Aliphatic Amines

The first attempts to separate aliphatic amines were performed on silica gel using chloroform-ammonia (39: 1), chloroform-methanol-17% ammonia (2:2: 1), butanol-acetic acid-water (4:1:5) and phenol-water (8 : 3) as eluents. However, highly volatile amines cannot be chromatographed with an ammoniacal solvent. A systematic collection of data on the chromatographic behaviour of a large number of aliphatic amine hydrochlorides, with particular emphasis on eluents, adsorbents and detection reagents, was published by Prandi (see hRF values in Table 1, columns 1, 2 and 3).

Silica gel is particularly useful for the adsorption chromatography of amines that have different polarities but does not resolve the fatty amine series. In particular, the RF values increase as the aliphatic chain length increases but this increase becomes smaller with increasing chain length.

Reversed-phase partition chromatography on paraffin oil-saturated silica gel is useful for the separation of fatty amines. RM[log(1/RFjvalues for such amines increase as the length of the aliphatic chain increases and there is a linear relationship between RM and the number of carbon atoms in the molecule.

Table 1 Retention data (hRF)* of aliphatic amine hydrochlorides under different experimental conditions3

Amine

Silica gel''

Po-s Kieselguhrc

Silica gel

Sil C18-50

AWPf

+ chlorophenold

# 4%HDBSe

EluentA

Eluent B

Eluent C

Eluent D

Eluent E

Eluent F

Methylamine

3

6

-

26

92

69

Dimethylamine

4

7

-

31

-

-

Trimethylamine

-

43

-

35

-

-

Ethylamine

7

16

-

46

85

75

Diethylamine

16

32

-

50

-

-

Triethylamine

-

75

-

53

-

-

Ethanolamine

4

10

-

-

-

-

Diethanolamine

5

16

-

-

-

-

Triethanolamine

18

36

-

38

-

-

Ethylethanolamine

11

23

-

-

-

-

Ethyldiethanolamine

30

52

-

-

-

-

2-Ethylhexylethanolamine

65

93

-

-

-

-

Propyldiethanolamine

52

69

-

-

-

-

Propylamine

16

35

-

57

-

-

Di-n-propylamine

51

80

-

-

-

-

Isopropylamine

17

36

-

63

-

-

Diisopropylamine

33

66

-

-

-

-

Propanolamine

4

8

-

-

-

-

Triisopropanolamine

52

85

-

-

-

-

Allylamine

-

-

-

60

-

-

Diallylamine

-

-

-

66

-

-

Butylamine

22

48

-

-

55

65

Di-n-butylamine

63

95

-

80

-

-

Tri-n-butylamine

-

-

-

85

-

-

Isobutylamine

31

58

-

-

62

70

Diisobutylamine

85

99

-

-

-

-

3-Methoxypropylamine

18

43

-

-

-

-

Pentylamine

29

55

-

-

34

60

Isoamylamine

30

56

-

-

45

62

2-Methylbutylamine

36

68

-

-

-

-

Hexylamine

34

65

86

-

24

53

Cyclohexylamine

33

63

-

76

-

-

3-Amino-2,2'-dimethylbutane

51

90

-

-

-

-

2-Amino-3-methylpentane

47

78

-

-

-

-

2-Amino-4-methylpentane

42

73

-

-

-

-

Heptylamine

36

70

82

-

14

elongated spots

Octylamine

37

74

78

-

7

3

2-Ethylhexylamine

54

88

-

-

-

-

Di-2-ethylhexylamine

100

100

-

-

-

-

tert-Octylamine

52

87

-

-

-

-

Nonylamine

39

77

74

-

-

-

Decylamine

40

78

70

-

1

0

Undecylamine

42

79

65

-

-

-

Dodecylamine

44

79

58

-

0

0

Tridecylamine

47

80

50

-

-

-

Tetradecylamine

50

82

43

-

0

0

Pentadecylamine

52

83

38

-

-

-

Hexadecylamine

55

85

30

-

-

-

Heptadecylamine

58

85

24

-

-

-

Stearylamine

60

85

18

-

-

-

1,2-Diaminoethane

2

4

-

22

82

49

1,2-Diaminopropane

3

10

-

-

81

58

1,3-Diaminopropane

-

-

-

-

84

36

1,4-Diaminobutane

-

-

-

-

84

32

1,5-Diaminopentane

-

-

-

-

85

26

1,6-Diaminohexane

-

-

-

-

79

19

1,7-Diaminoheptane

-

-

-

-

72

17

1,8-Diaminooctane

-

-

-

-

56

16

N-(3-Aminopropyl)cyclo-

5

18

-

-

-

-

hexylamine

Table 1 Continued

Diethylenetriamine

0

0 -

17

-

-

Spermidine

-

- -

-

81

10

Spermine

-

- -

-

72

2

Tetraethylenepentamine

0

0-

-

-

aEluents: A and B = chloroform-methanol-17% ammonia in the 82.5 : 15.5 : 2 (A) and 70 : 26 : 4 (B) ratios: C = acetone-17% ammonia (70 : 30 v/v); D = n-butanol-acetic acid-water (35 : 5 : 10); E = 1 M acetic acid # 1 M HCl in 30% methanol; F = 2 M NH4NO3.

bSilica gel G (Merck); detection reagents: 1% ninhydrin solution in ethanol-acetic acid (95 : 5); 1% potassium permanganate # 1% potassium persulfate (1 : 1); 25% iodine methanolic solution; 5% sodium nitroprusside in acetaldehyde-2% sodium carbonate (1 : 1 v/v) solution. Sample volume; 10 ^L of a 0.5% water-alchol solution of the amine hydrochloride.

cParaffin oil-saturated Kieselguhr G (Merck) layers were prepared by immersing the plates in a 5% solution of the oil in acetone. dHome-made plates were prepared by spreading a slurry of 50 g of silica gel G (BDH) in 2% o-chlorophenol solution (100 mL). The plates were dried for 24 h at 60°C before use. Detection agent: 3 g ammonium thiocyanate and 1 g cobalt chloride in 20 mL of distilled water (blue spots).

eThe Sil C18-50 impregnated layers (Macherey-Nagel) were prepared by immersing the plates in a 4% dodecylbenzenesulfonicacid (HDBS) solution in 95% ethanol.

fHome-made plates were prepared by spreading a slurry of 4 g ammonium tungstophosphate (AWP) and 2 g calcium sulfate hemihydrate in 50 mL of distilled water after stirring 10 min with a magnetic stirrer. Detection agent: 1% ninhydrin solution in a 5 : 1 (v/v) mixutre of pyridine and glacial acetic acid.

Sources: Adapted from Prandi C (1978) Thin-layer chromatography of aliphatic amines. JournalofChromatography 155: 149-157; Srivastava SP, Dua VK and Chauhan LS (1980) Chromatographic behaviour of aliphatic amines on phenol-impregnated thin layers. Journal of Chromatography 196: 225-235; Lepri L, Desideri PG, Heinler D and Giannessi S (1982) High-performance thin-layer chromatography of nitrogen compounds on layers of RP-18and Sil C18-50 untreated or impregnated with dodecylbenzenesulfonicacid and of ammonium tungstophosphate. JournalofChromatography245: 297-308.

Cellulose and aluminium oxide have also been used as adsorbents. The phenomenon of multiple-spot formation of amines on cellulose thin layers when using neutral or weakly acidic eluents is caused by the presence of carboxyl groups in the cellulose. Partial hydrolysis of the amine hydrochloride, volatilization of the liberated hydrochloric acid and the presence of charged groups in silica gel and alumina layers have also resulted in double-spot formation for specific compounds.

Phosphate and acetate-buffered silica gel and impregnated plates have been used. Hydrogen bond formation between the impregnated plates and aliphatic amines influences their chromatographic behaviour on metal salt-impregnated plates and on phenol-impregnated silica-gel layers.

The hRF values of some amines, obtained on silica gel impregnated with a 2% solution of o-chloro-phenol using a butanol-acetic acid-water (35 : 5 : 10) mixture as eluent are reported in Table 1 (column 4). No correlation exists between the pKa value of the conjugated acid of an amine and its RM value; it therefore seems that the chromatographic behaviour of such compounds is due to hydrogen bond formation between the amine and silica gel as well as o-chlorophenol.

Reversed-phase thin-layer chromatography of several aliphatic mono- and polyamines has been performed on layers of silanized silica gel untreated and impregnated with anionic and cationic surfactants. Ion-exchange and/or partition contribute to the retention of the amines depending on the type of stationary phase, the percentage of surfactant and the apparent pH of the eluent.

Table 1 (column 5) shows the retention data obtained on Sil C18-50 plates impregnated with a 4% dodecylbenzenesulphonic acid solution in 95% ethanol and eluted with 1 M acetic acid + 1 M hydrochloric acid in 30% methanol. On these plates the retention of polyamines is governed chiefly by an ion-exchange mechanism while aliphatic mono-amines can be differentiated according to the number of carbon atoms in their side chain.

Aliphatic amines have been studied on layers of weak and strong ion exchangers, Dowex 50-X4 (Na + and H + ), sodium carboxymethylcellulose and Rexyn 102 (Na + ), using hydrochloric acid and various buffer and salt solutions as eluents.

The use of ammonium tungstophosphate (AWP) as a layer material is particularly promising since on this exchanger different affinity sequences in comparison with the above-mentioned results are found (see Table 1, column 6). The behaviour of poly-amines is of interest since it seems to be correlated to the distance between the protonated amino groups involved in the ion-exchange process and, in the case of 1,2-diaminoethane and 1,2-diaminopropane, to the steric hindrance of the methyl group.

Quantitative analysis of the diamine hydrochloride recovered from acid-hydrolysed copolyamides prepared from diamine-diacid has been carried out on silica gel G eluting with phenol-w-butanol-formic acid-water (5:2:1:2 v/v) or phenol-formic acid-water (74 : 1 : 25 v/v). Densitometric scanning was performed using a Shimadzu spectrophotometer at 560 nm after spraying the plates with a 0.2% solution of ninhydrin in ethanol and heating at 90°C for 15 min.

Specific procedures have been developed for al-kanolamines. The high performance TLC (HPTLC) behaviour of closely related diethanolamines was studied on silica-gel layers eluted with binary solvents (methanol-chloroform, methanol-dichloromethane, methanol-acetone, acetone-chloroform) and on four types of reversed-phase, chemically bonded, silica gel with methanol-water as mobile phase. Alkanol-amines, in particular ethanolamines and iso-pro-panolamines, are used extensively in hydraulic brake fluids and cutting oils as corrosion inhibitors; the derivatives with fatty acids are used as emulsifiers and detergents. Their separation and identification is performed on neutral silica gel using methylene chlor-ide-95%-ethanol-ammonia (0.880) in the proportions 43 : 43 : 15 v/v as eluent. A solution of 0.2% ninhydrin, then alizarin in acetone, is employed to locate the separated alkanolamines. The method has been applied to commercial formulations.

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