Aromatic Amines

Table 4 shows the hRF values of several primary aromatic amines under different experimental conditions. Such compounds have been studied on silica-

gel G or alumina using organic and aqueous-organic solutions as eluents.

Further studies concern the chromatographic behaviour of aromatic amines on silica gel impregnated with silver compounds as rn-complexing metal and manganese, cadmium and zinc salts as complexing agents. On silica gel impregnated with manganese salts, the pKa values of the conjugated acids of sixteen isomeric methylanilines and chloroanilines

Table 3 Retention data (hRF or hRX* for eluent B) of derivatized amines under different experimental conditions3

Amine

DADB-b Silica

PABS-c Alumina

NBD-d Silica

DBAB-e Silica

DNP-f Silica

gel + Carbowax

gel G

gel HF254

EluentA

Eluent B

Eluent C

Eluent D

Eluent E

Ammonia

-

-

45

-

3

Methylamine

15

47

57

17

30

Dimethylamine

44

116

-

19

34

Ethylamine

24

71

63

30

59

Diethylamine

58

143

-

36

69

n-Propylamine

33

87

-

36

75

Di-n-propylamine

65

150

-

44

-

Isopropylamine

39

92

-

40

74

Diisopropylamine

-

-

-

47

-

Allylamine

-

81

-

39

62

Diallylamine

-

147

-

-

-

n-Butylamine

42

100

71

42

81

Din-butylamine

68

>150

-

48

-

Isobutylamine

50

106

71

43

82

Diisobutylamine

-

>150

-

48

-

sec-Butylamine

-

104

-

-

-

tert-Butylamine

-

105

-

-

-

n-Amylamine

50

108

-

-

85

Di-n-amylamine

-

>150

-

51

-

Isoamylamine

51

113

-

47

86

n-Hexylamine

-

115

-

47

88

Din-hexylamine

-

-

-

57

-

Cyclopentylamine

-

100

-

-

-

Cyclohexylamine

-

105

-

51

-

Dicyclohexylamine

-

-

-

56

-

Octylamine

-

120

-

-

90

Di-n-octylamine

-

-

-

64

-

Decylamine

-

127

-

-

92

Benzylamine

-

87

-

-

67

Dibenzylamine

-

141

-

-

-

1-Phenylethylamine

-

86

-

-

-

2-Phenylethylamine

26

83

-

-

66

Ephedrine

-

57

-

-

-

Amphetamine

-

94

-

-

-

ß-Phenylisopropylmethylamine

-

126

-

-

-

Mescaline

-

15

-

-

-

Tryptamine

3

-

55

-

-

Tyramine

8

-

48

-

-

3-Methoxytyramine

-

-

48

-

-

Histamine

-

-

30

-

-

Putrescine

-

-

31

-

-

Cadaverine

-

-

34

-

-

Spermidine

-

-

27

-

-

Spermine

-

-

30

-

-

*hRF = RF x 100; hRF = RX values x 100 (relative to the hRX of n-butylamine derivative taken equal to 100).

aEluents: A = n-hexane-ethyl acetate (7 : 3); B = 25 mL ethyl acetate and 100 mL petroleum ether (62-82° from Shell) saturated with water; C = toluene-acetic acid (4 : 1 v/v); D = cyclohexane-methylethylketone (70 : 30); E = pentane-benzene-triethylanine

b4-Dimethylamino-3,5-dinitrobenzoyl-; home-made plates were prepared by spreading a mixture of 10 g of Carbowax 400 (Fluka) and 40 g of silica gel G (Merck) in 80 mL of water.

c4-(Phenylazo)-benzenesulfonyl-; microchromatoplates (40 x76 mm) coated with alumina (Fluka, D5). d4-Chloro-7-nitrobenzo-[c]-(1,2,5)-oxadiazole; silica 60 (250 ^m) TLC plates (20 x 20 cm) were obtained from Sigma. <p>(A/,A/-Dimethylamino)benzene-p'-azobenzoyl. f2,4-Dinitrophenyl.

Sources: Adapted from Wirotama IPG and Ney KH (1971) Dunnschicht chromatographie von amines als 4-dimethylamino-3,5-dinitrobenzoyl amide. Journal ofChromatography 61: 166-168; Jart A and Bigler AJ (1967) Thin-layer chromatographic separation of primary and secondary amines as 4-(phenylazo) benzene sulfonamides. Journal ofChromatography 29: 255-258; Price NpG and Gray DO (1993) Mapping of derivatised biogenic amines by two-dimensional thin-layer chromatography. A comparative study. Journal of Chromatography 635: 165-170; Churaccek J (1970) Einige neue reagenzien zur chromatographischen identifizierung von sauren, alkoholen und amines. Journal of Chromatography 48: 241-249; Ilert HI and Hartmann T (1972) Dunnschichromatographie der 2,4-dinitrophenyl derivate wasserdampfflüchtiger amine und ihre anwerdung auf die trennung pflanzlicher amine. Journal of Chromatography 71:119-125.

Table 4 Retention data (hRF) of primary aromatic amines under different experimental conditions3

Amine Silica gelG Silanizedsilica # 4%N-DPCb AG-1XA(CH3COQ-)c pKa

EluentA EluentB EluentC EluentD EluentE EluentF EluentG

Table 4 Retention data (hRF) of primary aromatic amines under different experimental conditions3

EluentA EluentB EluentC EluentD EluentE EluentF EluentG

Aniline

-

22

72

92

-

-

4.58

o-Toluidine

42

17

16

61

77

-

-

4.39

m-Toluidine

29

10

14

68

79

-

-

4.69

p-Toluidine

20

5

14

75

81

-

-

5.12

2,4-Dimethylaniline

-

-

7

53

71

-

-

-

2,6-Dimethylaniline

-

-

11

21

50

-

-

-

o-Aminophenol

24

0

-

-

-

46

83

4.72

m-Aminophenol

13

0

-

-

-

30

83

4.17

p-Aminophenol

1

0

-

-

-

70

84

5.49

o-Anisidine

42

15

18

66

78

58

83

4.49

m-Anisidine

30

9

20

53

70

44

83

4.20

p-Anisidine

2

2

26

85

86

74

84

5.29

o-Chloroaniline

75

66

-

-

-

12

40

2.64

m-Chloroaniline

51

40

-

-

-

15

64

3.34

p-Chloroaniline

41

22

6

14

44

18

75

3.98

o-Bromoaniline

78

69

6

13

17

10

27

2.60

m-Bromoaniline

58

44

4

10

28

10

55

3.51

p-Bromoaniline

47

27

5

9

37

12

69

3.91

o-Nitroaniline

55

52

5

7

17

4

8

-0.29

m-Nitroaniline

44

36

9

10

28

21

31

2.50

p-Nitroaniline

37

29

6

9

23

2

7

1.02

o-Aminobenzoic acid

47

44

-

-

-

-

-

-

m-Aminobenzoic acid

28

12

-

-

-

-

-

-

p-Aminobenzoic acid

37

29

-

-

-

-

-

-

o-Phenylenediamine

0

0

37

81

86

67

83

4.47

m-Phenylenediamine

0

0

49

96

96

71

84

4.88

p-Phenylenediamine

0

0

60

95

97

79

84

6.08

2,4-Dichloroaniline

-

-

6

15

44

-

-

-

2,4-Dinitroaniline

-

-

3

5

15

0

0

- 4.53

2,4-Diaminotoluene

-

-

34

92

94

72

83

-

2,5-Diaminotoluene

-

-

-

-

-

79

84

-

2,6-Diaminotoluene

-

-

50

95

96

73

83

-

3,4-Diaminotoluene

-

-

10

23

34

67

83

-

2,4-Diaminoanisole

-

-

-

-

-

72

83

-

2-Amino-4-nitrophenol

-

-

-

-

-

1

10

-

2-Amino-5-nitrophenol

-

-

-

-

-

1

2

-

4-Amino-2-nitrophenol

-

-

-

-

-

22

65

-

2-Amino-4,6-dinitrophenol

-

-

-

-

-

0

0

-

4-Nitro-o-phenylenediamine

-

-

-

-

-

8

16

-

2-Amino-4-chlorophenol

-

-

-

-

-

3

41

-

2-Amino-3,4,6-trichlorophenol

-

-

-

-

-

0

1

-

a-Naphthylamine

-

-

3

9

38

-

-

-

4-Aminodiphenylamine (DPA)

-

-

2

31

53

-

-

-

2-Amino-DPA

-

-

2

11

23

-

-

-

3-Methoxy-4-amino-DPA

-

-

2

29

55

-

-

-

4-Methoxy-4'-amino-DPA

-

-

2

45

56

-

-

-

4,4'-Diamino-DPA

-

-

28

97

97

-

-

-

2,4-Dinitro-4'-amino-DPA

-

-

2

3

26

-

-

-

Benzidine

-

-

6

65

81

-

-

-

o-Tolidine

-

-

2

26

73

-

-

-

o-Dianisidine

-

-

2

7

49

-

-

-

aEluents: A = dibutyl ether ethylacetate-acetic acid (15:5:1); B = dibutyl ether-acetic acid-n-hexane (20:1:4); C = 0.1M CH3COONH4 #0.1 M NH4OH in water-methanol (20%) (pH=9.20); D and E = 0.1M and 2M, respectively, CH3COOH in water-methanol (20%); F = 0.1 M acetate buffer; G = 1 M acetic acid.

bHome-made layers prepared by spreading a mixture of 20 g of silanized silica gel 60HF (Merck) with 4% N-dodecylpyridinium chloride in 50 mL of 95% ethanol.

cHome-made AG1-X4 (CH3COO~) plates prepared by mixing 2 g of the resin (200-400 mesh) and 6 g of microcrystalline cellulose in 40 mL of water.

Sources: Adapted from Gillio-Tos M, Previtera SA and Vimercati A (1964) Separation of some aromatic amines by thin-layer chromatography. Journal of Chromatography 13: 571-572; Lepri L, Desideri PG and Heinler D (1979) Soap thin-layer chromatography of sulfonamides and aromatic amines. Journal ofChromatography 169: 271-278; Lepri L, Desideri PG and Coas V (1974) Chromatographic and electrophoretic behaviour of primary aromatic amines on anion-exchange thin layers. Journal of Chromatography 90: 331 -339.

were correlated with their RM values using benzene-ethyl acetate-acetic acid (2 + 2 + 1 v/v) as mobile phase. Separations via charge-transfer complexes with nitro compounds (picric acid, 2,4,6-trinitrophenyl-N-methylnitramine and 2,4-dinitroch-lorobenzene) have also been reported.

Reversed-phase planar chromatography has been performed on silanized silica gel untreated or impregnated with cationic and anionic surfactants. The aromatic amines, which are in the free base form at the pH of the eluent, exhibit a high affinity towards the silanized silica gel and are more strongly retained in the presence of N-dodecylpyridinium chloride on the stationary phase.

As the pH of the eluent decreases, a sharp increase in the RF values is observed on the impregnated layers (see Table 4, columns 3-5). Such behaviour is correlated with the protonation of one or more of the amino groups present in the aromatic amines. On the basis of hRF values many interesting separations of isomers can be carried out.

Primary aromatic amines can be separated, with difficulty, on polystyrene-based cation exchangers in aqueous-organic solutions and also by elution with concentrated mineral acids owing to the high affinity of such exchangers towards compounds which contain one or more aromatic nuclei. Therefore weak cation exchangers, i.e., carboxymethylcel-lulose (H+ or Na+ form) and alginic acid, or synthetic inorganic exchangers such as ammonium mo-lybdophosphate and tungstophosphate, have been used for separating such compounds. Better results can be achieved using polystyrene-based anion exchangers as shown by the hRF values obtained on AG-1X4 (CH3COO) plates (see Table 4, columns 6 and 7).

As regards the influence of the pH of the eluent, not that the protonated forms of the amines exhibit a lower affinity towards the exchanger than the free base forms.

An equation similar to that suggested for alkaloids can be used for studying quantitatively the influence of eluent pH on the chromatographic characteristics of aromatic amines:

where Ka is the dissociation constant of conjugated acid of the base and RFac and RFalk are the RF values of the protonated and the free base form of the amines, respectively.

The detection of aromatic amines has been accomplished with fluorescamine in glacial acetic acid (1mgmL~1), 5% p-dimethylaminobenzaldehyde in a 5 : 1 mixture of ethanol and glacial acetic acid, 0.2% chloranil in chlorobenzene or 9-chloroacridine in 95% ethanol.

Diazotization and coupling can be carried out directly on the layer, i.e., the plates can be exposed to nitrogen dioxide to diazotize the amines and then sprayed with a solution of 0.1 M ft-naphthol and 0.1 M triethylamine in benzene.

Derivatives of aromatic amines have also been used for separating and identifying these compounds. Therefore, 2,4-dinitrophenyl derivatives and dansyl derivatives have been studied on silica gel with different solvent systems.

Fifty-four aromatic amines used as antioxidants and/or antiozonants for elastomers have been separated on silica gel with a concentrating zone using benzene-ethyl acetate-acetone (100:5:1 v/v) and benzene-w-hexane (50 : 50 v/v) as eluents. The detection reagent is N-chloro-2,6-dichloro-p-benzoquinone monoimine in buffered alkaline medium.

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