Direct Measurement of Transition State Bond Cleavage inHydrolysis of Phosphate Esters of p-Nitrophenol

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Journal of the American Chemical Society

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We have found a simple technique to measure the degree of bond breaking in the transition state which should be applicable to any reaction where p-nitrophenol is the leaving group. The breaking of a bond to p-nitrophenol in a displacement reaction involves the development of delocalized negative charge in the phenol corresponding to the degree of bond breaking. The delocalization of this negative charge occurs through the contribution of a quinonoid resonance form in which the bond order between nitrogen and oxygen decreases and the bond order between nitrogen and carbon increases. Since nitrogen-oxygen bonds are more stiffening than nitrogen-carbon ones, 15N will enrich in the uncharged phenol, and a normal isotope effect will be observed on formation of the phenolate anion. The size of this isotope effect in a displacement reaction affords a measure of the magnitude of charge developed on the p-nitrophenol leaving group in the transition state.

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