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ionization potentials, electron affinities, and charge distribution, and that of Mulliken,25 which is related to ionization potentials and electron affinities. None provides a linear free energy relationship for the reactions under discussion here. Typically, the enhancement of the rate constant caused by CH3 substitution is underestimated compared to the predicted reduction of reactivity caused by the substitution of the electron-withdrawing halogen atoms.The reaction transition state in the R + HI reactions, whether formed directly from the reactants or formed from a rearrangement of an R.1-H complex,' involves an H atom with a partial positive charge located in the region between the I atom and the methyl radical. Enhanced electron density (as provided by methyl (25) Mullikcn, R. S. J . Chem. Phys. 1934, 2, 782.substitution for H) at the methyl radical carbon would stabilize such a transition state and facilitate reaction, while diminished electron density (such as that caused by halogen atom substitution for H) would destabilize such a transition state. This is the behavior that is observed.Additional studies of the kinetics of R + HI and R + HBr reactions are in progress to understand more fully the factors controlling reactivity. Acknowledgment.The kinetics and mechanism of the sonochemical reactions of p-nitrophenol have been investigated in oxygenated aqueous solutions. In the presence of ultrasound (20 W z , 84 W) pnitrophenol was degraded primarily by denitration to yield NO2-, NOf, benzoquinone, hydroquinone, 4nitrocatecho1, formate, and oxalate. These reaction products and the kinetic observations are consistent with a model involving high-temperature reactions of pnitrophenol in the interfacial region of cavitation bubbles. The main reaction pathway appears to be carbon-nitrogen bond cleavage. Reaction with hydroxyl radical provides a secondary reaction channel. The average effective temperature of the interfacial region surrounding the cavitation bubbles was estimated to be T 800 K.

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