Zucker–Hammett hypothesis

This hypothesis states that, if in an acid catalysed reaction, $log 10 k 1$ (first-order rate constant of the reaction) is linear in $H 0$ (Hammett acidity function ), water is not involved in the transition state of the rate-controlling step. However, if $log 10 k 1$ is linear in $log 10 H +$ then water is involved. This has been shown to be incorrect by Hammett himself.

Source:

PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1176