Register      Login
Australian Journal of Chemistry Australian Journal of Chemistry Society
An international journal for chemical science
RESEARCH ARTICLE

Neighbouring group participation by the carboxy group in the solvolysis of the dipeptides in acetic acid

RJL Martin, DS Schneider, CH Skovron and DLH Yiu

Australian Journal of Chemistry 30(5) 1025 - 1035
Published: 1977

Abstract

The acid-catalysed solvolysis of the dipeptides in acetic acid solution occurs readily both in the presence and absence of water. The rates increase with increasing methylation, which is consistent with a mechanism involving a ring-closure pathway by the terminal carboxy group. The experimentally determined rate law is:

f × rate × [H2O]/[dipeptidium(1+) ClO4-][ΣH+] = k1 + k2[H2O]2

where f is the salt effect factor and ΣH+ is the acetic acidium perchlorate concentration after correcting for the presence of oxonium perchlorate.

The k1 values increase with increasing methylation, as required for a ring-closure mechanism, and the inverse water term in this component of the rate is ascribed to a medium effect. The ratecontrolling step is defined as being the acid-catalysed ring fission of a rapidly formed intermediate, oxazalone acetic acid solvate. The k2 component of the rate is first order with respect to water and would appear to refer to the normal mode of solvolysis. But the much higher values of k2 for the heavily methylated compounds as compared with Gly-Gly are incompatible with the much lower activities shown by these compounds when reacting by the normal mode of solvolysis. Since k2 passes through a minimum with increasing methylation, it is suggested that it is a composite of the normal and ring-closure modes of solvolysis. The rate-controlling step for this latter reaction is the acid-catalysed reaction of water with the oxazolone acetic acid solvate. The increasing rates arising from increasing methylation are due to increasing stabilities of the oxazolone solvate ring systems resulting in a kinetically favourable displacement of this equilibrium towards the ring system.

Infrared analysis shows that the oxazolone of Aibu-Aibu readily undergoes a fast reaction with acidified acetic acid to form a stable acetic acid solvate with the complete disappearance of all oxazolone absorption frequencies. Aibu-Aibu on heating in acidified acetic acid solution rapidly forms the same oxazolone acetic acid solvate with no evidence of any oxazolone absorption frequencies. The solvolysis of optically active Gly-Val demonstrates that the oxazolone is not formed as an intermediate because the Val produced retains its optical activity with no evidence of any racemization.

https://doi.org/10.1071/CH9771025

© CSIRO 1977

Committee on Publication Ethics


Export Citation Get Permission

View Dimensions