Free-radical addition
Radical addition enables anti-Markovnikov addition to alkenes.
Free-radical addition is an addition reaction in organic chemistry that involves free radicals, species with an unpaired electron in their valence shell, making them highly reactive. These reactions occur with a variety of unsaturated substrates, including olefinic or aromatic compounds, with or without heteroatoms. The process depends on one or more relatively weak bonds in a reagent; under conditions such as heat or light, these bonds homolyse into radicals, which then induce further decomposition before recombination. The mechanism follows a radical chain process divided into initiation, where a radical is created from a non-radical precursor; propagation, where a radical reacts with a non-radical to produce a new radical; and termination, where two radicals combine to form a non-radical species. In free-radical addition specifically, two propagation steps occur: the adding radical attaches to a multiply-bonded precursor, yielding a radical with reduced bond order, and then this new radical abstracts a substituent from the adding reagent to regenerate the original radical. The adding radical typically attacks the most sterically accessible, or least substituted, carbon of an alkene, with the radical stabilizing on the more substituted carbon, leading to anti-Markovnikov addition—a phenomenon termed the "peroxide effect" by Morris Kharasch. Reaction with alkynes is slower than with alkenes. In a paradigmatic example, hydrogen bromide radicalizes to monatomic bromine, which adds to an alkene at the most accessible site to form a bromoalkyl radical; this radical then abstracts a hydrogen from another HBr molecule, regenerating the bromine atom. This anti-Markovnikov addition is synthetically valuable for hydrogen bromide, but the other hydrohalic acids do not undergo radical addition due to highly endothermic radical formation. The behavior generalizes to halogenated compounds with stable radicals, such as sulfur halides, which add to yield β-halo sulfones, sulfoxides, or sulfides, and to unsubstituted compounds like thiols, which can add across double bonds but risk catalyzing polymerization. For silicon, germanium, or phosphorus, energetics are unfavorable unless a pendant hydrogen is present. Nitrogen oxides add to give mixtures, such as vicinal dinitro compounds and nitro-nitrite esters. Aryl radicals require strong leaving groups, as in the Meerwein ary
- field
- Organic chemistry
- known_for
- Anti-Markovnikov addition via radical chain mechanism
- key_reagent
- Hydrogen bromide (HBr)
- mechanism_steps
- Radical initiation, chain propagation, chain termination
- regiochemistry
- Anti-Markovnikov addition (peroxide effect)
Lore & Background
The basic steps of any free-radical process follow a radical chain mechanism: radical initiation (creation of a radical from a non-radical precursor), chain propagation (a radical reacts with a non-radical to produce a new radical), and chain termination (two radicals react to form a non-radical). In free-radical addition, there are two chain propagation steps: the adding radical attaches to a multiply-bonded precursor, giving a radical with lesser bond order, and the newly-formed radical product abstracts another substituent from the adding reagent to regenerate the adding radical. The adding radical attacks the alkene at the most sterically accessible (typically least substituted) carbon, and the radical then stabilizes on the more substituted carbon, resulting in anti-Markovnikov addition, a phenomenon Morris Kharasch called the 'peroxide effect'.
Reader's Guide
Free-radical addition is significant as a synthetic technique for achieving anti-Markovnikov addition, particularly with hydrogen bromide, which is incredibly selective and does not produce detectable polymeric byproducts. The reaction does not occur with the other hydrohalic acids (HF, HCl, HI) because radical formation from them is extremely endothermic. The behavior of hydrogen bromide generalizes in two directions: halogenated compounds with a relatively stable radical can dissociate from the halogen (e.g., sulfonyl, sulfenyl, and other sulfur halides add radically to give β‑halo sulfones, sulfoxides, or sulfides), and unsubstituted compounds with a relatively stable radical can dissociate from hydrogen (e.g., thiol-ene reaction, thermal silane additions). Side reactions include radical cyclization and radical polymerization. With stable inorganic radicals, such as nitrate radicals from ceric ammonium nitrate, self-terminating oxidative radical cyclization can oxidize alkynes to ketones.
Did You Know?
- Free-radical addition of hydrogen bromide is a valuable synthetic technique for anti-Markovnikov carbon substitution.
- Radical formation from HF, HCl, or HI is extremely endothermic and chemically disfavored.
- In the paradigmatic example, bromine atoms add to an alkene at the most accessible site, giving a bromoalkyl radical with the radical on the more substituted carbon.
- Reaction is slower with alkynes than alkenes.
More in Organic Chemistry And Reaction Mechanisms 1-22
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