Secondary deuterium isotope effects for addition of nitrogen nucleophiles to substituted benzaldehydes
Kinetic secondary deuterium isotope effects, k/sub D//k/sub H/, for the water-catalyzed addition of semicarbazide to a series of substituted benzaldehydes are near 1.30 and are indepcndent of the nature of the polar substituent; corresponding values for the same reaction catalyzed by the hydrated proton and other acids are near 1.21. Values of k/sub D//k/sub H/ for the water- catalyzed addition of phenylhydrazine to substituted benzaldehydes decrease from 1.28 to 1.16 as the nature of the polar substituent is varied from p-methoxy to p- nitro. The corresponding values for the same reaction catalyzed by the hydrated proton decrease from 1.21 to 1.13 for the same substituents. These results indicate that the extent of carbon--nitrogen bond formation in the transition state for addition of nucleophiles to the carbonyl group is a function of the reactivity of substrate, nucleophilic reagent, and the strength of the catalyzing acid. The secondary deuterium isotope effect on the equilibrium constant for carbinolamine formation from benzaldehyde and hydroxylamine is near 1.36, larger than the largest of the kinetic effects observed. Under conditions in which carbinolamine dehydration is rate determining, values of k/sub D//k/sub H// for the overall rate of semicarbazone formation from benzaldehydes decrease from 1.31 to 1.20 as the nature of the polar substituent varies from hydrogen to p-nitro. This result demonstrates that the extent of carbon-oxygen bond cleavage in the transition state for carbinolamine dehydration is a function of the nature of the polar substituent of the substrate. Moreover, for those substituents which are not strongly electron withdrawing, the transitlon state closely resembles the carbinolamine itself. (auth)
- Research Organization:
- Univ., Sao Paulo, Brazil
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-29-000130
- OSTI ID:
- 4425481
- Journal Information:
- J. Amer. Chem. Soc., v. 95, no. 22, pp. 7369-7374, Journal Name: J. Amer. Chem. Soc., v. 95, no. 22, pp. 7369-7374; ISSN JACSA
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
Similar Records
Kinetic secondary deuterium isotope effects for substituted benzaldehyde cyanohydrin formation
Stopped-flow studies of carbon dioxide hydration and bicarbonate dehydration in H/sub 2/O and D/sub 2/O. Acid-base and metal ion catalysis