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Recommendations for measuring second-order rate constants and kinetic solvent isotope effects on acid-catalyzed reactions

✍ Scribed by James L. Jensen; Michael D. Carr; Kenneth S. Yamaguchi

Publisher
John Wiley and Sons
Year
1983
Tongue
English
Weight
778 KB
Volume
15
Category
Article
ISSN
0538-8066

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✦ Synopsis

Kinetic solvent isotope effects (KSIE) were measured for the hydrolyses of acetals of benzaldehydes in aqueous solutions covering the pH (pD) range of 1-6. For p-methoxybenzaldehyde diethyl acetal, kD+/kH+ = 1.8-3.1, depending on the procedure used to calculate the KSIE and on the pH (pD) range used as the basis for k H + ( k D + ) . It is shown that this variation is an experimental artifact, and is a characteristic of KSIE measurements in general. It is recommended that kL+ be calculated from a least-squares fit of data to the equation kobs

and that the KSIE be reported as k D + / k H + . The limitation remains, however, that the KSIE measured for a variety of substances over quite different pH (pD) ranges may not be comparable tomore than f20%. The source of these observations is discussed in terms of small changes in the activity coefficient ratios (a specific salt effect), including the solvent isotope effect on the activity coefficient ratio [eq. ( 3 ) ] .

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