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Radical Additions and Substitutions with Alkenes

Radical Substitution in Allylic Position

Depending on the conditions, the reactions of propene with either bromine or chlorine, respectively, yields different products. At low temperatures and a high halogen concentration, an addition to the double bond occurs. In contrast, a substitution of hydrogen at the methyl group is common among high temperatures and a low halogen concentration.

The alkylic position that is immediately adjacent to the double bond is called allylic position. The allylic position represents characteristic chemical properties. Some allylic species are specially stabilized due to the adjacent double bond.
Dissociation energies of different types of C-H bonds.

Allyl radicals and allyl cations are considerably resonance-stabilized. This may be illustrated by comparing the dissociation energies of different types of C-H bonds to each other (see table). The considerable resonance stabilization of allyl radicals and allyl cations is caused by the overlapping of the p orbitals of the three vicinal sp2 carbons in the allyl group.

Resonance of allylic systems.
Illustration of the spin density of an allyl radical.

The resonance of allylic systems results in the equal lengths of the propenyl radical's C-C bonds. Due to the resonance, all four terminal hydrogen atoms display allylic character. Only the hydrogen at the central carbon is vinylic. The specific characteristic of allylic systems is reflected in their chemical reactions. Generally, at room temperature, chlorine and bromine are added to the double bonds of alkenes. At higher temperatures, however, the addition reaction is increasingly superseded by the allylic substitution, especially when the halogen concentration is low. In such radical allylic substitutions, which are also used on a large scale (see Shell process), a fair yield is obtained.

Shell process.

The allylic substitution is carried out at a high temperature in the gas phase. Under these reaction conditions, the chlorine molecule is homolytically cleaved by thermal energy into two chlorine radicals. Principally speaking, the chlorine radicals may also be attached to the double bond by a radical addition. Nevertheless, due to the application of chlorine in a low concentration, radical substitution exceeds the addition. On a laboratory scale, selective halogenations - particularly brominations - in allylic position are obtained by the application of N-bromosuccinimide (NBS), which is a continous source of small amounts of bromine.

Exercises: Radical substitution in allylic position

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