Determination of Polymer-Supported Carbodiimides Waldemar Adam" and Faris Yany Department of Chemistry, University of Puerto Rico, R b Piedras, Puerto Rico 0093 1
During the course of our work on dehydrative cyclizations of a-hydroperoxy and a-hydroxy acids by polymer-supported carbodiimides 1, we required a convenient and reliable method 0:
~~
Table 1. Active Cardodiimide Content Carbodiimide
R = i-Pr
eH2-N=C=N-R
I
R=t-W
for determining the active carbodiimide content in 1. Previously the conversion of glacial acetic acid into acetic anhydride by I and GLPC analysis of the anhydride formed (1)was utilized, but we found this method tedious. As early as 1899 Dains (2) observed that simple carbodiimides evolve quantitatively carbon dioxide and monoxide when treated with oxalic acid according to the stoichiometry shown in Equation 1.
This method was employed in the quantitative analysis (3) of simple carbodiimides; but the determinations involved cumbersome techniques, such as chemical trapping ( 4 ) or GLPC analysis (5)of the gases produced or back-titration with standardized sodium methoxide ( 5 ) . Presently we report a convenient and reliable method for the quantitation of active carbodiimide in 1, consisting of treatment of 1 with oxalic acid and back-titration of the excess acid with standardized sodium hydroxide. The results are summarized in Table I. Within an experimental error of better than 5%, our simple oxalic acid method results match those of the more involved acetic anhydride method. Furthermore, the oxalic acid method also gives reliable results for unstable carbodiimides such as the dimethyl derivative, which can be handled only as solutions in inert solvents such as CHzClz since in its pure state the dimethylcarbodiimide readily polymerizes (6).The experimental details for the quantitation of the polymer-based carbodiimides are detailed below. Oxalic Acid Method. A 25-mL, 2-necked, round bottom flask, fitted with magnetic spinbar, glass stopper, and 3-way stopcock,was charged with 0.5 g finely powdered carbodiimide resin and 0.32 g (3.52 mmol) of oxalic acid. The flask was flushed with dry N2 gas, charged with 10 mL dry CHZCl2 (analytical grade), and allowed to stir at room temperature under a N2 atmosphere for 30 h. The optimum reaction times were determined by following the disappearance of the N=C=N Ir absorption at 2140 cm-l. Longer reaction times
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
Acetic anhydride method"
Dicyclohexyl
4.70 f 0 . 2 0 b
laa,c
3.65 f 0.15 2.55 f 0.10
Oxalic acid method 4.60 f 0.20 3.60 f 0.15 2.52 f 0.10
lbd * Ref. 1. Maximum content of pure carbodiimide is 4.85 mmol/g, based on mol wt of 206 g/mol. c Purchased from Dynapol, 1454 Page Mill Road, Palo Alto, Calif. 94304. Prepared by dehydrosulfurization of the corresponding polystyrene-bound thiourea (7). were required for the polymer-supported carbodiimides 1 because these polymeric reagents are insoluble in organic solvents, resulting in heterogeneous reaction mixtures. The solvent was carefully removed by evaporation a t reduced pressure (ca. 30 mm), the residue suspended in 30 mL distilled water and titrated with 0.1 N NaOH, using phenolphthalein as indicator. The mmol of active carbodiimide contents per 1.0 g of substrate are given in Table I. The dicyclohexylcarbodiimide used as control was back-titrated after ca. 100-min reaction time with oxalic acid. ACKNOWLEDGMENT We thank M. Weinshenker and G. A. Crosby of Dynapol for supplying us with samples of the polymer-supported carbodiimide la and detailed procedures for its synthesis. LITERATURE CITED N. M. Weinshenker and C. M. Shen, Tetrahedron Lett., 3281 (1972). F. Dains, J. Am. Chem. Soc., 21, 136 (1899). F. Kurzer and K. Douraghi-Zadeh, Chem. Rev., 67, 107 (1967). F. Zetsche and A. Fredrich, Chem. Ber., 72, 363 (1939). J. Zerembo and M. Wall, Microchem. J. Symp. Ser., 2, 591 (1962). G. Rapi and G. Sbrana, J. Chem Soc., Chem. Commun., 128 (1968). (7) R. Appel, R. Kleinstuck and K. D. Ziehn, Chem. Ber., 104, 1335 (1975).
(1) (2) (3) (4) (5) (6)
RECEIVEDfor review November 29,1976. Accepted January 13,1977. This work was supported by the Donors of The Petroleum Research Fund (Grant 8431-AC-1,4),administered by the American Chemical Society, the National Science Foundation (Grant CHE 72-04956-A03),and the National Institutes of Health (Grants GM-22119-02and RR-8102-03). W.A. is recipient of an NIH Career Development Award for the period 1975-80 (GM 00141-02).
