radical

Examples of radical in the following topics:

• Radical Chain-Growth Polymerization

  • Virtually all of the monomers described above are subject to radical polymerization.
  • When radical polymerization is desired, it must be started by using a radical initiator, such as a peroxide or certain azo compounds.
  • Because radicals are tolerant of many functional groups and solvents (including water), radical polymerizations are widely used in the chemical industry.
  • The 1º-radical at the end of a growing chain is converted to a more stable 2º-radical by hydrogen atom transfer.
  • Further polymerization at the new radical site generates a side chain radical, and this may in turn lead to creation of other side chains by chain transfer reactions.

• Intermolecular Addition Reactions

  • 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.
  • The first two equations show how different radicals may be generated selectively from the same compound.
  • 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.

• 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.

• Elimination Reactions

  • The use of thionoesters, such as a xanthates, as radical generating functions was described above, and these groups may also serve as excellent radical leaving groups.
  • Once again, the tolerance of radical reactions for a variety of functional groups is demonstrated.
  • An industrial preparation of vinyl chloride from 1,2-dichloroethane, made by adding chlorine to ethylene, proceeds by elimination of a chlorine atom from an intermediate carbon radical.
  • The isomer 1,1-dichloroethane does not undergo an equivalent radical chain elimination.

• 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.
  • Photolysis generates an oxy radical that is located close to the 18-methyl group.

• Background & Introduction

  • A radical is an atomic or molecular species having an unpaired, or odd, electron.
  • Early chemists used the term "radical" for nomenclature purposes, much as we now use the term "group".
  • The resonance structures drawn here may give the impression that the triphenylmethyl radical is planar (flat).
  • 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.

• Methods of Generating Free Radicals

  • The resulting oxy radicals may then initiate other reactions, or may decompose to carbon radicals, as noted in the shaded box.
  • Equations illustrating these radical producing reactions are displayed below.
  • The action of inorganic oxidizing and reducing agents on organic compounds may involve electron transfers that produce radical or radical ionic species.
  • The exceptional facility with which S–H and Sn–H react with alkyl radicals makes thiophenol and trialkyltin hydrides excellent radical quenching agents, when present in excess.
  • 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.

• Cyclization by Intramolecular Addition Reactions

  • If a radical is joined to a double bond by a chain of three or more carbons intramolecular addition generates a ring.
  • In the first two examples shown below, double bond substitution would favor formation of a six-membered ring, but five-membered ring formation by way of a 1º-cyclized radical dominates the products.
  • The second diagram below shows some interesting examples of tandem radical cyclizations will be shown.
  • The stereoelectronic factor in this reaction is defined by the preferred mode of approach of a radical as it bonds to the pi-electron system of an alkene function.
  • Because of this requirement, many cyclizations to moderately sized rings proceed by radical attack at the nearest carbon of the double bond, regardless of substitution.

• Substitution Reactions

  • This means that atom abstractions and radical additions should be exothermic, or only mildly endothermic.
  • 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 the phenyl radical intermediate bonds to sulfur, followed by homolysis of the tert-butyl substituent.
  • Here advantage is taken of e weak N–O bond to generate a carboxyl radical, which rapidly decarboxylates to an alkyl radical.

• Detection and Observation of Radicals

  • Only triphenymethyl and a few other stabilized radicals may be generated in concentrations suitable for examination by traditional laboratory methods.
  • However, an interesting chemical detection of the methyl radical was carried out by the Austrian chemist Fritz Paneth not long after Gomberg's preparation of triphenylmethyl radical.
  • The Paneth experiment involved gas phase thermal decomposition of tetramethyllead to methyl radicals and lead atoms in a glass tube.
  • In practice, esr spectra may be quite complex, as shown by the derivative spectrum of triphenylmethyl radical on the right.
  • For example, the esr signal from methyl radicals, generated by x-radiation of solid methyl iodide at -200º C, is a 1:3:3:1 quartet (predicted by the n + 1 rule).