$	Nitration ; Z = -OH, -NO ₂
${\ displaystyle \ mathrm {R {-} X + H-MRa \ longrightarrow R {-} H + X {-} MRa}}$	Reduction of halogen compounds, sulfonic acid esters and dithiocarbonic acid esters with trialkylstannanes and - silanes ; X = -Hal, -OSO ₂ R ′, -OCS ₂ R ′; M = Sn, Si

Regioselectivity and rate of reaction

The longer a radical exists, the more likely it is that it will react with a halogen molecule during this time. Thus, the increased stability of a radical also increases its reactivity. The rules for the stability of radicals apply analogously to those for the stability of carbocations . The stability increases from primary to secondary to tertiary C radicals. In addition, mesomeric boundary structures, i.e. the mesomeric effect , also have an effect here. At the same time, the reactivity also depends on the probability of the formation of the radical, i.e. on the probability of splitting off the hydrogen atom, which is expressed in the dissociation enthalpy .

In general, it is also true that the selectivity of the reaction increases when the reactivity decreases. Thus, for example, radical bromination is more selective than radical chlorination.

Free radical substitution on the aromatic (S _Ar )

In the case of aromatics, radical substitution leads to a reaction on the side chain, since a radical in the benzyl position is particularly stabilized, in contrast to an aryl radical , which is particularly unfavorable in terms of energy. When toluene reacts with bromine, the bromine radical binds to the alkyl radical . Another example is the Gomberg-Bachmann reaction .

If the reaction conditions are different (darkness, low temperatures or the presence of a catalyst ), an electrophilic substitution takes place.

Keep in mind:
SSS rule - radiation / sun, boiling heat, side chain
KKK rule - cold, catalyst, core

Individual evidence

↑ Albert Gossauer: Structure and reactivity of biomolecules. Verlag Helvetica Chimica Acta, Zurich, 2006, ISBN 3-906390-29-2 , pp. 70-72.
^ Francis A. Carey, Richard J. Sundberg: Organic Chemistry. A further textbook. Translated from the English by Doris Fischer-Henningsen u. a. 2nd Edition. Wiley-VCH, Weinheim et al. 2004, ISBN 3-527-29217-9 , pp. 655-656.
^ Marye Anne Fox, James K. Whitesell: Organic Chemistry. Basics, mechanisms, bio-organic applications. From the English by Elke Buchholz u. a. Spektrum Akademischer Verlag , Heidelberg 1995, ISBN 3-86025-249-6 , p. 296.

literature

Klaus Schwetlick: Organikum . 23rd edition. Wiley-VCH, Weinheim 2009, ISBN 978-3-527-32292-3 .

Web links

Explanations on radical substitution

[Gossauer-1] Albert Gossauer: Structure and reactivity of biomolecules. Verlag Helvetica Chimica Acta, Zurich, 2006, ISBN 3-906390-29-2 , pp. 70-72.

[2] Francis A. Carey, Richard J. Sundberg: Organic Chemistry. A further textbook. Translated from the English by Doris Fischer-Henningsen u. a. 2nd Edition. Wiley-VCH, Weinheim et al. 2004, ISBN 3-527-29217-9 , pp. 655-656.

[3] Marye Anne Fox, James K. Whitesell: Organic Chemistry. Basics, mechanisms, bio-organic applications. From the English by Elke Buchholz u. a. Spektrum Akademischer Verlag , Heidelberg 1995, ISBN 3-86025-249-6 , p. 296.