A Guidebook to Mechanism in Organic Chemistry

Methods of Formation

CH,===CH^-CH, -M- CH,—CH==CH,

(i) Methods of formation

There are numerous methods by which short-lived radicals may be
formed, of wHEn the most important are the thermal and photo­
chemical fission of bonds, oxidation/reduction reactions by inorganic
ions resulting in single electron transfers, and electrolysis.
(a) Photochemical fission: A well-known example is the decom­
position of acetone in the vapour phase by light having a wavelength
of « 3000 A:

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Me—C—Me -> Me* + *C—Me -»• CO+-Me

Another classic example is the conversion of molecular chlorine to

chlorine atoms by sunlight

ci—ci -+ a- + -a.

• that occurs as the first step in a number of photo-catalysed chlorin-
  ations (p. 248). Normally speaking, such photochemical decomposi­
  tion may only be effected by visible or ultraviolet light of definite
  wavelengths corresponding—hardly surprisingly—to absorption
  maxima in the spectrum of the compound. Reactions of this type also
  occur in solution, but the life of the radical is then usually shorter
  owing to the opportunities afforded for reaction with solvent mole­
  cules (see below).
  (b) Thermal fission: Much of the early work on short-lived radicals,
  including studies of their half-lives, was carried out on the products
  obtained from the thermal decomposition of metal alkyls:
  Pb(CH 8 —CHa) 4 -* Pb+4-CH,—CH,
  Further reference is made to this work when the methods for detect­
  ing short-lived radicals are discussed below. In the vapour phase, the
  life of such radicals can be ended not only by dimerisation
  CH,—CH,- + -CH,—CH, CH,—CH,—CH,—CH,