Kinetics and mechanism of hydrolysis of lactams in aqueous H₂S0₄ solutions

Conclusions

• 1. A study has been made of the kinetic relationships in the hydrolysis ofγ-butyrolactam,δ-valerolactam,ε-caprolactam, and enantholactam in 0.5–65% H₂SO₄ at 59.5–94°C, and an analysis has been made of the literature data for the hydrolysis ofβ-,γ-, δ-, andε-lactams and N-ethylacetamide in H₂SO₄, solutions.

• 2. The influence of the medium on the rate constants ofγ-, δ-, ε-, and enantholactams and N-ethylacetamide can be explained quantitatively on the assumption that the hydrolysis proceeds along two pathways. The reaction rates are determined as follows. Along the first pathway, the rate is determined by the product of concentratrations of the un-ionized form of the reagent and H₅0₂ ⁺ ions. Along the second pathway, the rate is determined by the concentration of the complex BH₃0⁺, which consists of the form of the lactam that is protonated at the carbonyl group and a water molecule coordinated on the carbon atom of the carbonyl group.

• 3. Values have been calculated for the rate constants determining the contributions of the individual pathways to the experimentally determined rate constant.

• 4. In the hydrolysis ofβ-propiolactam, the limiting stage is the decomposition of the complex consisting of the un-ionized form of the lactam (B) and the ion pair H₃0⁺·H₂SO₄ ⁻. The concentration of such a complex is determined by the product of the concentration of (B) and the thermodynamic activities of the acid and water. Under conditions of hydrolysis, the relative concentration of such a complex is small.