Determination of Primary and Secondary Aliphatic A m ines by N ea r-Inf ra red S pec t ro phot o metry FRED H. LOHMAN and WILLIAM E. NORTEMAN, Jr.’ Research and Development Department, Miami Valley laboratories, Procter & Gamble Co., Cincinnati, Ohio
b The developmeni of a method for the determination of primary and secondary aliphatic amines in the presence of tertiary amines by near-infrared spectrophotometry i s described. The determination of primary amines is based on their charocteristic combination absorption band at 2023 mF. Secondary amines cire determined b y their first overtone N-H stretching absorption band at 1538 mp after correction for primcry amine absorption at that wavelength. The mean error of the methoc i s -0.0370 and --o.35y0 (absolute:l for primary and secondary amines, respectively, in the range 1 to 25%. Amides, nitriles, alcohols, and esters up to a concentration of 10% in the amine mixture do not interfere seriously. Aliphaiic aldehydes interfere by reacting with both primary and secondary amines. The effect of the solvent on the spectra of primary amines is explained and the significance of these effects to the analytical method are discussed.
HEMICAL METHO IS for
the separate determination of primary and secondary amines in the presence of tertiary amines are based upon the application of various selective or partially selective masking agents such as salicylaldehyde for primary amines and acetic anhydrid? for primary and secondary amines collectively (4). hfore recently, a method based on the different reaction rates of primary and secondary amines with phenyl isothiocyanate has been described by Hanna and Siggia (W). These methods are not well suited to the determination of small amounts of primary or secondary amines in amine mixtures, and in addition are often rather inconvenient and tedious to carry out accurately. Spectrophotometri: methods based upon near-infrared absorption have been reported for aromatic primary and secondary amines by Whetsel, Roberson, and Krell (5, 6). These same authors studied the effects of various solvents Present address, Department of Chemistry, University of North Carolina, Chapel Hill, N. C.
on the constants A,, and emax, of several typical primary aromatic amines (7, 8). Holman and Edmondson (3) reported the wavelengths of the principal bands of octadecylamine; however, no quantitative study of the near-infrared absorption bands of aliphatic primary and secondary amines and their application to the analysis of amine mixtures has been reported. EXPERIMENTAL
Standard Materials, Reagents, and Apparatus. The amines used as standard materials in this work together with data on their purity are listed in Table I. Hexadecyldimethylamine was obtained by redistilling Armour’s Armeen1GD and collecting the fraction boiling between 116’ and 121’ C. a t 0.4 mm. of Hg. Coconut dimethylamine, Armour’s Armeen CD, 97% minimum purity, was used as supplied. Absorbance measurements were made with a Gary Model 14 spectrophotometer and 10-cm. fused silica cells. Slit conditions and scan settings were those specified in the “Procedure” section below. Procedure. Preparation of Calibration Curves. Primary Amine (choose the primary amine or primary amine mixture appropriate t o the particular amine or amines .to be determined). Prepare a series of standard solutions by weighing within = t O . O O l gram, 0.0, 0.250, 0.400, 0.550, 0.700, and 0.850 gram of primary amine into 50-ml. beakers. Dissolve in about 30 ml. of chloroform and quantitatively transfer them into 50-ml. volumetric flasks. Weigh 5.000
grams of tertiary amine (appropriate to the sample being analyzed) into each flask and dilute to the mark with chloroform. Fill a 10-cm. cell with the solution and scan the spectrum from 2100 mp to 1950 mp and from 1600 mp to 1500 mp using the zero concentration standard as a reference solution. The slit control should be set to give a mechanical slit width of 0.4 mm. a t 2023 mp for 10 cm. of CHC1,. A scan rate of 0.5 mp per second and a chart speed of 1.33 inches per minute should be employed for the scan. During the scan stop the scan (but allow chart to run) for about 20 seconds a t exactly 2023 mp and 1538.5 mp. Calculate the net absorbances a t 2023 and 1538.5 mp as indicated in Figure 3. Plot these values against concentration in grams per 50 ml. to give calibration curves for primary amine a t 2023 and 1538.5 mp. Secondary Amine (choose the standard secondary amine most appropriate to the secondary amine(s) to be determined). Prepare a series of standard solutions by weighing within +0.001 gram, 0.0, 0.100, 0.300, 0.550, 0.800, and 1.000 gram of secondary amine into 50-ml. beakers. Proceed with the preparation of these solutions as described under “Primary Amine.” Fill a 10-cm. cell and scan the spectrum as before from lGO0 mp t o 1500 mp using the zero concentration standard as reference solution. Interrupt the scan for about 20 seconds (one chart division) a t 1538.5 mp. Calculate the net absorbance a t 1538.5 as shown in Figure 3 and plot the absorbances us. concentration of seconday amine (grams per 50 ml.) to give the calibration curve for secondary amine.
Purities of Standard Amines Per cent of amine based on
Material Decylamine 97.3 D ode cylamine 97.9 Tetradecylamine 96.3 Hexadecylamine 97.7 Octadecylamine 90.4 Methylhexadecylamine 100.9 Methyldodecylamine 99.1 Methy 1-n-amylamine 93.6 Gas chromatography on 5-ft., 3.2% Carbowax 20M
Acid titration, %
97.7 99.5+ 98.1 99.5+ 97.9 99.5f 96.9 99.5 96.9 93.6 95.1 98.1 97.7 99.9 90.7 88.9 0.4% KOH on Gas Chrom.-P.
VOL 35, NO. 6, MAY 1963
Analysis of Mixtures. Weigh to kO.001 gram 5.0 to 6.0 grams of the tertiary amine mixture to be analyzed into a 50-ml. beaker. Dissolve in about 30 ml. of chloroform and transfer the solution quantitatively to a 50-ml. volumetric flask and dilute to the mark with chloroform. Scan the spectrum from 2100 mp to 1950 mp and from 1600 mp to 1500 mp using the instrument conditions specified under “Preparation of Calibration Curves,” and using as reference solution a solution of 4.5 to 5.0 grams of the appropriate tertiary amine in 50 ml. of chloroform. Calculate the net absorbances a t 2023 mp and 1538.5 mp as shown in Figure 3. From the net absorbance a t 2023 mp obtain the concentration of primary amine in grams per 50 ml. from the 2023 mp calibration curve. Calculate per cent of primary amine in the sample. Convert the measured concentration of primary amine (grams per 50 ml.) to its equivalent absorbance a t 1538.5 mp using the calibration curve for primary amine a t 1538.5 mp. Subtract this from the measured net absorbance a t 1538.5. Convert this secondary amine absorbance to concentration (grams per 50 ml.) of secondary amine using the calibration curve for secondary amine a t 1538.5 mp. Finally, calculate the per cent of secondary amine.
TETRADECYLAMW in CCI4
TETRADECYLPNINE in Chloroform
Figure 1 Near-infrared spectrum of tetrodecylamine in CHC13 and CC14, each 0.40 gram per 50 ml.
HEXADECYLAMINE in Chloroform
RESULTS AND DISCUSSION
Selection of Solvent. Carbon tetrachloride is the solvent most often suggested for near-infrared spectrophotometry because of its inertness and wide range of transmittance. However, it is not a very good solvent for the long-chain aliphatic amines: further, solutions of primary amine in carbon tetrachloride showed a heavy precipitate after about 2 hours’ standing. In contrast, solutions of the various amines in chloroform were stable for several days. In addition, chloroform solutions offer an advantage in sensitivity for primary amines over other solvents. Figure 1 shows the spectrum of tetradecylamine in both solvents. The combination band for primary amine in carbon tetrachloride (2.0-micron region) is a doublet with maxima a t 1998 and 2020 mp, and molar absorptivities at both wavelengths equal to 0.56 liter per mole em. In contrast, the same band in chloroform is a singlet with A, at 2023 mp and a molar absorptivity of 1.701 liter per mole em. The first S-H overtone bands a t 1527 mp for primary amines in carbon tetrachloride and chloroform are comparable in position and intensity. Spectra of Amines. The spectra of typical primary, secondary, and tertiary aliphatic amines in chloroform are shown in Figure 2. The bands of interest are the K-H combination stretching and bending ab-
WAVE LENGTH (N)
HEXADECYLDIMETHYLAMINE in Chloroform
Figure 2. Near-infrared spectra of typical primary, secondary, and tertiary aliphatic amines in CHC13 Tetradecylamine, 0.4 gram per 50 ml. Hexadecylmethylamine, 0.75 gram per 50 ml. Hexadecyldimethylamine, 0.5 gram per 50 rnl.
sorption a t 2023 mp which is unique n-ith primary amines, and the first overtone symmetric S-H stretching absorption between 1520 and 1540 mp which is eshibited by both primary and secondary amines. Tertiary amines do not eshibit any specific absorption bands in these wavelength regions. Clearly, these spectrz, suggest the possibility of determinicg primary amine alone, without interfwence, a t 2023 mp and secondary amine a t 1538 mp after correction for prirn2.r. amine absorption. The wavelengths 01’maximum absorption and peak molar absorptivities for the combination and the first overtone stretching bands for a number of primary and secondary amines in chloroform are liited in Table 11. The values reported are all based on measurements on three different concentrations of each amine. KOcorrections for purity have been made in the absorptivity valucs since most ms.terials were better than 95%-,pure, and since the absorptivity values will vary somewhat from time to time depending upon the condition of t’he instrummt. Peak absorbance.~w r e measured for the two bands and corrected for background as shown in Figure 3. chlciroform absorption band a t 2060 nip mikes background ahsorbnnce measurerlent’s on that side of the comhination band unreliable, particularly in t,lie presence of tertiary amine. Conformity to Boer’s Law. Primary Amines. The conformity t o Beer’s 1aiT a t the combination absorption a t 2023 mb: was investigated in detail for solutions of tetradecylamine in CHC13. These solutions obey Beer’s 1aFv from the lowest rnrasurable concentration up to a primary amine concentration of 1.0 gram per 50 ml. The presence of 5.0 gram per 50 nil. hexadecj-ldimethylaniine iii these solutions caused no deviation from Beer’s law, although it lowered the molar absorptivity of this absorption slightly. Addition of an equnl anount of tertiary amine to the referecce solution causes a decrease in molar dmrptivity due to an increnwd slit n-idtn, but no deviation from Beer’i Inn-. The first overtone K-H stretching absorption a t 1538.5 mp has also been shown to conform to Beer’s law from the lowest measurzhle concentration up to a concentration of 1.0 gram per 50 ml. Tertiary arrines up to a concentration of 5.0 grarns per 50 ml. cause no deviations from B2er’s law or change in molar nbsorpti Since the general background for amines is quite low in this region, the addition of 5.0 grams per 50 ml. of hesadecyldimethylamine to the reference solution had no effect on the molar absorptivity of primary a,mines a t 1538.5mp. Secondary Amines. The conformity A \
BACKGROUND CORRECTIONS SHOWN FOR A MIXTURE OF PRIMARY AND SECONDARY AMINES
2023 r n ~
Figure 3. Procedure for obtaining background absorption at analytical wavelengths
to Beer’s law a t the first overtone N-H stretching absorption was investigated in detail for hexadecylmethylamine in chloroform. These solutions, too, were found to obey Beer’s law up to a concentration of 1.0 gram per 50 ml. Tertiary amines up to a concentration of 5.0 gram per 50 ml. have no effect on the molar absorptivity a t 1538.5 mp when added to the sample or the reference solution. Additivity of Primary and Secondary Amine Absorbances at 1538.5. Since
primary amines have an absorption band in the 1.5-micron region which interferes to an appreciable extent with the determination of secondary amines a t 1538.5 mp, it is necessary to subtract from the measured absorbance at 1538.5 the absorbance contributed by primary amine. To demonstrate the additivity of primary and secondary amine absorbances, and the absence of any measurable interaction between them in solution, known mixtures of primary and secondary amine were prepared and
Table II. Molar Absorptivities of Wavelengths of Maximum Absorption for Primary and Secondary Amines in Chloroform [Instrument conditions: 10-em. silica cells; slit control 20 (0.4 mm. a t 2023 mp, 0.2 mm. at 1538 mp); slit height, full; scan, 0.5 mp/sec.]
Amine Decylamine Dodecvlamine Tetracfecylamine Hexadecylamine Octadecylamine Butylamine0 Isobutylaminea 2-Ethvlhexvlamine~ 3-Ami”no-l-propanola n-Amylmethylamineb Dodecylmethy lamine Hexadecylmethylamine Di-n-hexylaminea Di-n-dodecylaminea Di-isopropanolaminea a
mp 2025 2023 2023 .
1st overtone symmetric T-H stretching
2023 2023 2023 2021 2022 2019
1.637 . 1,738 1.752 1.624 1.101 1.307
1526 1526 1527 1526 1526 1526 1525 1526 1527 1538 1637 1538 1543 1543 1535
0.743 0 779 0,779 0.764 0.738 0.754 0.721 0.747 0,724 0.682 0.797
0.806 0.576 0.551 0.360
Used as supplied by Eastman Organic Chemicals, Inc.
* Used as supplied by K&K Laboratories, Inc. Table 111.
Additivity of Primary and Secondary Amine Absorptions at 1538.5 mp
Hexadecylmethylamine Concn., g./50 ml. Absorbance 0.400 (0.254) 0.400 (0.254) 0.400 (0.254)
Tetradecylamine Concn ., g./50 ml. Absorbance 0.300 (0.145) 0.400 (0.194) 0.500 (0.240)
Calcd. A1538 0.399 0.448 0.494
A1538 0,397 0.439 0.492
VOL. 35, NO. 6, M A Y 1963
their absorbances measured a t 1538.5 mp. Table I11 shows that the calculated absorbances of these mixtures are in agreement with the measured absorbances within experimental error. Analysis of Standard Mixtures. Standard mixtures of primary, secondary, and tertiary amine were weighed, dissolved, and diluted to exactly 50 ml. with chloroform. All mixtures contained 5.0 grams of tertiary amine and varying amounts of primary and secondary amines. Results on four series of standard mixtures of varying complexity analyzed according to the procedure described above are shown in Table IV. The mean error of these results is -0.03% for primary amine and -0.35% for secondary amine in the range 1 to 25% primary or secondary amine. Interferences. Table V shows a list of organic compounds representing five different classes, and the error, if any, caused by the presence of 10% of that substance in a mixture of amines. The amine mixture employed contained tetradecylamine, amylmethylamine, and coconut dimethylamine in a weight ratio of about 1 : l : l O . Although a slight positive error in % primary amine due to the interference is indicated for each substance, the aldehyde presents the only case of serious interference. Primary amines, of course, react with aldehydes to form the Schiff base, which explains the very low value for primary amine. However, the aldehyde also causes serious negative errors for secondary amine apparently due t o a reaction involving the addition of the secondary amine t o the carbonyl group of the aldehyde to form a tertiary, hydroxyamine (1). The product has not been isolated, but the near-infrared spectrum does show the 1.40-micron 0-H band which is not present in the spectrum of either of the reactants alone. Further, this serious interference is not encountered with the bulkier di-dodecylamine. The spectrum of undecyl aldehyde alone confirms that the observed interference is not spectral in nature. Concerning the indicated error for secondary amine in the presence of the other interferences, a result for primary amine which is in error by +0.30% would cause a negative error for secondary amine equal t o about O . l O ~ o . In consideration of this, there appears to be no error in the determination of secondary amine caused by any of the interferences listed except undecyl aldehyde. Effect of Solvent on the Combination Band. It was remarked earlier t h a t the primary amine combination band is markedly different for CC1, solutions as compared to CHCla solutions (Figure 1). These solvent effects are by no means confined t o
Analysis of Standard Mixtures of Primary, Secondary, and Tertiary Amines
Sample composition Tetradecylamine Hexadecylmethylamine Hexadecyldimethylamine Clo-Cls alkylamines Hexadecylmethylamine Hexadecyldimethylamine C1O-Cl8 alkylamines Hexadecylmethylamine CI0-CJ8alkyldimethylamines Clo-Hl~alkylamines Amylmethylamine Coconut dimethylamine
Primary amine, 70 True Found Error 1.32 1.32 0.0 2.68 2.73 f0.05 5.36 5.42 +O ,06 8.93 8.93 0.0 -0.1 16.4 16.3 11.9 -0.1 12.0 8.58 -0.12 8.70 3.73 +0.03 3.70 1.66 +0.02 1.64 14.2 -1.60 15.8 9.77 + O . l l 9.66 4.76 +0.42 4.34
Secondary amine, % True Found Error 10.5 10.5 0.0 8.03 8.02 -0.01 5.36 5.36 0.0 1.78 1.81 +0.03 1.64 0.87 -0.77 8.00 7.19 -0.81 4.35 3.85 -0.50 21.1 -1.1 22.2 13.1 -3.3 16.4 13.1 12.1 +1.00 2.00 +o. 19 1.81 +o. 10 15.9 15.8
2.61 6.65 10.7 15.5
19.3 (Iff scale 15.1 -0.30 15.4 11.3 -0.20 11.5 6.40 -0.02 6.42 -0.35
2.57 6.42 10.3 15.4
SO.04 +O 23 + O , 40
CC1, and CHCls, but are also exhibited in an even more dramatic way by benzene, hexane, and hexane-CHCl3 mixtures. Figure 4 shows the primary amine combination band for tetradecylamine in a variety of solvents ranging from the nonpolar, inert hexane to the very polar CHCla. Whereas
Figure 4. Effect of solvent on the nearinfrared combination band of primary amines a. CHCh b. CClr C. Benzene d. Hexane e. 257' CHC13 in hexane (All solutions except that in CHCla were clear, saturated solutions. Variations in peak absorbances are due principally to variations in amine concentration.)
solutions in CHCla show a single maximum a t 2023 mp, solutions in hexane show a single maximum a t 1998 mp. Mixtures of these two solvents exhibit two maxims (as do benzene and CCk solutions), one a t each of these two wavelengths; the relative intensities of these bands depend solely on the proportion of the two solvents in the mixture. The explanation of these phenomena is to be found in an examination of the fundamental -NH2 stretching and scissoring vibrations which are simultaneously excited to produce the combination band. Figure 5 shows these two fundamental bands for solutions of tetradecylamine; CC14 and CHCla were a satisfactory pair for studying the stretching vibrations, while hexane and hexane-CHC13 were the most suitable solvents for studying the scissoring vibrations. (CC14 solutions could not be studied because a strong solvent band obscures the -SH2 scissoring band,) The intensity differences shown for CCla and CHC13 solutions in Figure 5A and for hexane and hexane-CHC13 solutions in Figure 5B are due principally to concentration differences. Saturated solutions were employed for all spectra, except in the case of CHCI,, in order to get the strongest possible bands for study. The -XHz stretching bands around 3380 and 3450 em.-' are not appreciably altered in going from CClr to CHC13, although a slight shift (from 3390 cm.-' to 3375 cm.? is observed in CHC13 solutions for the symmetrical N-H stretching (3380 em. -l) band. The 3450 em.-' band is almost too weak to be meaningful; however, the point is really proved by the first overtone bands in the near infrared spectrum (Figure 1) a t 1527 and 1490 mp, which do not show any marked differences in
the two solvents in question. The strong band a t 3200 cn.-l in the CHClr solution spectrum is not associated with the -KHz stre1,ching mode and is probably solvent absorption due to a slight difference in the path length of the fixed-thickness cells employed for the reference and sample solutions. An examination of the -SHz scissoring band at 1615 em.-’, on the other hand, shows that changes very sin ilar t o those in the combination band are produced on going from hexane to the CHC13-hexane mixture. The frequency separation for the two maxima of the fundamental scissoring band is 42 crn.-l compared to a separation of 62 em.-’ for the maxima of the combination band in the same solvent mixture. The discrepancy in frequency ssparations for the two bands may be accounted for by the probable fact that the combination band separation in CHC13 solvents probably includes the 15 cm.-l shift in the N-H stretching band in CHCls as compared to CC1,1. In any case, it seems certain that it is principally this perturbation of the -NHz scissoring mode by the solvent3 which causes the very striking differences in the nearinfrared combinatio 1 band in these solvents. The general natu-e of the solventamine interaction producing these spectral effects can be inferred from the behavior of the coribination band in the hesane-CHC13 mixtures. Starting with a solution in pure hexane only one maximum a t 1998 mp is present. The addition of CHCl3 to the solvent causes the appearance of the second maximum a t 2023 nip. An increase in the per cent of CIIC13in the solvent mixture causes a dewease in the intensity of the 1998-mp niasimum and a proportional increase in the intensity of the 2023-mp band; these changes are proportional to the concentration of CHC13 in the mixture. These observations suggest the presence of two differeni, solvent-associated states of the amine in benzene, CC14 and the CHC13-hexane mixed solvent. One of these states appears to result from an amine-solvent association which is essentially chemical in nature and in which the -XHz scissoring mode is perturbed to such an extent that a distinct band shift occurs; excitation of this state gives rise t 3 the 2023-mp band in CHC13 and CHC’&-hexane, and the longer wavelength band of the two found in benzene and CCld. The other state is characterized by the more usual type of solvent-solute interaction which manifests itself through band broadening rather than barid shifting; e-icitation of this state gikes rise to the 1998mp band which is present in all of these solvents except CHCl,. This interpretation is further substantiated by the o’wervation that the
a W a
2 e 5z
15% CHCl3 IN HEXANE
(cm-’) stretching and scissoring bands in different
Figure 5. solvents.
Infrared fundamental -NH2 A.
Assymmetrical and symmetrical -NH2 stretching bands in CCla and CHCIa -NH2 scissoring band in hexane and 25% CHC13 in hexane
Effect of Possible Interferences on the Determination of Primary and Secondary Amines
Added material (interference) None (1) (2)
Dodecvl nitrile hlethd Dalmitate Dodecanoamide Tetradecanol Undecyl aldehyde Undecyl aldehyde
Primary amine, yo True Found Error
Secondary amine, yo True Found Error
8.21 7.46 7.68 8.41 8 25 7 60 9 18 ...
8.59 7.91 8.33
8.23 7.53 7.98 8.64 8 54 7 81 1.67
ratio of the intensities of the two combination bands of CC1, solutions is altered by heating the solutions. An increase of 20’ C. in the solution temperature causes a small, but significant increase in thc intensity of the 1998-mp band relative to the 2020-mp band. Chloroform, as employed in the method described here, apparently forms only one solvent-associated form with the amine, and at the same time exerts the strongest perturbing influence on the -XH2 scissoring mode. It would be expected as a consequence of this that deviations from Beer’s law due to amineamine interactions would be greatly reduced in solvents such as chloroform. Unfortunately, the fatty amines are not sufficiently soluble in any of the other solvents to test this point. Applicability of the Method. Ordinarily, it is necessary to calibrate the method with the particular amines t o be determined. However, within a homologous series such as the C10-C18 n-alkylamines the molar absorptivities are sufficiently constant to permit the use of a mean molar absorptivity for the analysis of such mixtures. If the
$0.02 +0.07 +0.30 +0.23 $0 29 +O 21 -7 51
8 18 9 23 7 37 9.06
8.42 7.63 8.10 7.82 7 93 9 14 6 63 6.02
-0.17 -0.28 -0.23 -0.28 -0 25 -0 09 -0 74 -3.04
method is to be used for the determinittion of relatively small amounts of primary and secondary amines in some sample mixture-such as a tertiary amine or an amid?-it is advisable to make the calibration in the presence of such components. This is suggested because of the relatively high and variabIe background absorption of substances of this nature in the 2-micron region. LITERATURE CITED
(1) Fieser, L. F., Fieser, M., “Advanced Organic Chemistry,” p. 497, Reinhold, New York, 1961. (2) Hanna, J. G., Siggia, S., ANAL. CHEM.34,547 (1962). (3) Holman, R. T., Edmondson, P. R., Ibzd., 28,1533 (1956);, (4) “Organic Analysis, Vol. I, pp. 14178, Interscience, New York (1956).
(5) Whetsel, K., Robemon, W. E., Krell, M. W., ANAL.CHEM.29, 1006 (1957). (6) Zbid., 30,1594 (19%). (7) Zbid., p. 1598. (8) Ibid., 32,1281 (1960).
RECEIVED for review October 8, 1962. Accepted February 14,1963.
VOL. 35, NO. 6, MAY 1 9 6 3