Sonolysis of the carbon-halogen bond

Homolysis of carbon-halogen bonds occurs easily by sonication of alkyl halides, as liquids or in solution. The energy of an aliphatic C-Cl bond is ca. 325 kJ moLL A thermolytic mechanism in the bubble is generally invoked, but if a radiomimetic effect exists, ionization can occur in analogy with the radiolysis of chlorocarbons. 

53 Ando, T. Fujita, M. Kimura, T. Leveque, J.M. Luche, J.L. Sohmiya, H. Ultrasonics Sonochemistry 1996,3, S223-S227. 

Using suitable conditions and substrates, however, sonolysis can be selective, and its uses for synthesis become realistic, as shown below. 

Maleic acid and its esters are isomerized by irradiation of their solutions in the presence of various alkyl bromides, sometimes bromine itself. The C-Br (or Br-Br) bond is sonolyzed to bromine atoms, which add reversibly to the double bond.56 Geometric stabilization of the adduct radical and then elimination of bromine produces fumaric compounds (Fig. 13). When the initiator is bromine, oxygen inhibits the reaction (R = H or CH3), an argument in favor of a radical chain process with the initiation step, the cleavage of the Br2 molecule, occurring in the cavitation bubble. The lifetime of the bromine atoms is s, which at the 

Some of the atoms reach the olefinic compound at the solution interface, where the addition-isomerization sequence occurs. If oxygen is present, quenching of Br occurs in the bubble. Using a different initiator, such as dibromoethane or -hexyl bromide, isomerization is slower than with bromine, but is not inhibited by oxygen. The authors interpret this result as the participation of oxygen in the cleavage of the C-Br bond, without giving a mechanism. 

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This reaction was reinvestigated recently in carbon tetrachloride solution at 20 kHz.57 Sonolysis of the solvent was not detected and, if present, does not induce any isomerization. Of the several alkyl bromides tried, f-butyl bromide is the most efficient, a result which is explained by its higher volatility and the lower dissociation energy of the C-Br bond. It was concluded that the rate limiting step is actually the sonolysis of the carbon-halogen bond inside the bubble.58... 

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