Gas chromatographic study of interactions between aliphatic amines

field and Yoke's (7) findings from the stoichiometry of the complexes formed by condensation of ethyl-, diethyl- or tri- ethylamine on cobalt, zinc, a...
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Gas Chromatographic Study of Interactions between Aliphatic Amines and Metallic Ions R. C. Castells and J. A. Catoggio Departamento de Quimica Analftica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata,

47 esq. 115, La Plata, Rep. Argentina INTERACTIONS between metallic ions and aliphatic amines have not been intensively studied up to the present time. It is generally accepted that the coordinating capacity follows the decreasing sequence N H , > NH2R > NHR:! > NR3. Hatfield and Yoke's ( 1 ) findings from the stoichiometry of the complexes formed by condensation of ethyl-, diethyl- or triethylamine on cobalt, zinc, and calcium halides, agree with this order. Drago et al. ( 2 ) were able to precipitate from methanol, Ni(I1) complexes with six molecules of primary monoamine, but not with secondary or tertiary amines ; energies of the nickel-ligand bonds for the complexes they obtained, decrease the longer the hydrocarbon chain. Zinc dimethyldithiocarbamate in toluene forms, with aliphatic amines, complexes the stabilities of which diminish according to : ethylamine > n-butylamine > t-butylamine > di-n-butylamine (3). Gas chromatography has been applied on several occasions to the study of complex formation (4-10); Purnell (11), in a thorough treatment of the subject, discussed possible applications of gas chromatography to this field. With this information in mind, the study of interactions between aliphatic amines and metallic ions was attempted, making use of columns the stationary phases of which were solutions of metallic stearates in an inert solvent, Le., Apiezon M. However, intense adsorption of amines on the support made it necessary to modify the systems under study. For the sake of obtaining symmetric peaks, Quadrol, N,N,N',N'tetrakis(2-hydroxypropyl)ethylenediarnine, had to be substituted for Apiezon M. A series of solutions of metallic stearates were used as stationary phases when studying the gas chromatographic behavior of aliphatic amines. It was hoped to establish a sequence of affinities for those amines tested, ascertaining the effect of their structural characteristics; and possible mechanisms of interaction of soap solutions in Quadrol through the behavior of different metallic stearates toward amines. EXPERIMENTAL

Reagents. Metallic stearates were prepared by reaction of solutions of sodium stearate with a salt of the metallic ion (1) W. E. Hatfield and J. T. Yoke, Inorg. Chem., 1, 463 (1962). (2) R. S. Drago, D. W. Meek, R. Longhi, and M. D. Joensten, ibid.,2, 1056 (1963). (3) E. Coates, B. Rigg, B. Saville, and D. Skelton, J. Chem. SOC., 1965, 5613. (4) E. Gil-Av and J. Herling, J. Phys. Chem., 66, 1208 (1962). ( 5 ) M. A. Muhs and F. T. Weiss, J . Amer. Chem. SOC.,84, 4697 (1962). (6) R. J. Cvetanovic, F. J. Duncan, W. E. Falconer, and R. S. Irwin, ibid., 87,1827 (1965). (7) R. J. Cvetanovic, F. J. Duncan, W. E. Falconer, and W. A. Sunder, ibid.,88, 1602 (1966). (8) W. E. Falconer and R. J. Cvetanovic, J. Chromatogr. 27, 20 (1967). (9) D. F. Cadogan and H. J. Purnell, J . Chem.SOC.,1968,2133. (10) R. S. Juvet, Jr., V. R. Shaw, and M. A. Khan, J. Amer. Chem. SOC.,91, 3788 (1969). (11) H. J. Purnell, "Gas Chromatography 1966," A. B. Littlewood, Ed., Institute of Petroleum, London, 1967, p 3. 1268

(12). Stearic acid (Fisher reagent chemical, mp 68-69.5 "C) and analytical reagent quality metallic salts were used. Stearates of copper, nickel, zinc, lead, strontium, calcium, cadmium, and palladium were prepared. Those of lead and strontium were insoluble in Quadrol; palladium stearate was already reduced by the solvent at 70 "C and the copper soap, once dissolved, slowly decomposed on heating, thus excluding their use. The thirteen amines tested (Table I) were standard commercial chemicals bought from Aldrich Chemical Company. They were analyzed chromatographically in a l/c-inch o.d., 2-m long column packed with 15% Carbowax 20 M on 60-80 mesh Chromosorb W previously deactivated by deposition of 5 % potassium hydroxide; as only traces of impurities were found, they were used without further purification. High purity nitrogen was used as carrier gas. Equipment. The inlet pressure of nitrogen was kept constant by two regulators; pressure was read on an open-end mercury manometer and atmospheric pressure was taken as the outlet one. Carrier gas flow was measured with a bubble flowmeter; flow was constant to within =t0.5%. The oven, built in the laboratory, consisted of two concentric cylinders through which air was forced at high speed; the temperature gradient between extreme points was smaller than 1 % of the working temperature. Power fed to the heating elements was controlled by an electronic circuit, so that temperature was constant to +0.05 "C. A commercial Aerograph 600-D chromatograph injector, flame ionization detector, and electrometer was used. The output from the electrometer was fed into a Sargent SR recorder. Packings and Columns. U-shaped, borosilicate glass columns, 1-m long and 0.45 cm i.d., were used. Packings were prepared by adding the support to the stationary phase in toluene and distilling the solvent on a rotary evaporator. Regardless of soap concentration in Quadrol, packings were prepared so as to contain 20% of stationary phase on 60-80 Chromosorb W. Packing weight in each column was determined by weighing the container before and after filling the column. Eleven columns were studied, containing the following stationary phases: plain Quadrol, 10-1 molal zinc stearate in Quadrol, and lod1, 5 x lo+, and 10-2 molal solutions of calcium, cadmium, and nickel stearate in Quadrol. Working Conditions. Samples of each amine, 0.1- or 0 . 2 4 , were injected by means of a I-pl Hamilton syringe. Every column was operated at 70.3, 90.3, and 110 O C ; in each case, columns packed with Quadrol and with 10-1 molal soap solutions in Quadrol were used at flowrates of 50, 70, 90, and 110 ml/min; while those packed with and 5 x 10-2 molal solutions were used with flowrates of 50 and 70 ml/min. Specific retention volumes, V,, calculated from experimental values obtained at each temperature under different flowrates, agreed satisfactorily in every case. V , values were calculated by applying the equation:

v, = A t R - tM) F C l W L

(1)

(12) W. F. Whitmore and M. Lauro, Ind. Eng. Chem., 22, 646 (1930).

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970

Amine n-Propylamine +Butylamine n-Heptylamine i-Buty lamine 2-Aminobutane 2-Aminopentane 2-Aminoheptane 3-Aminoheptane 1,4-Dimethylpentylamine r-Butylamine Diethylamine Dipropy lamine Trimethylamine

Amine n-Propylamine n-Butylamine n-Heptylamine i-Butylamine 2-Aminobutane 2-Aminopentane 2-Aminoheptane 3-Aminoheptane 1,4-Dimethylpentylamine t-Butylamine Diethylamine Dipropylamine Triethylamine

Amine n-Propylamine n-Butylamine rz-Heptylamine i-Butylamine 2-Aminobutane 2-Aminopentane 2-Aminoheptane 3-Aminoheptane 1,4-Dimethylpentylamine i-Butylamine Diethylamine Dipropy lamine Triethylamine

Table I. Specific Retention Volumes, V,, at 70.3 “C (ml/gram) Molality of Cast? Molality of NiStn ZnSt? 0.01 0.05 0.10 O.lOm 0.05 0.10 Quadrol 0.01 105.7 121.8 130.6 164.0 100.6 102.1 107.4 91 .O 305.0 388.0 248.5 229.5 282.0 212.2 235.2 243.0 ... ... ... ... ... ... ... 135.5 127.0 1’38.9 145.0 143.1 161.0 175.9 200.0 115.2 117.9 116.5 120.0 119.7 128.5 133.6 158.0 243.1 224.0 258.2 265.0 253.0 275.0 280.5 335.0 980.0 1089.0 1169.0 1170.0 1148.5 1225.0 1259.6 1531.0 875.0 796.0 937.5 947.0 921.1 984.0 1018.4 1122.0 732,Q 61.4 63.9 178.2 52.1

Quadrol 46.1 97.3 738.0 62.2 53.7 102.5 381 .Q 318.5

Quadrol 23.3 46.5 289.5 30.9 27.2 48.5 158.2 136.4

854.0 56.6 67.0 198.5 54.7

876.0 63.8 72.3 207.0 59.0

845.0 60.1 70.3 197.3 53.0

904.0 64.4 76.1 213.0 60.1

944.5 68.6 77.3 223.8 67.2

1134.0 81.1 88.7 242.0 67.1

Table 11. Specific Retention Volumes, V,, at 90.3 OC (ml/gram) Molality of Casts Molality of NiStn ZnStz 0.01 0.05 0.10 0.01 0.05 0.10 O.lOm 50.6 52.4 54.2 54.0 59.4 60.9 72.2 105.8 110.3 112.0 112.7 126.3 131.4 159.0 807.7 878.0 883.0 860.0 965.0 1044.4 , , , 67.0 69.0 72.2 71.0 75.3 81.6 89.1 57.1 58.5 61.4 60.6 62.5 64.7 72.1 109.7 113.9 118.0 115.2 121.4 125.2 142.8 414.8 444.0 449.0 433.8 459.0 483.8 558.0 344.2 368.0 369.0 360.0 378.0 396.9 428.0 316.8 31.4 34.2 87.3 29.3

293,6 29.8 32.0 80.6 27.4

124.5 15.3 16.4 38.0 14.4

804.0 59.2 67.4 189.8 53.3

337.5 31.6 35.5 91.7 30.0

342.0 32.6 36.6 94.5 31.4

332.2 32.9 36.7 90.6 28.1

349.0 34.1 38.6 94.1 30.9

366.5 35.2 38.8 99.3 34.2

Table 111. Specific Retention Volumes, V,, at 110.0 Molality of Castz Molality of NiStz 0.01 0.05 0.10 0.01 0.05 0.10 29.4 31.5 32.9 26.2 29.2 29.1 56.7 59.7 63.0 52.4 57.2 56.6 378.0 341 . O 410.4 329.0 366.1 349.0 38.8 41.3 37.9 33.6 38.7 37.8 33.6 34.4 32.3 29.9 33.5 32.3 59.5 57.8 54.8 59.1 61.2 59.6 193.4 188.3 173.O 208.3 197.3 190,O 152.7 165.0 157.9 169.5 177.0 162.9 140.8 17.2 18.0 41.6 16.0

154.8 19.0 20.1 47.9 18.2

149.4 18.5 20.2 46.9 19.2

147.0 18.3 20.2 45.9 16.0

where j , is the correction factor for gas compressibility; Fc, nitrogen flowrate reduced to 0 “C and corrected for wet gas; w L , the weight of stationary phase in the column, and t R and ru, the retention times of the solute and methane, respectively.

153.8 18.6 19.7 47.2 17.8

162.1 19.1 21.7 49.4 19.7

426.0 38.6 41.8 102.3 33.2

Molality of CdSt, 0.01 129.9 315.0

0.05

179: 3 139.2 292.5 ... ...

528,8 248.7 542.0 ... ...

.

.

.

I

68.6 75.7 211.5 61.3

0.01 59.8 126.6 966.0 79.3 63.9 123.4 462.3 374.9 355.4 34.8 39.3 92.8 31.8

0.10 677.6 1641.0 ... 859.8 353.0 760.7

404.0 1042.0 I

I

...

...

...

... 105.6 88.2 238.6 63.9

86.4 82.5 222.7 61.8

Molality of CdStz

-

134.0 306.1

0.10 229.9 510.8

171.2 94.7 190.1 749.0 513.5

278.3 134.1 267.8 1075.5 669.0

588.0 41.1 40.6 95.9 32.4

836.0 48.5 45.5 104.2 32.2

0.05

...

...

“C (ml/gram) ZnSt, 0.10 m 36.6 72.1

Molality of CdStn

44.1 37.0 67.2 228.0 185.0

0.01 31.5 59.7 373.4 39.6 33.8 59.9 193.6 162.1

51.1 104.3 674,O 63.0 41.6 76.0 254.3 192.0

0.10 75.4 163.7 1131.0 95.6 55.3 100.8 338.2 239.0

179.0 21 .o 22.4 49.8 18.3

154.7 18.9 20.5 45.6 17.3

204.3 20.6 20.6 47.2 17.6

274.3 23.3 21.6 49.1 19.2

...

0.05

DISCUSSION

bp, and this effect is still more evident for triethylamine, which emerges before t-butylamine, in spite of almost doubling its bp. Even though ethylamine was not studied, its V , was extrapolated from the graph of the retention volumes at 90.3 “C against the bp of primary amines with the amine group o n the terminal carbon, so as to compare it with those corresponding to diethyl- and triethyl-amine. These values of V , are included in Table IV, together with the values of the dissociation constants of the organic bases in aqueous solutions (13) KB,and vapor pressures for the three amines at 90.3 “C(14).

Effect of Metal Ion. Primary amines are eluted from the Quadrol column in order of increasing boiling points; secondary amines are eluted before the primary ones of same

(13) “Handbook of Chemistry,” N. D. Lange, Ed., Revised 10th edition, McGraw-Hill, New York, 1966, p 1209. (14) T. E. Jordan, “Vapor Pressure of Organic Compounds,” Interscience, New York, 1954.

RESULTS

Specific retention volumes of the 13 amines tested, in each of the 11 columns used, at 70.3, 90.3, and 110 “C, are listed under Tables I, 11, and 111, respectively.

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970

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Amine Ethylamine Diethylamine Triethylamine

Table IV. Data of Ethylamines Vg(mlk) Kb X 10' Vp (mm Hg) 20.5 5.60 7,200 32.0 12.60 2,400 21.4 5.65 800

Table V. Affinity Quotients, Amine Castz n-Propylamine 0.98 n-Butylamine 1.13 n-Hept ylamine 2.48 i-Butylamine 0.80 2-Aminobutane 0.65 2-Aminopentane 1.03 2-Aminoheptane 1.92 3-Aminoheptane 1.87 1,CDimethylpent ylamine 1.77 t-Buty lamine 0.17 Diethylamine 0.10 Dipropylamine 1.36 Triethylamine 0.64

AQ, at 90.3 NiSt2 2.92 3.50 3.55 1.74 0.89 1.51 1.65 1.41 1.43 1.09 1.49 1.09 2.50

"C CdSt, 40.15 46.10 ...

1270

a : Triethylamine b: n- Propylamine c : i - Butylamine d : Dipropylamine e : 2-Aminopentane

(7,

X

E

I

P 250

37.00

14.35 16.25 18.83 10.86

200

19.80 5.28 1.02 0.96 0.55

,--,La

There should be strong interactions in solution to account for this elution order, and it is symptomatic that it agrees with the K b values sequence. It is probable that hydrogen bond formation between the amino group and the hydroxyl groups of the lateral chains in the Quadrol molecule is related to this behavior. The elution order for amines from columns packed with calcium stearate solutions in Quadrol as stationary phases, is the same as for the plain Quadrol column, though V , values are slightly higher. With nickel stearate columns, the elution order changes compared to plain Quadrol, and indeed the extent of changes is larger in columns containing solutions of zinc and cadmium stearates. Also, an increase occurs in the retention volumes of t,he primary amines, but there are only slight changes in those of the secondary and tertiary amines. It is evident that steric interference plays a major effect o n amine-metallic ion interactions. This is the reason the retention volume of a primary amine is larger when the nitrogenfunction is located on the terminal carbon atom than when it is o n the second carbon atom of a straight chain one atom longer, in spite of being these less volatile (e.g., n-propylamine as compared with 3-aminobutane). F o r analogous reasons an amine with the functional group o n carbon two (1,4-dimethylpentylamine) delays with respect to one carrying the amine group o n carbon three (3-aminoheptane). In comparing the behavior of primary, secondary, and tertiary amines, the extrapolation of the V , for the ethylamine indicates that from columns with calcium, nickel, and zinc stearates the elution order of the ethylamines is the same as from pure Quadrol. From columns containing cadmium stearate the eluting order would, instead, be: triethylamine, diethylamine, ethylamine. This is the reverse order of their volatilities, but agrees with that expected o n the basis of basic nitrogen atom accessibility and with reactivities found by Hatfield and Yoke ( I ) . Effect of Stearate Concentration. If the stationary phase is a solution of an additive A , in a n inert solvent, capable of forming a 1 :1 complex with a solute, it can be shown (11, e.g.) that the partition coefficient, K , of the solute between stationary and mobile phases is related to the additive concentration in the stationary phase, CA,through the equation:

K=K"fK"K~CA

-

(2)

Ste

150

-----------looe d C

50 b

a

0 o .os

0.00

0.10

Molality of Stearate Figure 1. Plot of specific retention volume us. concentration of metallic stearate for named amines eluted at 90.3 OC where KO is the partition coefficient between pure inert solvent and the mobile phase, and K , is the stability or formation constant of complex A X . K and K" can be calculated from the Vovalues of the solute in columns with and without additive, respectively, making use of the following expressions : K = Voa P L a Tc/273 (3) K"

=

Vgo p~~ T,/273

(4)

Here VOais the specific retention volume of the solute in a column with additive and pLa is the density of the additive solution used as stationary phase; V,, and pLo are equivalent terms for the column where plain diluent is used as stationary phase.

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970

When plotting K as a function of CA,a straight line should be obtained. From the slope (KOKJ and the intercept a t zero concentration (KO) it is possible to calculate K I . Unless ionic strength is kept constant, it is usual to obtain decreasing slopes a t high concentration values, because of salting-out effects. As will be shown later, our system has been complicated by previous Quadrol-cation interaction, and in fact we shall propose the insertion of the amine as a n additional ligand o n a n already formed metallic ion:Quadrol complex, to give a complex of higher coordination number. The eventual definition of the stability constants for these complexes would be irrelevant because of the uncertainty as to their actual constitution. O n the other hand, if it is accepted that the specific gravity of different solutions of the same additive in a given solvent remains constant, a valid approximation for sufficiently dilute solutions, then Equation 2 can be written:

Voa

+ Voo K1 CA

(5) Then, o n plotting specific retention volumes as a function of stearate concentration, the quotient between the slope of the resulting graph for a given solute and the retention volume in plain Quadrol, will give a measure of the solute affinity for the stationary phase. These quotients d o not define the formation equilibrium constants for complexes being formed between the amines and the stearate solutions in Quadrol, but are useful in comparing the affinities of different amines for a given phase. We shall designate these quotients as “affinity quotients,” i.e., AQ. Table V gives values for the studied amines, a t 90.3 O C , in columns of calcium, nickel, and cadmium stearates. V g us. cation concentration graphs, a t 90.3 OC, are almost straight lines for every amine in cadmium stearate columns. This is not always the case when using columns with calcium and nickel stearates, a few solutes showing a slight curvature. In these cases the slope of the straight line connecting the V , values for and 5 X molal concentrations was used, thus all the graphs are linear over that concentration range. This criterion agrees with that of Muhs and Weiss (9,who take into account the slope over a low concentrations range, where salting-out effects can be neglected. Figure 1 shows some typical graphs of this kind. Notwithstanding the meaning of affinity quotients, it is possible to deduce from the values for different ions, that interactions of primary amines decrease in the order cadmium >> nickel > calcium. With the exception of a few apparently anomalous values for secondary and tertiary amines with calcium and nickel stearates-measurements being exposed to higher errors in these cases because of weaker interactions--AQ values make it possible to classify studied amines within the following groups, according to the decreasing ion-amine affinity : (1) Primary amines with the amine group on the terminal carbon interact most energetically, affinity increasing with carbon chain length. ( 2 ) i-Butylamine exhibits smaller affinity than the first group, in spite of the amine group being located o n carbon one; this must be attributed to steric effects because of chain branching, as its K b is of the same order as that of n-butylamine. (3) Primary amines with functional group o n carbon two; within this series, too, affinity increases with carbon chain length. (4) 3-Aminoheptane, the AQ of which is smaller than those for the preceding group, undoubtedly because of the greater hindrance that the ethyl group o n the carbon atom bearing the amine function, introduces as compared with the methyl with= Vgo

in the previous class. The behavior of this amine might be considered, most certainly, representative of normal primary amines, with the functional group o n carbon three. ( 5 ) A group including t-butylamine, secondary amines, and the tertiary amine studied, within which affinity decreases in the order mentioned. O n the basis of these AQ’s it is now possible to explain all the observed inversions when comparing the elution sequence of amines from stearates’ columns with that from the Quadrol column. The fact that calcium turns out to be the least reactive ion could be foreseen as a consequence of its noble-gas electronic structure and its charge:radius ratio, which places that ion in Schwarzenbach’s class A, which includes ions whose interactions with ligands are due primarily to electrostatic forces (15). Thus, its complex with o-phenanthroline has a stability constant of 3.1, as opposed to a 2.7 X l o 6 value for the same constant of the zinc homolog (16). I t is not possible to explain the sequence Cd > Zn > Ni o n the basis of previous studies; there are no data for stability constants of their complexes with aliphatic monoamines, most probably because of the hydrolysis in aqueous solution [the case, at least, for nickel ( 2 ) ] . Both cadmium and zinc pertain to Schwarzenbach’s class B, covering ions with complete d sublevels, giving complexes through the establishment of mainly covalent bonds, the stabilities of which depend much more on the electronegativities of both cation and ligand’s donor atom, than o n the charge :radius ratio of the central atom. Nickel would fall within Schwarzenbach’s class C, which includes divalent transition elements with incomplete d sublevels, which share the characteristics of groups A and B, in different proportions. If the stabilities of complexes with ligands related to our aliphatic monoamines [e.g., ammonia and diamines (16)] are considered, the sequence found should be reversed. From these considerations it is not possible to explain the sequence exclusively o n the basis of the amine-cation interactions. As Quadrol is a diamine, an interaction aminecation :Quadrol complex is postulated, instead. Under these circumstances, there would be two reasons for the order found. (1) Possible differences between the initial structures of the ion :Quadrol complexes or, between the final amine-ion : Quadrol complexes, would determine that complexation with cadmium is energetically favored as compared to that with zinc o r with nickel. I n this sense, complexes of these ions with coordination number 5 are known, where nickel forms part of a tetragonal pyramidal structure (17), whereas cadmium and zinc appear with trigonal bipyramid structures (3, 18). ( 2 ) If these structural differences exist, differences in the respective crystal ionic radii (Ni, 0.78 A ; Zn, 0.74 A; Cd, 0.97 could explain the greater affinity of amines for cadmium than for zinc or nickel.

A)

RECEIVED for review February 9, 1970. Accepted June 2, 1970. (15) G. Schwarzenbach, Experieritiu Suppl., 5 , 162 (1956). (16) K. B. Yatsimirskii and V. P. Vasil’ev, “Instability Constants of Complex Compounds,” D. Van Nostrand, Princeton, N. J., 1960, p 148. (17) C. M. Harris, R. S. Nyholm, and N. C. Stephenson, Nature, 179, 1127 (1956); J. Clzern. Soc., 1956,4375. (18) D. E. C. Corbridge and E. G. Cox, J. Chem. Soc., 1956,594.

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