completion points to the extreme inertness of the hydrated Rh(II1) ion. In fact, if these solutions were merely heated to 100 “C, and not boiled, there would be very little color development. This may be the reason for the fact that Nath and Agarwal did not observe the effect of continued boiling upon formation of the Rh(II1)-nitroso R complex. Nath and Agarwal used a Klett-Summerson colorimeter with filter number 54 (which transmits in the 520- to 580-nm region), so their study and ours were performed in essentially the same spectral region. Their source of rhodium(II1) was its chloride (obtained from JM) which was assayed gravimetrically according to Scott (4). Our solution studies show that the Rh(II1)-nitroso R complex has a ligand to metal ratio of 3 t o 1. This is depicted in Figure 3 where it is also evident that the complex dissociates appreciably at room temperature. The formula for the complex is also substantiated through the isolation and analysis of its sodium derivative. I n a potentiometric titration of this (4) W. W. Scott, “Standard Methods of Chemical Analysis,” D. Van Nostrand Co., New York, N. Y., 1964, Vol. I, p 746.
derivative, we found no evidence for acidic hydrogen atoms which shows that the nitroso R is coordinating through the oxygen atoms of its naphthol group, a fact that is also supported by the sodium analysis. All of this evidence suggests that the nitroso R is functioning as a bidentate ligand in accord with the not unexpected coordination number of six for Rh(II1). The amount of sodium sulfate which crystallized with the Rh(II1) complex amounts to approximately a 10% “contamination” by weight in the final product. The Rh(II1)-nitroso R complex is so stable that solutions of it show no change in absorbance on standing for 24 hours. ACKNOWLEDGMENT We thank Dr. Curtis D. Herron of Matthey Bishop, Inc. for the standard rhodium sulfate solution which was used in this research.
RECEIVED for review July 27, 1970. Accepted October 26, 1970. We are grateful to the United States Naval Academy Research Council for financial support for this work.
Determination of Polymeric Isocyanate in the Presence of Reactive Halides Phillip M. Beazley Marathon Oil Company, Denver Research Center, Littleton, Colo. THEISOCYANATE CONTENT of polymers derived from reactive halide intermediates ( I ) can not be determined by established amine titration procedures (2-4) because the residual halides react with the amines used. Kinetic studies showed that dicyclohexylamine reacts slowly with reactive halides and quickly with isocyanate. A titration technique using this amine was then developed. Excess dicyclohexylamine is reacted with the isocyanate in dimethylformamide, D M F , for a prescribed time at room temperature. The excess amine is then titrated with HCl in isopropanol using bromcresol green indicator. Because the total amount of amine taken is known, the amine consumed by the sample is calculated by difference and is equivalent to the isocyanate content of the sample.
EXPERIMENTAL Reagents. STANDARD 0.1N HCI. Dilute 9 ml of concentrated HCl to 1 liter with reagent grade isopropanol. Standardize this solution with tris-hydroxymethyl aminomethane using methyl red indicator. DICYCLOHEXYLAMINE. Dilute 50 ml of dicyclohexylamine, Eastman White Label 4627, t o 1 liter with dry D M F ( 5 ) . (1) P. A. Argabright, V. L. Sinkey, and B. L. Phillips, U. S . Patent
3,458,448 (1969).
(2) Sidney Siggia, “Quantitative Organic Analysis via Functional Groups,” 3rd ed., John Wiley and Sons, New York, N. Y., 1963, p 558. (3) R. Venkataraghavan and C . N. R. Rao, Chemist-Analyst, 51, 49 (1962). (4) Joe A. Vinson, ANAL.CHEM., 41, 1661 (1969). ( 5 ) E. Lieber, C. N. R. Rao, and T. S . Chao, ibid., 29,932 (1957). 148
e
BROMCRESOL GREEN INDICATOR. Slurry 100 mg bromcresol green, Matheson, Coleman and Bell, Inc., with 1.5 ml of 1 N N a O H and dilute t o 100 ml with distilled water after all the indicator has dissolved in the hydroxide. Procedure. Dissolve the sample containing about 1 mequiv of isocyanate in 5 ml of dry dimethylformamide. Pipet 10 ml of amine solution into the sample and after 2 minutes, add 40 ml of isopropanol and 8 drops of indicator. Titrate the unreacted amine with 0.1NHCI until the indicator remains yellow for at least 15 seconds. R u n a blank to determine the total amount of amine taken and calculate the equivalent isocyanate content of the sample by subtracting the amount of unreacted amine from the total amount of amine taken. RESULTS AND DISCUSSION The reaction rates of benzyl bromide and o-tolylisocyanate with piperidine, dibutylamine, dicyclohexylamine, and diisopropylamine were compared. Equal portions of benzyl bromide in D M F were added to equal amounts of amine in D M F at room temperature. After 5 minutes, the reactions were quenched and the unreacted amines were titrated with HC1. The following results were obtained. % Benzyl bromine Amine reacted Piperidine 96 84 Dibutylamine Diisopropylamine 10 Dicyclohexylamine 6 I n the same way, the reactions of o-tolylisocyanate with piperidine and dicyclohexylamine were compared.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
% Isocyanate reacted
Amine Piperidine Dicyclohexylamine
no C1. Product B contained 0.06 mequiv/gram Br and 0.11 mequiv/gram C1 based upon elemental analyses.
97.7 97.0
Amine A mixture of 61.9 mole % o-tolylisocyanate and 38.1 mole per cent benzyl bromide was next reacted with the same amines in the same way for various times. Both piperidine and dibutylamine reacted readily with the benzyl bromide but dicyclohexylamine reacted with very little of the bromide. Mole % mixture reacted DicyclohexylPiperidine Dibutylamine amine
Time, min
94.4 95.4 98.1
1 2
4
68.7 71.3 79.1
60.6 61.4 62.2
Piperidine Dicyclohexylamine
B
x
Time, min
Mequivlgram RNCO
1 2 4
5.88 6.05 6.04
From these data, it is evident that 2 minutes was sufficient time for dicyclohexylamine to react with all the isocyanate in the sample and yet not with a significant amount of benzylic halides. To further test the applicability of dicyclohexylamine, the same product sample was spiked with 1.53 mequiv/gram of benzyl bromide, which represents the most reactive halide the product contains, and reacted with the amine for 2 minutes. The determined isocyanate content was 6.07 mequiv/ gram. The error caused by this large amount of benzyl bromide was only 0.3 %. Since the determined standard deviation of the method is 0 . 2 x , the error caused by the bromide is nearly insignificant. Two products containing different quantities of chlorine and bromine were analyzed using piperidine and dicyclohexylamine. Product A contained 0.33 mequiv/gram Br and
6.20 5.90
5.66 5.45
I n each case, the result with piperidine was higher than the result with dicyclohexylamine by an amount nearly equal to the halide content of the products.
Product A
A sample of polyisocyanate product (1) which contained 0.40 C1 and 0.47 Br (probably in the form of metal halides and benzylic halides) was reacted with dicyclohexylamine for various times.
Mequiv/gram RNCO A B
Br C1, mequiv/gram
Piperidinedicyclohexylamine, mequiv/gram
0.33 0.17
0.30 0.21
+
Finally, several isocyanates of diverse reactivity were assayed as received from Aldrich Chemical Co. using this procedure with piperidine and dicyclohexylamine.
Compound 2,4-Toluene diisocyanate Phenyl isocyanate Allyl isocyanate n-Buty lisocyanate Cyclohexyl isocyanate
Isocyanate found. 'Z DicyclohexylPiperidine amine 99.7 99.2 97.0 98.2 96.3
99.4 98.9 97.2 87.4 91.2
I t is evident that only cyclohexyl isocyanate and n-butylisocyanate did not react completely with dicyclohexylamine within the 2 minutes allowed. This method is, then, applicable to isocyanate-halide mixtures providing that the halides are no more reactive than benzyl bromide and the isocyanates are at least as reactive as allylisocyanate. ACKNOWLEDGMENT The author is grateful for the samples supplied by B. Phillips, and the helpful discussions with P. A. Argabright.
RECEIVED for review July 30, 1970. Accepted October 27, 1970.
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