ponents are inherently unresolvable, and the effective resolution is a consequence of the LD and birefringence. The resolved band peaks are shifted slightly from the centers of the true Gaussian components. A definite shift of the apparent CD peak away from a near-by LD band is observed in Figure 3. Both red and blue shifts can be obtained through this mechanism, and it is evident that only the isolated C D band (with a concentric LD band) will in general be unshifted. Figure 4 shows a more complex spectrum having three Gaussian C D and concentric LD components. Here underestimation and both red and blue shifts are observed. The assumption that the angle a = 45 degrees is reasonable, because it is possible to adjust this angle accurately. From Equations 9-1 1, only the coefficient C of Eqtion 5 is dependent upon x when CY = a/4, and calculations show that the C D is most severely underestimated when x = a/4, and its peak value may be as little as 80% of that at x = 0 or a/2. The general characteristics of apparent C D spectra are only slightly dependent upon X. When a # a14, the X-dependence derives entirely from effects upon the denominator in Equation 5.
*
Equations 5 and 10 show clearly that an optically inactive specimen, no matter how anisotropic, can give rise to no apparent C D bands of any kind when this method is used. A rather weak spurious band resembling a classical Cotton effect for optical rotation can occur, nonetheless, when there is a strong LD band and rather feeble optical activity such as that found well away from a CD band. The long wavelength end of the apparent CD plot of Figure 3 shows such a small spurious band (in effect, two bands, since there is a sign reversal), and the effect persists even for values of Wz a good deal smaller than those in the neighborhood of this CD band. The appearance of such a small spurious band can rather safely be interpreted as arising from an isolated LD band in an optically active specimen, and a confirmation of this interpretation may be obtained by separate measurement of LD. RECEIVED for review September 16, 1968. Accepted November 6, 1968. Work supported in part by the U. S. Army Research Office, Durham, under Contract DA-31-124-ARO-D305 and in part by the Joint Services Electronics Program under Contract DA-28-043 AMC-O0099(E). D. I. Sverdlik is a National Institutes of Health Predoctoral Fellow.
Imidazole Catalysis of Organic Hydroxyl Group Determination by Reaction with Pyromellitic Dianhydride in Dimethylformamide B. H. M. Kingston,' J. J. Garey, and W. B. Hellwig General Electric, Silicone Products Department, Waterford, N . Y . Imidazole is superior to pyridine, other substituted imidazoles and pyridines, and several tertiary amines in the catalysis of the reaction between pyromellitic dianhydride and organic hydroxyl groups. In dimethylformamide solution, quantitative reaction occurs within 5 minutes at room temperature or at 70-80 O C depending on reagent concentration. Hydrolysis of the excess anhydride is instantaneous and gives a colorless solution. Visual titration is carried out with either thymol blue or phenolphthalein. The overall precision for hydroxyl number determination is +0.3%. Imidazole does not catalyze the reaction of aldehydes and phenols with pyromellitic dianhydride; however, it does cause partial reaction of tertiary alcohols and alkoxysilanes. Infrared and NMR evidence indicates the formation of an intermediate which reacts with the alcohol to give the ester product. Two possible mechanisms for the catalysis are proposed.
USEOF PYROMELLITIC DIANHYDRIDE (PMDA) as a reagent for the determination of alcohols and amines in the presence of phenols was first demonstrated by Siggia, Hanna, and Culmo ( I ) . A sample was reacted with a PMDA-tetrahydrofuran (THF) solution in the presence of pyridine on a steam bath. Subsequently, Harper, Siggia, and Hanna (2) modified the procedure by dissolving the PMDA in dimethylsulfoxide (DMSO), adding pyridine and then running the reaction at 100 "C for 15 to 30 min. 1
Present address, 13 Coronet Court, Schenectady, N. Y . 12309
(1) S. Siggia, J. G. Hanna, and R. Culmo, ANAL.CHEM.,33, 900 (1961). (2) R. Harper, S. Siggia, and J. G. Hanna, ibid., 37,600 (1965).
86
ANALYTICAL CHEMISTRY
Other common procedures for determination of hydroxyl groups involve acetylation ( 3 , 4 ) ,and phthalation (3, but each has its disadvantages. Acetic anhydride reacts rapidly at room temperature but suffers interference from aldehydes, while phthalic anhydride requires heating for several hours. Review of these and other procedures for hydroxyl number determination have been written by Mathur (6) and more recently by Smith, Wagner, and Patterson (7). The work described within this paper uses PMDA in dimethylformamide (DMF) with imidazole (IMDA) as the catalyst. In contrast to the methods cited above, determination of alcohols by IMDA catalysis offers the combined advantages of a stable reagent solution, rapid reaction, instantaneous hydrolysis of excess PMDA, a colorless titration solution, a sharp visual endpoint with either thymol blue or phenolphthalein, and excellent precision. With a molar ratio of PMDA to organic hydroxyl group of at least 2.5 (thet used by Siggia, et a / . ) (1) and a ratio of PMDA to IMDA of 1.1) 0.4, quantitative recovery of cyclohexanol (100.4 occurs at room temperature in five minutes. However, if the ratio of PMDA to organic hydroxyl group is reduced to 1.2, precision improves considerably while quantitative reaction requires heating at 70-80 "C for 5 minutes. Imidazole does not effect the reactivity of PMDA with aldehydes and phenols as previously reported ( I ) and confirmed by several deter(3) J. S. Fritz and G. Schenk, ANAL.CHEM., 31, 1808 (1959). (4) J. A. Magnuson and R. J. Cerri, ibid., 38, 1088 (1966). (5) P. J. Elving and B. Warshowski, ibid., 19, 1006 (1947). (6) N. K. Mathur, Tulunta, 13, 1601 (1966). (7) W. T. Smith, N. F. Wagner, and J. M. Patterson, ANAL.CHEM., 40, 413 (1968).
minations in this system. Tertiary alcohols react to about 60% completion; alkoxysilanes react partially. EXPERIMENTAL
Reagent I. Pyromellitic dianhydride, 0.19M. PMDA (Eastman Chemical, practical grade) was heated at 160 "C for 24 hours and then 41.0 grams were dissolved in 900 ml of Baker reagent grade DMF. After liZ hour of stirring, IMDA (Eastman Chemical, White Label), 21.0 grams, were added along with enough D M F to bring the solution to 1000 ml. Storage of the PMDA-DMF solution for at least one month proved possible if the IMDA was not added until the time of use. Procedure I. Fifty ml of 0.19M PMDA-IMDA solution were pipetted into a glass-stoppered 500-ml erlenmeyer flask. A sample containing 0.007 to 0.008 equivalent of organic hydroxyl was weighed and added to the reagent. The flask was stoppered, swirled, and placed in a hot water bath at 70-80 "C for 5 minutes. A 150-ml portion of distilled water and 5 drops of 2% Thymol Blue (DMF) were added. The mixture was titrated with 0.5N KOH until thymol blue gave no further color change. A blank was carried through the same procedure. Reagent 11. Pyromellitic dianhydride, 0.26M. Dried PMDA, 60 grams, was dissolved in 900 ml of DMF. After hour of stirring, IMDA, 54 grams, was added, followed by enough D M F to bring the solution to 1000 rnl. Procedure 11. Procedure I was followed with these exceptions: a sample contained 0.004 to 0.005 equivalent of organic hydroxyl; reaction time was 5 minutes at room temperature. Comparison of Solvents and Catalysts. Pyromellitic dianhydride displays a rather limited range of solubility. Among these solvents which will dissolve enough PMDA to make at least a 0.2M solution are pyridine (Py), dimethylsulfoxide (DMSO), dimethylformamide (DMF), and tetrahydrofuran (THF). Table I illustrates the relative reactivity of PMDA in the first three. The reactions were run for five minutes at room temperature (26.5 "C) with 7 to 11 mmol of cyclohexanol. Table 11 describes the catalytic effect of various amines, imidazoles, and pyridine on the reaction rate of PMDA in Table I. Comparison of % Recovery of Cyclohexanol in Reaction with PMDA in Various Solvent Systems % Recovery Solvent [PMDA]/[ROH] (5 min at RT) 1.9 81.7 Pyridine Dimethylformamide 1.2 2.3 Dimethylsulfoxide 1.8 7.2
D M F solution. By use of the 15-minute reaction figures of Table I1 as a reference point, IMDA appears to be at least twice as effective and at most approximately fifty timis as effective a catalyst as the other amines studied. Pyridine displays about one tenth the efficiency of IMDA, whereas, 2,6-lutidine is about one fifth as effective. Apparently, electron donation from the methyl groups of lutidine seems to more than counterbalance any steric factor. All of the substituted imidazoles show less reactivity than IMDA itself. Surprisingly, the location of methyl substitution makes little difference in the extent of reaction after the first fifteen minutes. Reagent Concentration and Preparation. The concentration of PMDA recommended in procedure I is primarily based on the accuracy afforded by a 50-ml aliquot of reagent, a 14- to 16-ml titer between blank and sample and the convenience of titrating with a 100-ml buret. A higher ratio of PMDA to organic hydroxyl group does allow for a 5min reaction at room temperature but also reduces precision. The average of five determinations with a molar ratio of 2.4 to 4.2 PMDA to cyclohexanol was 100.4% i. 1.1%. In contrast Table I11 presents data obtained from the analysis of various alcohols where the ratio of PMDA to ROH was approximately 1.3. Phenolphthalein was the indicator and the reaction was run at 75-80 "C for at least 5 min. Further experimentation showed that precision can be increased slightly to the choice of a different indicator and by care in the preparation and storage of the reagent. Simultaneous addition of PhlDA and IMDA to D M F results in the formation of a fine white precipitate which was identified by IR and elemental analysis as the monoanhydride-disalt of PMDA and IMDA. However, if the PMDA and D M F are allowed to stir for one half hour before the addition of IMDA, the final solution remains clear. The intensity of the reagent color seems somewhat dependent on its exposure to air. A nitrogen cover maintains an extremely pale yellow PMDA-DMF solution which darkens only slightly upon the addition of IMDA. The admission of air to the latter solution leads to a gradual deepening of color to a red-brown over a four-day period. Hydrolysis of the Nz blanketed reagent gives an almost colorless solution whereas the hydrolysis product of the other solution remains a definite yellow. The improvement in precision caused by the combination of a virtually colorless hydrolyzate and the use of thymol blue is indicated in Table IV. The ideal ratio of PMDA to IMDA to ROH for maximum rate of reaction was investigated by the following experiments. A series of reactions was run in which the molar ratio PMDA to alcohol remained constant while the IMDA concentration was increased. The results showed a rapid rise in reaction rate (measured by titration after 5-minute reaction at 26.5 'C) until the PMDA to IMDA ratio reached 0.5. After this point, the rate of increase dropped significantly. A second set of experiments was carried out in which the ratio of PMDA
Table 11. The Effect of Various Catalysts on the Reaction of PMDA in DMF with Cyclohexanol 70Recoverya Catalyst Moles catalyst Moles PMDA [PMDA]/[ROH] 5 min 15 min 0.002 0.013 2.03 20.5 49.2 Imidazole 26.3 2-Methyl imidazole 0.002 0.013 2.05 10.1 0.002 0.013 1.95 6.0 25.9 N-Methyl imidazole 0.002 0.013 2.17 6.0 7.0 Triphenyl imidazole Quinuclidine 0.002 0.013 2.14 1.o 1.0 0.002 0.013 2.01 1.o 7.3 Triethylamine 0.002 0.011 2.00 2.8 3.1 Pyrroleb 0.002 1.92 2.4 4.8 0.013 Pyridine 0.002 0.013 1.97 4.2 11.6 2,6-Lutidine a
b
45 min 78.6 61.3 63.2 14.6 3.9 20.8 5.0 18.2 36.4
Each figure represents the average value of two determinations. Average deviation &3.0%. Reactions run at room temperature. Pyrrole forms a salt with the hydrolyzed anhydride which does not titrate as an acid with KOH in this system.
VOL. 41, NO. 1, JANUARY 1969
87
I
Table 111. Determination of Alcohols with Reagent I Alcohol Purity No. of detn Cyclohexanol 100.2 f 0 . 6 8 99.4 + 0 . 2 6 I-Butanol 2-Propanol 99.8 i 0.5 6 Benzyl alcohol 99.4 i 0.3 5 Allyl alcohol 98.4 i 0.3 5 1,3-Butanediol 100.3 i 0.3 5 1,3-Propanediol 99.8 i 0 . 2 5
0.20
g 0.30 4
0.40 ln
0.70 1.0
Figure 1. Infrared spectrum Left: PMDA, 0.5M DMSO. Right: Complex I, 0.5M DMSO
0
Figure 3. Mechanism of imidazole catalysis by formation of Complex I
C'
0
Indicator Phenolphthalein Thymol blue Either
88
C-N II
droxyl number determination. 1,2-Propanediol with 1heptanal gave 99.6 % recovery. DISCUSSION
Discussion of Reaction Mechanism. The reaction mechanism of imidazole as a catalyst in the PMDA-ROH system is not completely straightforward. Both NMR and IR
0
0
0
0
II C-OH
0
0
Table IV. Comparison of Precision of Several Procedures in Determination of Cyclohexanol PMDA Concentration, M Atmosphere Temperature, "C Time, min Purity 0.19 Air 75-80 15 100.2 rt 0 . 6 75-80 5 100.1 f 0 . 3 0.19 Nitrogen 26 5 100.4 f 1 . 1 0.26 Nitrogen
ANALYTICAL CHEMISTRY
I
4
0 Figure 2. Complex I
c-0
R
0 1 I
I1
0
II
5 6 WAVELENGTH (MICRONS)
5 6 7 WAV E LENGTH (MICRONS)
to IMDA was held constant at 0.45 and the PMDA to alcocol ratio was gradually increased from approximately 0.5 to 2.0. The reaction rate rose rapidly until the latter ratio was just over 1.0 and then leveled off. Thus, ideal reaction conditions seem to require a minimum molar ratio of PMDA to IMDA to ROH of 1:2:0.8. The molar ratio suggested in procedureI is 1:1.6:0.7andinprocedureTI,1:2.9:0.3. Scope of Reaction. Since quantitative reaction of hydroxyl groups with PMDA-IMDA in D M F was found to occur for a variety of commercial polyols as well as pure alcohols (Tables 111 and V), a limited study of the reactivity of alkoxysilanes toward several PMDA-IMDA reagents was pursued. Both Reagent I and Reagent IIa at room temperature (Table VI) indicated the possibility of determining hydroxyl numbers of polyols in the presence of alkoxysilanes. Unfortunately, these reagents did not react quantitatively with alcohols in mixtures of pure alcohol and alkoxysilane. Further, those conditions of reagent concentration and temperature needed for quantitative hydroxyl group determination gave rise to a 2-50 % reaction of the alkoxysilane. Experimentation with other types of compounds showed that phenols react less than 0.3 %, tertiary butyl alcohol reacts to about 60%, and aldehydes do not interfere in hy-
0
0.50 0.60
No. of detn 8 7 5
0
A
N t R'OH
HN
\=/
A
0 II
II
- 0 0
HN
wN H .... O R ' +
0
0 II
II
--m
0
0 II
, c ~ c - o H
+ HNwNH
4- N0NH LJ
C - OR! II II 0 0 Figure 4. Imidazole catalysis by proton transfer from ROH to IMDA C'
~~
Table V.
Determination of Hydroxyl Numbers of Polyols by Procedure I Poly01 Hydroxyl number No. of detn Niax Poly01 PPG-425 258.4 =t0.1 2 3 2 . 2 ?r 0.3 6 Jefferson Poly01 WL-1700 56.2 =k 0.7 4 Wyandotte-Pluracol GP 3030 12.7 j= 0.2 4 Jefferson Poly01 WL 4280 analyses confirm the formatior. of some kind of complex (I) between IMDA and PMDA in the presence of either DMSO or DMF; however its structure is not entirely clear. The infrared spectrum of Complex I gives bands at 6.95 p and 6.25 p which seem to correspond well to the bands at 6.10 p and 6.23 p of the reaction product of PMDA and di-n-butylamine. Unlike the latter, Complex I also maintains some bands in the anhydride region, although these differ from the PMDA anhydride bands (Figure 1). The possibility that IMDA has reacted to form an acid and an amide by proton transfer to PMDA is also supported by NMR data. The N-proton of IMDA gives a peak at 12.70 ppm downfield from tetramethylsilane, whereas Complex I absorbs at 14.50 pm. The aromatic protons of PMDA shift from 8.78 ppm to 8.62 ppm in Complex I. The remaining protons on IMDA all shift downfield approximately 0.20 ppm in Complex I. Such spectral information strongly suggests that Complex I is that reaction product of PMDA and IMDA which contains one unreacted anhydride group, an acid and an amide (Figure 2). The mechanism by which IMDA catalyzes the addition of an alcohol to PMDA is believed to proceed according to the following pattern (8, 9) (Figure 3).
Table VI.
Methyltriethoxysilane Phenyltriethoxysilane Dimethyldiethoxysilane
Comparison of Reaction Rate of
+ ROH with Premix PMDA + IMDA PMDA I
Recovery after 5 min at RT PMDA ROH ROH IMDA PMDA IMDA
+ IMDA I1
+ ROH 11
75.2x 1.74
42.8%
72.1% 1.91
54.5%
1.74
0.50
1.23
2.79
3.26
0.86
2.13
5.33
6.35
1.95
ACKNOWLEDGMENT Special thanks are due to D.G.I. Kingston for his suggestion of the use of imidazole at the outset of our experimentation. Appreciation is also extended to C . Hirt and W. Wheeler for their assistance with the NMR work. RECEIVED for review July 8, 1968. Accepted September 30, 1968. (8) J. L. Bender, and B. W. Turnquest, J. Amer. Chem. Soc., 79, 1652 (1957). (9) T. C. Bruice, and G. L. Schmir, Zbid., 79,1663 (1957).
Reaction of Alkoxysilanes in Various PMDA-IMDA-DMF Solutions Reagent I Reagent I Reagent I1 70-80 "C for 25 "C for 25 "C for 5 minutes 5 minutes 5 minutes 21.8 42.5
IMDA I
It also seems likely that IMDA serves as a proton acceptor for the alcohol. This is indicated both by the increased reaction rate when IMDA and the alcohol are premixed and then added to PMDA (Table VII) and by the upfield shift in the NMR peak of the N-proton of IMDA from 12.70 to 9.80 ppm when it is combined with cyclohexanol in DMF.
1.2 3.9
29.5 less than 0 . 3 Contains one third the amount of IMDA suggested in the experimental section.
y-Chloropropyldimethoxysilane a
Table VII. Premix IMDA
2.1
Reagent IIQ 25 "C for 5 minutes less than 0 . 3
95.9 10.9 2.0
7.0 less than 0 . 3
1.o
VOL. 41, NO. 1, JANUARY 1969
0
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