free radical

Examples of free radical in the following topics:

• Non-Ionic Reactions

  • Here we shall consider two other classes of organic reactions: Free-Radical Reactions & Pericyclic Reactions.
  • One type of "free-radical reaction", alkane halogenation has already been described.
  • Indeed, the study of free-radical polymerization of alkene monomers has opened the door to modern polymer chemistry.
  • In contrast to ionic reactions, both free radical and pericyclic reactions may occur in the gas phase, as well as in solution in various solvents.

• Background & Introduction

  • A radical is an atomic or molecular species having an unpaired, or odd, electron.
  • Credit for the first isolation and characterization of a "free radical" goes to Moses Gomberg, a young instructor at the University of Michigan.
  • Gomberg concluded that the colored solutions contained reactive triphenylmethyl free radicals, formed by thermal dissociation of their dimer (Keq = 2 • 10–4 at 25º C).
  • Other relatively stable radicals, such as galvinoxyl have been prepared and studied.
  • The term "free radical" is now loosely applied to all radical intermediates, stabilized or not.

• Odd-Electron Molecules

  • Molecules with unpaired electrons are termed 'free radicals.'
  • While typically highly unstable, and therefore highly reactive, some free radicals exhibit stability of days, months, or even years.
  • There is persistent radical character on nitrogen because it has an unpaired electron.
  • Nitric oxide (NO) is an example of a stable free radical.

• Addition Reactions

  • Two non-polar addition reactions also exist: free radical addition and cycloadditions.
  • In free radical additions of halogens to alkanes (or alkenes), a radical halogen can attack an alkane to produce another radical, in this case a radical version of the alkane.
  • The radical alkane can attack another compound, producing another radical that can continue on to attack another compound.
  • Polymerization can either proceed via a free-radical or an ionic mechanism.
  • Top to bottom: electrophilic addition to alkene, nucleophilic addition of nucleophile to carbonyl, and free radical addition of halide to alkene.

• The Configuration of Free Radicals

  • Since the difference in energy between a planar radical and a rapidly inverting pyramidal radical is small, radicals generated at chiral centers generally lead to racemic products.
  • Initial formation of a carboxyl radical is followed by loss of carbon dioxide to give a pyramidal bridgehead radical.
  • This radical abstracts a chlorine atom from the solvent, yielding the bridgehead chloride as the major product.
  • Rapid decomposition to other radicals may occur, but until one or both of these radicals escape the solvent cage a significant degree of coupling (recombination) may occur.
  • Cage recombination of radicals may be sufficiently rapid to preserve the configuration of the generating species.

• Intermolecular Addition Reactions

  • Addition reactions to carbon:carbon double bonds are among the most important free radical reactions employed by chemists.
  • As the following equations demonstrate, radical addition to a substituted double bond is regiospecific (i.e. the more stable product radical is preferentially formed in the chain addition process).
  • The following diagram provides other examples of radical addition to double bonds.
  • Indeed, free radical polymerization of simple substituted alkenes is so facile that bulk quantities of these compounds must be protected by small amounts of radical inhibitors during storage.
  • These inhibitors, or radical scavengers, may themselves be radicals (e.g. oxygen and galvinoxyl) or compounds that react rapidly with propagating radicals to produce stable radical species that terminate the chain.

• Substitution Reactions

  • For many years organic chemists considered free radical reactions to have limited applications, and to be of little interest outside some fields of industrial chemistry.
  • Since most free radical intermediates are very reactive and have short lifetimes, all the steps in a practical chain reaction sequence must be fast compared with possible competing reactions.
  • The alkyl halide reduction described above is one example of a radical substitution reaction.
  • Phenylsilane may be substituted for the stannane as a radical carrier.
  • Here advantage is taken of e weak N–O bond to generate a carboxyl radical, which rapidly decarboxylates to an alkyl radical.

• Methods of Generating Free Radicals

  • Free radicals, produced by homolysis of C–C bonds, are known to be intermediates in these transformations.
  • The resulting oxy radicals may then initiate other reactions, or may decompose to carbon radicals, as noted in the shaded box.
  • The action of inorganic oxidizing and reducing agents on organic compounds may involve electron transfers that produce radical or radical ionic species.
  • If free radical reactions are to be useful to organic chemists, methods for transferring the reactivity of the simple radicals generated by the previously described homolysis reactions to specific sites in substrate molecules must be devised.
  • Carbon halogen bonds, especially C–Br and C–I, are weaker than C–H bonds and react with alkyl and stannyl radicals to generate new alkyl radicals.

• Radical Recombination Reactions

  • Radical coupling (recombination) reactions are very fast, having activation energies near zero.
  • The only reason radical coupling reactions do not dominate free radical chemistry is that most radicals have very short lifetimes and are present in very low concentration.
  • Consequently, if short lived radicals are to contribute to useful synthetic procedures by way of a radical coupling, all the events leading up to the coupling must take place in a solvent cage.
  • The oxy radical abstracts a hydrogen atom from a nearby carbon, and the resulting radical couples with •NO to give a nitroso compound.
  • Stabilized free radicals have sufficiently long lifetimes to permit coupling outside solvent cage confinement.

• Halogenation

  • The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms.
  • It should be clear from a review of the two steps that make up the free radical chain reaction for halogenation that the first step (hydrogen abstraction) is the product determining step.
  • By this reasoning we would expect benzylic and allylic sites to be exceptionally reactive in free radical halogenation, as experiments have shown.
  • The methyl group of toluene, C6H5CH3, is readily chlorinated or brominated in the presence of free radical initiators (usually peroxides), and ethylbenzene is similarly chlorinated at the benzylic location exclusively.
  • Since carbon-carbon double bonds add chlorine and bromine rapidly in liquid phase solutions, free radical substitution reactions of alkenes by these halogens must be carried out in the gas phase, or by other halogenating reagents.