Equilibrium and Kinetics of Glyconitrile Formation in Aqueous Solution

Abstract

The equilibrium constant (Ka) for the dissociation of glyconitrile to formaldehyde and hydrogen cyanide in aqueous solution was measured relative to the known dissociation constant of lactonitrile. Over the temperature range 25-70° log KG = 1.66 — 2184/7' (KG = 2.1 X 10-6Mat25°). The enthalpy of dissociation was found by calorimetry to be +9.96 kcal mol-1. The rate of addition of hydrogen cyanide to formaldehyde in dilute, acetate-buffered (pH 3.8-4.7) aqueous solution was measured between 25 and 45°. In this pH range the reaction is first order in both CN- and unhydrated formaldehyde (at 25° k2 = 3.5 X 105 M 1 sec-1, and ΔH≠ = 5.02 kcal mol-1). No general acid or general base catalysis was observed. In aqueous solution at 25° the dissociation constant for mandelonitrile was found to be 5.2 X 10-3 M, and the second-order rate constant for the addition of CN-to benzaldehyde was measured to be 1.13 X 102 M-1 sec-1. The equilibrium and rate constants for each of the steps in the formation and dissociation of the cyanohydnns of formaldehyde, acetaldehyde, acetone, and benzaldehyde are evaluated. A plot of log K' (dissociation of the cyanohydrin to the unhydrated aldehyde) vs. ∗ gives a straight line, as do plots of log k2 and log k-2 vs. ∗, except for mandelonitrile where the deviations are attributed to a resonanee effect. The prebiotic significance of the reaction between formaldehyde and hydrogen cyanide is discussed. © 1973, American Chemical Society. All rights reserved.