radical

(noun)

a very reactive substance with an unpaired electron

Related Terms

  • electrophilic addition
  • addition polymerization
  • cracking
  • alkane
  • addition reaction

(noun)

A group of atoms joined by covalent bonds that are characterized by a free, unpaired electron that imparts their reactivity.

Related Terms

  • electrophilic addition
  • addition polymerization
  • cracking
  • alkane
  • addition reaction

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).
Subjects
  • Accounting
  • Algebra
  • Art History
  • Biology
  • Business
  • Calculus
  • Chemistry
  • Communications
  • Economics
  • Finance
  • Management
  • Marketing
  • Microbiology
  • Physics
  • Physiology
  • Political Science
  • Psychology
  • Sociology
  • Statistics
  • U.S. History
  • World History
  • Writing

Except where noted, content and user contributions on this site are licensed under CC BY-SA 4.0 with attribution required.