Gas-chromatographic separation of aromatic amines, nitriles and hydrocarbons—I

Tuhlu.

Vol

22. pp

I?I- 12X Prrg.tmon

Press. 1975 Prmted

m Great Bntam

GAS-CHROMATOGRAPHTC SEPARATION OF AROMATIC AMINES, NITRILES AND HYDROCARBONS-I SABRI

M.

FARROHA

and

College of Science, Umversity (Rece~red 21 Ma)

SAMIR S. EMEISH of Baghdad,

1973. .4cceptrd 25 March

Iraq 1974)

Summary~A mixture of mono- and di-substttuted nitriles was separated on SIX different liquid phases, but a mtxture of mono- and di-substituted antlines was separated on only two liqutd phases. The forces mfluencing the acparatton and elution order of these solutes are drscussed. The absence of selecttve interactions between aromatic hydrocarbons and the varrous liquid phases was proved thermodynamically. An ortlto methyl substituent increases the log of the specific retention volumes of the nitrile solutes by an approximately constant amount.

Gas-chromatographic separations of aliphatic nitriles have been reported by many workers.‘,’ The behaviour of tolunitriles was studied on different liquid phases.3*4 A quantitative study of the dimerization of nitriles and the consequent effect on gas-chromatographic soluteesolvent interactions have been reported.“,’ The behaviour of aromatic amines was studied by James,’ and others have reported resolution of substituted aromatic amines on various liquid phases.8,9 Aromatic hydrocarbons have been extensively studied and several liquid phases found to be selective (e.g., the tetrahalophthalate esters).“.’ ’ Janak and Hi-ivnat have explained several cases of aromatic selectivity in terms of 7r-interactions.” In the present work, the factors influencing the elution characteristics and resolution of mixtures of aromatic hydrocarbon, aniline and aromatic nitriles are discussed. EXPERIMENTAL

The bulk of the experimental work was performed with a Beckman CC-45 gas chromatograph equipped with a thermal-conductivity detector. An optimum nitrogen flow-rate of 25 ml/min at NTP was used. A current of 125 mA and an attenuation of 8 was found suttable for the best experimental peak shape and height. Columns. from Perkin-Elmer, were made in coils of stainless-steel tubing (2 m long and 3 mm o.d). The compositton of the hquid phases and the packing specifications of the columns are shown m Table I.

Table 2 summartzes some of the physical properties of the materials studred, whtch were obtained from Fluka AC (aromatic hydrocarbons and aromatic nitriles) and Hopkm and Williams Ltd. (aromatic ammes).

Mixtures of equal weights of each of the aromatic hydrocarbons, mtriles and ammes were blended in 3-ml cylindricnl Pyrex glass cells. Samples ranging from 0.5 to 0.7 ~1 for the aromatic amines and nitriles and 0.2 ~1 from the aromatic hydrocarbons were injected wtth a I-PI Hamilton syringe.

The spectfic rctcntton volumes ky were calculated by the method of Littlewood et ul.’ ’ and are gtven m Table 3 Plots of log Vf VS. I/T were generally linear. No difference was noticed m the retention ttme of the components whether they were injected indtvtdually or in a blend. and the results were reproducible. 112I

I?’

SABRI

Table

M. FARROHA and SAMIR S. EMUSH I. Speclficatlons

of llquld phases

Liquid phase 1.2.3.-Trls(2-cyano-ethoxy)-propane on Chromosorb W AM-DMCS (TC‘EP) Carhowa\ hO00 + sodium dodecylbenzene sulphonate on Chromosorb P (C‘DBS) Benzldme on Chromosorb W Poly(propylene glycol) (LB-550-x) on Chromosorb W (PPG) Dmonyl phthalate on Chromosorb W (DNP) DI-n-decyl phthalate on Chromosorb P (DPH) Tritely1 phosphate on Chromosorb W (TTP) Sillcone oil MS 550 on Chromosorb W (SO) SIllcone gum rubber E30, on Chromosorb G AW-DMCS Aplezon L + Bentone 34 on Chromosorb W (APL-34)

The activity

coeffeclent

Weight

O0 w/w

of the solute m the solvent at infimte dllutlon.

of phase y

0.2

8 25 I0 IS IO 15 IO IO 1.5 30

I.61 0.15 0.37 ti25 0 72 0.2 0.25 0 IO7 04x

y’, was calculated’s~‘O

from the equation

where bl is the molecular weight of the hquld phase and P; IS the vapour pressure of the solute The excess partial molar free energy AC:,. enthalpy AH:. and entropy Asp, of mixing at infimte dilution calculated from the equations. A[;” z RT ln II” ,

Table 2. Physlcal

(‘1

properties*

Dipole moment

Electron polarizablhty. c1113x IO”

CdC..

Compound Benzene Toluene Iii-Xylene Mesitylene Benzonitrile cl-Tolunitrile ,Ir-Tolunitrlle IT-Tolumtrde 7.5.Dlmethylbenzonitrile 2,4-Dimethylbenzomtrile 2.3-Dimethylbenzonitrile Aniline o-Toluidine o-Chloroamlme In-Chloroanilme /Khloroamlme 6-Chloro-2 methylamlme 5-Chloro-7-mrthylai~llnt: 4-Chloro-2-methylanilme

B.p..

C

Dehr I‘L’ 0.00 0.37 0.37 000

I.501 12”’ 1.49693”’ 1.4977? 1.49937’0

I 5’65

190.7 204.0 1140 217.0 104/1X mmHg ll2~12mmHg I 12125 mmHg

4.39 4.22 4.59 4 76 4.39 441 143

I.5189 I.5191 15196 I.5211 I.5255 I.5211 I.5178

I.5657 1 5665

184.4 199.8 209.0 230.0 23 I ,o 215.0 237 0 241 0

I ,4x I.34 I.71 2.75 3.17 I.77 3.24 3 37

I 5863

I 7084 I 6787 I7151 I 7246

80.1 1 IO.6 139 I 164.7

were

* The physical properties were collected from different sources.‘3.‘J.’ accordmg to Smythlh and /I;” values were measured at this laboratory

1~5170 1.51X6 I.5222

1.5674 I 5709 I ,5x03 I 5705 I ,563-l

I 5776 1~5x95 I 5942

I 6707

I.5679 I.5759 1.5755 ’ The dipole

16x73 I ,6X66 moments

were calculated

Separation

of aromatic

Table 3. Specific retention Solutes

)

mtriles and hydrocarbons

I??

Vf (ml/g) of solutes on the liquid phases studied

PPG

APL

DNP

1x0, (90, (100, (110)I90

(100)

(90, (100 ) (II” 1 (80 ) 43 40 32 51 5x 54 42 103 X7 16 56 210 I30 IOX 78 438

(YO ) (loo ) (80 I 39 34 57 73 61 119 146 114 256 290 220 572

(90, 47 91 190 407

,,?O,,130 ) ,140,

,110, 332 499 6”X 676

(IZO) 123 325 396 436

(IOU) 657 II00 141” I5K”

(II0 b (120, 425 294 692 461 XXII 512 9x1 636

TTP

+ 34

40 76 150 309

32 67 I47 341

28 52 I,,, 242

23 41 Xl 173

I9 30 59 II6

) (IO+ 1 (IlO, 77 63 50 115 101 75 204 I7h I?4 396 365 ?II

439 508 62 75X

2998 34X 431 506

142 lb3 I95 226

(loo, 416 726 x97 1012

II71

75X

506

226

13x8

42”

583

2348

1420

902

5%

x34

509

318

1359

40x

5113

1356

X69

5x1

152

1577

IU34

649

2629

15x6

I”“7

656

I JOY

7”5

437

1359

NIX

504

672

294

1799

II69

735

29X3

7U.l

10x4

651

4U9

1635

945

6”:

1140 ) ,150) II8 X? I75 IIY 141 I65

(16(l) 62 x5 II7

,110, 652 775 960 I171

A”lll”e r,-T”luld,ne o-Chlorodnd,ne 6.Chloro-2. methylamlme ,,i-Chloroamlme p-Chloro.m,lme 5.Chloro-2. methylandme 4.Chloro-2. mcthylandme

volume

TCEP (X0 4x 14 ,,I I55

amines,

1592

lOI6

(120, x35 I “J? I.117

1130) 531 655 X38

(IJO) 137 28’) 257

1472 3675 3675

929 223 2x3

399 920 920

,111, l,l21,, 624 39X Y”6 59, ,619 YX9 23x4 3703 3921

1130, (IOU, 215 451 396 813 h-15 l2Y5

182X 2155 2274

XJI 1342 1411

2237 ?XX4 2xX4

,7X,

II25

(11”) 301 524 XIX

(130, 20x 362 57X

I307 1713 1713

X6, 1100 1100

1130, 105 314 390 42’)

1l”U I III0 111x I 1l"U I(110 1 (l2U ) 265 1713 II4 403 261 IX4 434 114 Ihl 629 3Xh 270 4x4 3(1? 1’)s X6X 535 356 564 34X 2Xx X6X 535 356

2x3 561 632

?I)(, 34x 3X4

, I UU , , I I” ) , I ?I1 , 453 311 2”Y 792 510 332 I409 x5x 540

IJX 23x 2?5

2126 379Y 3799

I?44 2139 ?l3Y

774 1253 1253

4657

2787

II35

52X8

3043

19U3

5124

297X

IX66

966

480

319

623’)

33?U

1732

4657

27x7

II35

5573

3225

1977

5124

297x

IX66

920

572

377

623’)

3320

1732

DISCUSSION

Resolution of the mixture of aromatic nitriles was achieved on six out of the ten liquid phases used; these phases were Benzidine, TCEP, CDES, PPG, DNP and APL-34, and the order of the peaks was benzonitrile, o-, HI-,p-tolunitriles, 2,5-, 2,4- and 2,3_dimethylbenzonitriles (except that 2,3-DBN emerges before 2,4-DBN from APL-34). The elution order Table 3. co~ltirwd SOlUteS

c,-Tolunmde r,,-Tulun~lrde p-Tolumtr~le 2.5.Dlmcthllbenromtrde 2.4.DImethyl hcnzomtrk 2.3.Dtmrfhylhenzonltrdc A!llllne <,-Toluldme wChloroamlme h-Chloro-lmrthgldndme ttr-Chloroandlne p-Chloroamlme 5-Chloro-2. mrth~lamlme J-Chloro-?methyldndmr

Benzldme

1110 1 592 803 1078 I .?hX I423

(x0 I

208

274 274

435 557 557

(90 IX7 306 391 391

703

405

904

62X

43x

2.50

I77

755

423

9VI

662

JSS

‘Xi

2111

,,Wh

71,,

5”

122

230

15’)

(120, 396 534 701 xxx

1130, ?7K 366 4?5 550

, 1U” 1 ??U 351 439 457

604 726

916

1277

xx

190 1 35 53 XX 15x

,100

26 44 7Y 132

,

?+I

(IOU) 40 54 KU I34

I

11110 135 219 270 270

X69

49x

1110) 61X X67 I”58

(9” I 255 44X 65U

(I”” , ?I).? 351 50')

290

435 6”Y

3”6 4UX

236 303

1413 2896 303x

II42 I?11 I.?12

x52 940 94”

506 506 506

I044 IO.14 III3

696 696 764

4X9 4XY 512

x71

194x

1257

X5’)

x71

,94X

I.?57

X59

4642

I774

4642

239

16x6

1110) I?5 ?UX

19” l ~100~ I4 II ?? IX ill ?I 72 5h

DPH

,110, 141

40

29

CDBS

190) 44 61 IO2 I63

WI I 45 h3 IIX 235

L_

SGR IXU) 4x h9 I?4 231

27 35 51

I754 2012

SO

) 1120

, (WI, ,I00, ?44 I47 135

,X0

I27 I63 IV5 220

I (13~) 97 II6 IJO 155

1110, 9 I4 21 39 1140 67 x3 9x I IO

IX”, 49 X9 IX? 3x5

(9U, 44 74 I44 2x0

1100) 35 if> I”4 I’)1

j II(X) )(IIO 1 21 3x5 529 529

I56 246 3x 32X

I24

927

579

I39

927

57’)

927

579

124

SABRI M. FAKROHAand SAMIRS. EMHSH

of these solutes is in accord with the order of their increasing dipole moments. While the elution order of the tolunitriles agrees with the increasing order of their vapour pressure and electron polarizability, the elution order of the dimethylbenzonitriles is the reverse of their order of both volatility and electron polarizability. The mixture of anilines was separated on only two liquid phases (PPG and APL-34). in the elution order: aniline, o-toluidine, o-chloroaniline. 6-chloro-2-methylaniline. 11~ chloroaniline, p-chloroaniline, 5-chloro-2-methylaniline and 4-chloro-3-methylaniline. The components are eluted in order of decreasing vapour pressure and increasing dipole moment. Theoretically, nitriles would be expected to have longer retention times than anilines,“-‘3 but the results obtained showed the contrary. The anilines have higher C’y values than nitriles or any liquid phase, and the difference in IJ’~values between aniline; and nitriles is greatest on PPG and lowest on SGR. The vapour pressure of the anilines and the nitriles cannot be considered as contributing to the observed results, because benzonitrile (b.p. 190.7”) was eluted before aniline (b.p. 184.4’) and o-tolunitrile (b.p. 204’) emerged before ~-toluidine (b.p. 199.8”). However, the results are explicable in terms of hydrogen-bonding between the aromatic amines and nitriles and the liquid phases. The effect of hydrogen-bonding is predominent on PPG but is unimportant on SGR. so the difference in Vz values for amines and nitriles is large on PPG and small on SGR. Thus. the aniline and benzonitrile have the same Vg value on SGR: o-toluidine and u-tolllnitrile also have identical Vz values. The factor dominating the order of elution of the aromatic hydrocarbons is the vapour pressure. These solutes are preferentially retarded on TTP and DNP but this is not due to n-interactions for the following reasons. The liquid phases CDBS, benzidine and DPH contain aromatic rings but the specific retention volumes on CDBS are lower than those obtained on any other liquid phase and lower on benzidine than on APL-34. If 7c-interactions were involved, the excess partial molar enthalpies of solution would be negative. but on TTP or DNP (see Table 5) positive enthalpies of solution were obtained. It has been pointed out by Littlewood 24 that n-interactions are not involved in the solution of atomatic hydrocarbons in DNP, TTP, TCEP or BDP. Furthermore, Maczek and Phillips” have shown that the order of elution of benzene and n-octene is the same on all these liquid phases.

The excess partial molar thermodynamic quantities for the aromatic hydrocarbons and some anilines (o-toluidine and o-, m- and p-chloroanilines) have been calculated. It is to be noted that these quantities are susceptible to many uncertaintiesZ6-‘” but they are useful for purposes of comparison. The vapour pressures of the solutes were calculated by using the Antoine equation and are listed in Table 4. The activity coefficients for the solute at infinite dilution in DNP. PPG and TTP were calculated by using equation (3) and are tabulated in Table 5. The activity coefficient in different solvents and at any temperature increases as the number of methyl groups increases. thus: mesitylene > i?t-xylene > toluene > benzene. o-Toluidine has a higher :‘Ovalue than the chloroanilines in all solvents and at all temperatures. The higher 7’ and AGy values of the anilines in TTP and DNP compared with those in PPG are due to the increased x-electron density in the ring systems in the first two liquid phases.‘h

Actw~ty coeffictent ,ve(on TTP)

80 90 0592 0.543

0796 @784 0678

80 90 IO0 w90 0 824

1037 1041 D9Oa

1173 I 101 v975

0948 0849 0746

Actw~ly coefficrcnt j+e(on DNP)

80 90 100

80 90 loa

Vapour pressure. rnmHy 291 2 407 7 556 3

Aroma&c T&WI%? 757 7 lO2lO 13.505

Temp. OC

I IO I?0

I 743 1 iii

I 376 I I72

IlO I20 I30

I IO IX 130

I10 11t1 I 10

(

1 567 1 559 I 394

I 57x 1461 I 305

45 I hX.4 IOi 0

MeSltylUE

1 281( 1 286 I ?I?

I 391 I ?I? 1212

1142 1646 223 6

hydrocarbon m”xylsne

I hl7 I 707

0 574 v 625 0 630

OX10 xi22 OX61 2 146 - ‘Oh

0 943 a 993 0988

33 3 so I 761

I I71 I 229 1229

42 2 63 I 915

Table 4. Vapour pressures and activity coefficients at various temperatures

I 549 I 550

0 565 OW? 0637

0953 0 998 IO27

148 238 3b3

0647 0694 0696

1 I56 1211 I173

123 196 315

126

SABRI

Table 5. Thermodynamic

80

M. FARROHA and SAMIR S.

data for aromatic

90~

hydrocarbons,

AC, “, cul/nlolr 100‘ 110

120

EMEISH

and monosubstituted

anilines

on drfferent phases

130’

on DNP Benzene Toluene m-Xylene Mesrtylene o-Toluidine o-Chloroaniline nr-Chloroamlme p-Chloroamlme

-37 -118 -217 113 69 -19 221 188 142 273 197 320

Benzene Toluene in-Xylene Mesitylene o-Tolmdine o-Chloroaniline wChloroanrline p-Chloroamline

-160 -175 -288 29 -72 26 177 182 142 315 320 246

Benzene Toluene rn-Xylene Mesitylene o-Tolmdme o-Chloroanilme m-Chloroamlme p-Chloroamline

-378 -7 230 400

8 15 -I? 4.3 47

2.x I

161 1 76 I .96 120 -45 -37 110

161 -6 7 L6

165 -10 3 I’X

-

-4.1 _ 39 -35 -39

I 45 I.54 I.38 I.39

on PPG 1.5 -03 06 11.6

0.39 -0.10 0.03 0.13 -160 -422 -434 -331

-153 -361 -393 -284

-120 -370 -361 -290

- 044 -2.56 - 2.60 -2.14

-07 - 5.6 -4 I -4.7

on TTP -452 -143 II8 167

2.10 4.67 4Ui X.40 591 365 2x7 423

618 417 342 493

7.0 1’9 IO 6 22 0 - 0.85 - I.62 -I 81 - I.50

- 3.1 -52 -55 -50

The excess partial molar free energies, enthalpies and entropies of mixing are given in Table 5. The enthalpy of mixing of the aromatic hydrocarbons in any solvent is always positive, implying the absence of selective interactions, while it is always negative when anilines are involved, which results from the strong specific soluteesolvent interaction. The efict of an o- or m-methyl substituerlt OHreterltiorl ~ohme The effect of a substituent in an aromatic ring on the specific retention volume is a reflection of the solution properties involved in gas chromatography. The effect of a methyl group ortho to a nitrile or an amine group is demonstrated by comparing the logarithms of the specific retention volumes of the nitriles and anilines. The pairs used were: Benzonitrile m-Tolunitrile p-Tolunitrile Aniline o-Chloroaniline rrl-Chloroaniline I?-Chloroaniline

o-Tolunitrile 2.5-Dimethylbenzonitrile 3,4-Dimethylbenzonitrile o-Toluidine 6-Chloro-2-methylaniline 5-Chloro-2-methylaniline 4-Chloro-2-methylaniline

The results show that there is always an increase in the specific retention volume of the component with an o-methyl group. The differences in the logarithms of the specific retention volumes of the pairs of solutes are listed in Table 6.

Separation

of aromatlc

amines,

mtriles and hydrocarbons

137

Table 6. Llquld phase

T. “C

a

h

c

d

r

f

9

DNP

100 I10 120 130

0.224 0.211 0.196 0.186

0.221 0.208 0.197 0.184

0.22 1 0.208 0,199 0 190

0.269 0.242 0.240

0.239 0.204 0.204

0.150 0.240 0.269

0.150 0.240 0.269

PPG

100 110 120 130

0.185 0 176 0, I63

0.189 0, I77 0.169

0.193 0.185 0.173

0.162 0,176 0,159

0,168 0,267 0293

0.155 0150 0,152

0.153 0.152 0.146

SGR

80 90 100

0.252 0.214 0.21 I

0.21 I 0.206 0.21 I

0.25 I 0,229 0.227

0.252 0.204 0.243

0.234 @233 0.207

0.27 1 0.256 0.245

0.243 0.216 0.216

so

90 100 110

0 204 0,167

0.204 0. I 70

0.219 0.189

0.245 0.240 0.222

0.245 0.223 0.25 1

0.265 0.249 0.236

0.267 0.254 0.236

TTP

100 110 120

0,194 0.169 0.166

0.195 0179 0.150

0,195 0.179 0.228

0.243 0,215 0,201

0,179 0.161 0, I56

0.215 0.191 0.142

0.215 0.191 0 I42

TCEP

100 120 130 140 150

0.075 0,063 0.070 0.060

0.086 0.08 I 0,069 0,065

0,064 0.060 0.060 0.048

0.096 0.09 1 0.088 0.08 I

O+I48 0.045 0.048 0.048

0.103 0.098 0.09 1 0.087

0.103 0.098 0.09 I 0.087

100 110 120 130 140 150 160

0.215 0204 0,199 0, I76

0.236 0.225 0.213 0.201

0.330 0.307 0.181 0,159 0.170 0.160 0.133

0.07 1 0.096 0.103

0.135 0.140 0.127

0.163 0.173 0168

Benzldme

I10 120 130

0.133 0.131 0120

0,121 0,116 0.104

0.141 0.097 0.121

CDBS

120 130 140

0.106 0,098 0,089

0.109 0.104 0.102

0.113 0.111 0 102

Apiezon L+B

([f) = log (h) = log (c) = log (d) = log (e) = log (f) = log (Q) = log

1 F (o-tolunitrile) - log VF (benzonitrile). Vf (2.5-DBN) - log VI (m-tolunitrile). Vz (2,4-DBN) - log Vx (p-tolunitrile), Vz (o-tolumtrile) - log Vg (aniline). Vz (6-chloro-2-methylanihne) - log Vt (o-chloroaniline) Vz (5-chloro-2-methylaniline) - log 1’: (m-chloroanilme). “‘z (4-chloro-2-methylaniline) - log VI (p-chloroandme).

The increase in the logarithm of the specific retention volume of a solute containing a cyano group when a methyl group is substituted in the ortho position is approximately constant. It has been suggested that the nitrile group does not suffer from steric interferences because.of its cylindrical shape, and the same constancy is not shown by the differences in the specific retention volumes of the analogous aniline pairs. In Table 7. the minimum and maximum difference values of columns a, b and c at the temperatures of experiment on each liquid phase are quoted. The lowest minimum differ-

128

SABRI M. FARROHA and SAMIR SEMEEISH Table 7.

Liquid phase DNP PPG SGR so Apiezon L + B TTP CDBS Benzidine TCEP

Temp. range, c 100-130 100-1’0 X&l00 90-l 10 IO@130 lo&l20 120-140 1l&130 1I@140

Minimum difference value

Maximum difference value

0.184 0, I63 0 206 0,167 0.159 0.150 0.089 0.097 0,048

0.224 0 193 0,252 0,219 0.330 0.228 0. I 09 0. I 4 I 0.086

ence value is 0.048 obtained on TCEP in the temperature range 1l&140” and the highest maximum difference value is 0.330 obtained on Apiezon L + Bentone 34 in the temperature range 1O& 130”. Ackrlowlrrlyelnellt~-We are extremely grateful to Prof. A. E. Habboush, for her Interest and helpful suggestions and discussion of the results, and are Indebted to the head and staff of the Chemistry Department. College of Science. Umversity of Baghdad for providing necessary requirements during the course of work. REFERENCES and G. Gambh, Chim. It7d.. 1963, 45, 44. 2. M. Taramasso and A. Guerca. J. Gus Chromutog, 1963.3, 417 3. A. E. Habboush and R. C. 0. Norman, J. Chromatog.. 1963, 7, 43X. 4 A. E. Habboush and A. Tameesh. hid, 1970.53, 151. 5. A M. Saum, J. Polymer Ser.. 1960,42, 57 6. A. N. Genkm. B. 1. Boguslavskaya. L. S. Bresler and M. S. Nemtsov. Dokl. Akad. Nnuk SSSR (Englrsh Tranal.). 1965, 164, 1089. 7. A. T James. Allal Chrrt~., 1956, 28, 1364. 8. H. G. Henkel. J. Gas Cltrornutog.. 1965, 3, 320. 9. A. R. Cooper. C. P. Crowne and P. G. Farell. J. Chromutog., 1967, 29, 15. IO S H. Langer. C. Zahn and J. Pantazoploz. hid., 1960,3, 154. I I H Pines and C T. Chen, J. A/n. Churn. Sot.. 1960. 82, 3562. 12 J. Janak and M. Hiivnac”_ J. Chromutog . 1960, 3, 3562. 13. Handbook of Chemistry and Physics, 51 st Ed.. ed. R. C. West, Chemical Rubber Co., Cleveland, Ohio, 197s 71. 14. Dictionary of Organic Compounds. 4th Ed., eds. J A. Pollok and R. Stevens. Spon, London. 1900. 15. J. W. Baker and J Gaunt, J. Cheat Sot., 1949, 9. 16 C P. Smyth. Dirkcrr~c &harrow und Structure, p. 333. McGraw-Hill, New York. 1955. 17. A. B Littlewood, Gus Chromutographp. p 3 1. Academic Press, New York, 1962. IX. A B Littlewood, C S G. Phillips and D Price. J C’hrri7. Sot., 1955. 1480. 19. M. R. Hoare and J. H Purnell, Trurzs. Faruduy SM., 1966,52,222. 20. D. E. Martire and L. Z. Pollara, in Adruwrs !?I Chromurogruplly. Vol. 1. eds. J. C. Giddings and R A. Keller. Arnold, London, 1965. 21. W. A Sheppard, The Clwrmstry ofFur~ctror~al Groups. Interscience, London, 1970. 22. G J. Kormek and W. G. Schneider, Curl. J Chrm.. 1957, 35, 1157. 23. W G Schneider. J Cltrra. P11ys.. 1955. 23, 26. 24. A. B. Littlewood. J Cu.\ Chromatog . 1963. I, 16. 25. A. 0. S. Maczek and C. S. G. Phillips, J. Chrornutog., 1967, 29, 15. 26. S. H. Langer and J. H. Purnell, J. Ph.rs. Cheat., 1963, 67, 263. 27. R. L Martin, Anal. Chrm , 1963. 25, 116 28. C W. P. Crowne and M. F. Harper, J. Chromzrog., 1971.61, 1 29. D. E. Marttre. .4nul. Chrrn., 1966. 38, 244.

1 E. Muguaini