5828
COMMUNICATIONS TO
THE
Thus the acid-dissociation of this type of compound may be explained on the basis of either “protonated” or “half-bond” intermediates.
EDITOR
Vol. 79
TABLE I VALUESOF K AND kaaSb kr
K ,M M - 1 , set.-' 680 30 DEPARTMENT OF CHEMISTRY EDWARD A. HEALY Benzoylglycine methyl ester 7.90 25 f 2 THEUNIVERSITY OF CONNECTICUT 67Ozk30 8.40 25 f 2 STORRS, CONNECTICUT R . KENTMURMANN Acetyl-L-valine methyl ester 7.90 2 . 8 f 0 . 1 7 . 2 i 0 . 2 RECEIVED JUNE 27, 1957 In aqueous solutions a t 25.0” and containing calcium chloride. * Constants evaluated from a minimum of eight experiments conducted under conditions where the conizI and THE CALCIUM CHLORIDE AND HYDROXYL ION centration of ester was varied from 5 to 40 X CATALYZED HYDROLYSIS OF SEVERAL ACYLATED that of calcium chloride from 0.6 to 3.0 $1. a-AMINO ACID ESTERS Ester
fiH
(I
Sir: The availability of a pH-Stat1r2has permitted us to determine the base catalyzed hydrolysis of several acylated a-amino acid esters, to the acylated a-amino acid and alcohol, in the presence of calcium chloride and in the absence of conventional buffers. Under these conditions i t has been found that the initial rates of hydrolysis are first order with respect to ester and hydroxyl ion but decrease from first order with respect to calcium chloride as the mean ion activity of the latter species is increased. From the reactions S
+M
ki
+
ks
+
MS; MS OH--+ M kb ka P; S OHP, it follows that v‘ = d[P]/dt = k 3 [ M S ][OH-] 4- k4[S] [OH-]. Experiments conducted under conditions where [MI >> [MS], when [MI is the mean ion activity of calcium chloride, allowed us to establish the relationship v’ = k,[MS][OH-] K~[u][OH-] - kq[MS][OH-1, when 1.1 = [SI [hlS]. Assuming a steady state process for MS, it follows that v’ - a,[.]. [OH-] = ZI = { ( k 3 - K~)[OH-][M][U])/{ [MI ((kl k~[OH-])/kl)]. Since k4[u][OH-] was determinable and the reactions were first order with respect to hydroxyl ion, k , >> &[OH-] and the second term in the denominator of the preceeding equation may be taken as the equilibrium constant K = k 2 / k l = [M][S]/[MS]. Setting k3’ = k3IOH-1, k’4 = k4[OH-] and k3” = (k3’ k4’) = (k3 - k4)[OH-] i t follows that [M]/(v/ K/&” and a plot of the left [u]) = ([M]/k3“) hand term vs. [MI will give a line of slope 1/k3” and intercept K/k3”. Such linear relationships were observed and led to the values of K and k3 given in Table I. The constancy of K , for benzoylglycine methyl ester a t pH 7.90 and 8.40, confirms the conclusion that k, >> &[OH-] and K =
+
.-t
+ +
+
+
+
k2/k1.
While calcium chloride acting in conjunction with hydroxyl ion is not a very efficient catalyst for the hydrolysis of acylated a-amino acid esters, it is just this property that permits the separate evaluation of K and k3. I n addition the use of a pH-Stat1t2 has avoided complications arising from specific buffer effects such as those encountered by Bender and Turnquest4 in their studies on the cupric ion catalyzed hydrolysis of several a-amino acid esters, a reaction examined earlier by Kroll.6 (1) C . F. Jacobsen and J. Leonis, Compt. rewd. tvav. lab. Cavlsberg, SPY.C h i m . , 27, 333 (1951). ( 2 ) J. B. Nielands and M. D. Cannon, A n d . Chrm., 27, 29 (1953). 1 3 ) I
