Conformation and reactivity. Part VII. Kinetics of the alkaline hydrolysis of the methyl and ethyl trans-decalincarboxylates, and 4-t-butylcyclohexanecarboxylates, certain trans-4-substituted cyclohexanecarboxylates, and methyl and ethyl cyclohexanecarboxylate in alcohol–water and dioxan–water mixtures

Abstract

Rates of alkaline hydrolysis of the methyl and ethyl trans-decalincarboxylates, cis- and trans-4-t-butylcyclohexanecarboxylates, trans-4-X-cyclohexanecarboxylates, (X = OH, CO₂Me [or Et], CO₂^– or CN), and cyclohexanecarboxylates have been measured. For the methyl esters the solvents were 1 : 1 methanol–water, 7 : 3 methanol–water, 1 : 1 dioxan–water, and 3 : 1 dioxan–water, and for the ethyl esters 1 : 1 ethanol–water, 7 : 3 ethanol–water, 1 : 1 dioxan–water, and 3 : 1 dioxan–water, with or without added 0·2M-potassium chloride. Measurements were made at 29·4 and at 50·0°, but it was not possible to study all the esters in all the solvent systems at both temperatures. In a few experiments 1 : 1 diglyme–water was the solvent.

In alcoholic solvents a trans-4-t-butylcyclohexanecarboxylate reacts more rapidly than the corresponding cyclohexanecarboxylate, while the reverse is true for reactions in dioxan–water or diglyme–water. Results for the other carboxylic esters are considered in relation to possible explanations of this observation. Comparison of the results for methyl trans-decalin-2α-carboxylate and methyl cis-4-t-butylcyclohexanecarboxylate suggests that the latter contains about 1 % of a conformation with an equatorial ester group. Methyl trans-decalin-1α-carboxylate shows unexpectedly low reactivity.

Solvent effects on reactivity are discussed in terms of the RO^–+ H₂O ⇌ ROH + OH^– equilibrium, of the existence of OH^– as solvated ions or as ion-pairs, and of solvation of the transition state; with water as the more important solvating species. The last factor is relevant to the discussion of salt effects on reactivity. The effects of polar substituents on the reactivity of methyl and ethyl trans-4-X-cyclohexanecarboxylates (X as above) are elucidated. Arrhenius parameters for some of the reactions are discussed.