Radical substitution

Radical substitution (short SR ) is a reaction mechanism in organic chemistry, in which at one sp3 -substituted carbon atom, a hydrogen atom is replaced, often by halogen or oxygen atoms (such as in the autoxidation ). The reaction proceeds as a free radical chain reaction through three reaction steps:

The free-radical substitution occurs only when radicals can be formed. This must be done a homolytic cleavage of a covalent bond, such as bromine, by UV light, or by heat at or benzoyl peroxide AIBN. The initiating radicals formed either take themselves in propagation part ( bromine) or transfer their radical function of the reactants.

Description of the reaction steps using the example of halogenation

Start reaction

At the start of the reaction, the halogen molecule X2 is homolytically split into two halogen radicals:

At room temperature this homolytic cleavage of the halogen fluorine results in a very rapid and violent reaction during the overall reaction, the halogens chlorine or bromine to react, however, only when the reaction mixture is exposed. Cleavage of iodine is not possible at room temperature.

Follow-up reaction ( chain propagation, chain reaction )

The subsequent reaction, the halogen radical ( X • ) reacts with the hydrocarbon (R -H) to the hydrogen halide (H -X), at the same time is formed thereby an alkyl radical (R • ):

The alkyl radical then attacks another halogen molecule and splits it homolytically. The alkyl radical represents a halogen atom binds via a carbon- halogen bond, it produces a haloalkane and a halogen radical:

Termination reaction

If two radicals each other, they can recombine to form a covalent bond. This ends in each case the chain reaction, also could result in undesirable by-products:

Examples

Has the greatest practical importance radical substitution with mechanistic consideration of combustion processes of alkanes, such as methane. Mixtures of methane and oxygen are kinetically stable but highly reactive free radicals R • if present. The latter react with oxygen (more precisely, the diradical • O -O • ) and trigger a chain reaction, which is known as combustion. Here, the fuel ( eg, methane ) by a reaction ignition ( match, electric spark etc.) is set in motion. In this initial reaction is a free radical R • a C -H bond of CH4 molecule cleaved to form a methyl radical ( • CH 3). The subsequent steps of the combustion reaction of methane are far more complex, the end to form the reaction products carbon dioxide and water.

Y = F, Cl, Br

Regioselectivity and reaction rate

Longer exists, a radical, the more likely it is that it reacts at this time with a halogen molecule. Thus is also increased by an increased stability of a radical its reactivity. The rules for the stability of radicals are analogous to those for the stability of carbocations. Thus, the stability of the primary increases through secondary to tertiary carbon-centered radicals. In addition, the mesomeric effect act also resonance structures, ie from. At the same time the reactivity also depends on the probability of the emergence of the radical, ie the probability of the elimination of the hydrogen atom, which is reflected in the dissociation.

Also generally true that increasing the selectivity of the reaction, when the reactivity is lowered. Thus, for example, the radical bromination is more selective than the free-radical chlorination.

Radical substitution on the aromatic ring ( SAR )

Radical substitution results in aromatics to the reaction on the side chain, since a radical is especially stabilized benzyl, in contrast to a aryl radical which is energetically particularly unfavorable. In the reaction of toluene with the bromine radical bromo binds to the alkyl radical. Another example is the Gomberg - Bachmann reaction.

Are the different reaction conditions ( darkness, low temperatures or the presence of a catalyst ) is an electrophilic substitution instead.