ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1655
Gas-Solid Chromatographic Behavior of 65 Linear or Branched Alkenes and Alkanes (C2-Cl0) on Graphitized Thermal Carbon Black Zdzislaw Krawiec,‘ Marie-France Gonnord, and Georges Guiochon Laborafoire de Chimie Analytique Physique, €cole Polyfechnique, 9 1 128 Palaiseau Cedex, France
Jacques R. ChrCtien Insfitut de Topologie et de Dynamique des Systbmes Universife Paris VII, 1 Rue Guy-de-la-Brosse, 75005 Paris, France
The adsorption of a series of 49 linear, branched, or crowded alkenes (up to Cl0), 1,3-butadiene, and 15 of their isotopological alkanes on graphitized carbon black has been studied. Kovats indices ( I ) and their variation with temperature (dZ/d T ) , column capacity ratios ( k ’ ) , adsorption enthalpies ( A H ) , and entropies (AS)are reported. The adsorption properties of graphite in regard to the structural effects on adsorption enthalpy and entropy and to the separation of different types of isomeric or isotopological compounds are discussed.
The importance of alkenes in the petrochemical field motivates the constant interest of chromatographers in the systematic determination of Kovats’ retention indices (1-3) and their generalization by developing computerized data handling methods ( 4 , 5 ) . I t also motivates the constant interest of physical chemists for studies dealing with the reactivity of ethylenic compounds in the area of heterogeneous catalysis (6, 7 ) ,of homogeneous catalysis (8) with its developments in metathesis (9), or in the area of electrophilic addition reactions (10, 11). The reactivity of ethylenic compounds can be affected by the strains between the alkyl groups (12) or by peculiar steric effects ( 7 ) whose fundamental role decreases or cancels the potential influence of electronic effects (13). Ab initio calculations supply much information on these strains and privileged conformations but they are limited to the ideal case of isolated molecules (14). To express satisfactorily the behavior of real molecules reacting in a given medium, the development of complementary methods is required. For example, a quantitative interpretation of the rapid nucleophilic step of electrophilic bromination of methyl-substituted ethylene has been developed by using I3C NMR chemical shifts of the corresponding intermediates ( 1I ) . Graphitized Thermal Carbon Black (GTCB) is a nonspecific and homogeneous adsorbent. Gas chromatography permits the measurement of retention data at zero surface coverage. Such data have been the experimental support of the development of the molecular statistical theory of adsorption (15-17) and provide good material for a systematic study of intramolecular strain between alkyl groups and of the perturbation of the privileged conformation by an external field, independently of electronic effects in the neighborhood of the carbon-carbon double bond. Kiselev e t al. (18, 19) have studied the geometrical n-alkenes isomers by adsorption chromatography on GTCB. The aim of this work is to extend this study to branched alkenes in order to understand the general structural effects
in alkenes and their consequences in gas adsorption chromatography. For that purpose we have determined the gas chromatographic retention data of a series of 50 linear, branched, or crowded ethylenic compounds (49 alkenes plus the 1,3-butadiene) on GTCB and calculated the adsorption enthalpies and entropies and the Kovats retention indices (20). The same data were also determined for 15 isotopological alkanes to improve the evaluation of the contribution of the carbon-carbon double bond and of its environment.
THEORY AND DATA HANDLING The retention indices, differential energy, and entropy of adsorption of all the compounds studied are presented in Table I. They have been derived from experimental data as described in the Experimental part. The net retention volume, Vh., is calculated from the equation: where t R is the retention time of the compound, t M that of an unretained compound, and Fc the carrier gas outlet flow rate a t the column temperature T , and j is the James and Martin factor for ideal gases:
with P = PJP,, ratio of the inlet to the outlet column pressures. The flow rate, Fc, is calculated from the flow measured with the aid of a soap film flow-meter, FA, at ambient temperature, TA,by assuming the carrier gas is ideal, through the relat ionship:
(3) where Pw is the partial pressure of water a t TA. With these corrections, the flow is measured with an accuracy of about 1%. The specific retention volume, V,, defined as the net retention volume per adsorbent unit surface area, is thus
..
VA =
A
m-* 1
(4)
where A is the total surface area of adsorbent in the column. The Henri adsorption coefficient is then given by: kA
= RT/V,
(5)
at the low surface coverages at which we are working (around monolayer (15). The free enthalpy of adsorption a t zero coverage is given by:
On Leave from Instytut Nawozow Sztucznych, 24-110 Pulawy, Poland. 0003-2700/79/035 1- 1655$0 1.OO/O
vn. I mL x
-
AA = -RT In VA 0 1979 American Chemical
Society
(6)
1656
0
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Table I. Kovats Index at 150 'C, Kovats Index Variation with Temperature, Column Capacity Ratio, Adsorption Differential Energy and Entropy for Alkenes rind Alkanes on Graphitized Thermal Carbon Rlack (GTCR), at Low Surface Coverage - A S cal
no.
compound
I, 1 5 0 ° C dIldT
k'
-AH
kcal mol-'
mol-' K-)
bp 760 mmHgb
2 ethane 3 propylene
7.1 x lo-: 200 0 7.3 x 10-7 296 -0.010 1.8 x l o - '
3.4 3.9 5.3
18.62 19.64 21.32
-103.71 -88.60 -47.70
4 propane
300
0
lo-'
5.3
21.25
-42.10
5 1-butene
391
0.058 5.0 x l o - '
6.5
22.11
-6.26
6 n-butane
400
0
5.9 x l o - '
6.6
21.66
-0.48
7 (Z)-2-butene
406
0.015 5.8 x 10.'
7 .O
23.04
3.72
414
0.045 6.4 x l o - '
7.0
22.74
-4.41
418
0.011 6.6 x
7.3
23.35
0.88
10 3-methyl-1-butene
456
0.009 1 . 3 x
7.3
21.84
20.06
11 1-pentene
480 -0.001
7.8
22.37
29.96
1 2 (Z)-2-pentene
486
0.045 1.6 X 10"
7.8
22.46
36.94
1 3 2-methyl-2-butene
494
-0.089 1.9 X 10"
8.6
24.03
38.56
1 4 2,3-dimethyl-2-butene
495 -0.065
1.9 X 10"
8.5
23.84
55.61
1 5 3,3-dimethyl-l-butene
497
1.9
lo3'
8.4
23.63
41.24
1 6 n-pentane
500
0
1.9 x l O + O
8.2
23.11
36.041
17 2,2-dimethylbutane
527
0.196 2.6 x 10'O
7.1
20.19
49.74
1 8 2,3-dimethylbutane
553
0.054 3.5 x
8.2
22.22
57.988
1 9 4-methyl-1-pentene
555
0.042 3.7 x 10"
8.7
23.05
53.869
20 2-ethyl-1-butene
559
0.079 3.9 x
8.9
23.39
64.682
0.114 4.2 x
8.3
22.0
63.382
1 ethylene
8
1,3-butadiene
9 (E)-2-butene
-0.045
21
3-methyl-pentane
565
22
3-methyI-(Z)-2-pentene
569 -0.031
1.9 x
1.7
X
X
4.6 x
lo-' loto lo+'
lota loca loto loio loto loto
10.0
25.66
67.702
9.9
25.28
70.438
8.4
22.07
60.271
23 3-methyl-(E)-2-pentene
572
0.009 4.7 x
24
574
0.125 4.6 x
25 ( E ) -3-hexene
576
0.043 4.8 x l O + O
9.2
23.61
67.088
26 (Z)-2-hexene
576
0.030 4.8 x 10"
9.3 (9.9)Q
23.97
68.891
577 -0.041 4.8 x 10"
10.3 ( l O . O ) n
26.22
63.485
9.7
24.58
76.740
9.7
24.55
77.60
27
2-meth ylpentane
1-hexene
28 4,4-dimethyl-(E)-2-pentene
586
29 2,3,3-trimethyl-l-butene
591 -0.021
30 (E)-2-hexene
591
0.022 5.7
31 4,4-dimethyl-l-pentene
597
0.119 6.1 x 10+O
8.6
21.95
72.51
32 4,4-dimethyl-(Z)-a-pentene
600
0.130 6.4 x
9.5
23.90
80.430
33 n-hexane
600
0
lo+" lo+'
34 2,3-dimethyl-l-pentene
613
35 2,2-dimethyl-pentane
0.028 5.3 x 10+O 5.7 x 10" X
10"
9 . 6 (10.2)* 24.35
67.884
9.9 (10.3)" 24.73
68.74
0.045 7.4 x l O + O
9.2
23.05
84.28
615
0.234 7.4 x 10+O
8.6
21.63
79.197
36 3-methyl-2-ethyl-1-butene
619
0.051 8.0 x
9.9
24.29
86.365
37
643
0.058 1.1 x 10"
9.8
23.79
89.5
5-methyl-(Z)-2-hexene
6.4
X
l O + O
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1657
Table I (Continued) - A S cal
no.
graph
compound
38 5-methyl-1-hexene
=--.-*--.-a
i a,.--.-aG a
39 2,3,3-trimethyl-l-pentene 40
5-methyl-(E)-2-hexene
41 2-methylhexane 42
a-a-a -0
a= ,'
a-a-*-a-a::
/*\
=-
1-heptene
43 2,3,4-trimethyl-2-pentene 44
I, 150 "C dI/dT
-..q 'a--.
3,3-dimethyl-2-ethyl-l-butene
45 5,5-dimethyl-(Z)-2-hexene
*\=/*-*
