Relationship of Sonochemistry with Tribochemistry

The tribochemical effect, according to which chemical transformations result from mechanical stresses, is complex. Largely ignored by organic chemists, this non-conventional chemistry is the object of a considerable amount of work in inorganic chemistry. A complete discussion of these reactions (also named mechanochemical or mechanical chemistry) is clearly out of the scope of this work. It is sufficient here to mention a few important data, taken from recent reviews, to which the reader will refer for a deeper approach.  

Illustrations can be chosen among the vast amount of data, and applications to organic synthesis were reviewed.i As an example, when submitted to mechanical treatment, solid o,o -di-t-butyl catechol and the corresponding quinone react to give the semiquinone radical anion. Polymers, synthetic or natural (lignin), undergo cleavage, but polymerization can also be induced.  

Current explanations of tribochemical reactions state that the more obvious consequence of mechanical treatments, the increase of the surface area of a solid, is a minor factor, which contributes only to 10% of the reactivity increase. The more important effect is due to the accumulation of energy in lattice defects which can relax either physically by the emission of heat, or chemically by the ejection of atoms or electrons, formation of excited states on the surface, bond breakages, and other chemical transformations. Mechanical stress can be applied as single or periodic shocks, rapid loads, etc. An example is that of the mechanochemical decomposition of aluminum hydride, which increases with the frequency of the applied stress (Fig. 2).i  

In the above example, the decomposition ratio (for which the exact method of determination does not appear clearly in the paper) increases notably when the frequency reaches a critical value. The violent shock undergone by a surface hit by a cavitation bubble should be similar in nature if not in intensity to other mechanical shocks, with analogous chemical consequences. Such analogies of the sonochemistry of solids with mechanochemistry were suggested previously.  

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