Correlation between Electron Capture Response and Chemical Structure for Alkyl Halides Tsugio Kojima* and Yasukazu Tanaka Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Yosida, Kyoto, Japan
Masaru Satouchi Department of Industrial Chemistry, Shiga Prefectural Junior College, Hikone, Shiga, Japan
The dissociative electron capture reaction for alkyl halldes was measured by use of an electron capture detector and the activation energy was obtained by means of the Arrhenlus plot of the electron capture coefficlent. The free electron In this reaction can be regarded as a nucleophlle and the halide anion as a leaving group; then It is conslderedthat the dlssoclative electron capture reaction proceeds by a mechanism similar to bimolecular nucleophlllc substitution (S N ~ )The . relationship between the chemical structure and the activation energy for alkyl halides can be completely explained by assuming the ~ A gas chromatographtransition state of an S N reaction. electron capture detector system can afford the retentlon Index and the activation energy simultaneously, and both closely relate to the chemical structure of the sample molecule. Therefore, it can be expected to facilitate the qualitative analysis of alkyl halides by measuring these values.
a sample molecule, variations in these energies can be explained in terms of the stabilization of the anion radical or the transition state through the inductive and resonance effects, and the molecular strain. Since the effects are closely correlated with the chemical structure of the sample molecule, the electron affinity and the activation energy obtained could be used as information for identification. It has been proved that the structure of various compounds can be estimated by measuring the electron affinity and the activation energy (3-5). Although an electron capture detector is constructed from very simple units compared with other instruments for identification, it functions not only as a highly sensitive detector but also as a qualitative detector. This paper describes the correlation between the activation energy and the chemical ,structure of haloalkanes on the basis of the transition state model analogous to that in an SN2 reaction and proposes its applicability to qualitative analysis.
Various organic compounds, containing a halogen, oxygen, or sulfur atom, and polynuclear aromatic hydrocarbons exhibit strong electron capture characteristics. By taking advantage of this property, the gas chromatographic trace analysis of a wide variety of environmental pollutants such as poly(chlorobiphenyl), alkyl mercuric chloride, and peroxyacetyl nitrate became possible with an electron capture detector connected to a chromatographic column. The principle of this detection method is summarized as follows. The background current I b which is the current observed when the detector is operating with no solute passing through it, is proportional to the concentration of secondary electrons produced through inelastic and elastic collisions between primary electrons from the P-source and molecules or atoms of carrier gas. When an electron-capturing solute entering into the detector captures a near-thermal secondary electron, the current across the cell decreases to I , because of the consumption of the free electron, and the difference in the detector current, I b - I,, is observed as the output of the detector on the recorder. Wentworth and co-workers ( 1 , 2 ) have described the electron attachment phenomenon on the basis of kinetic derivations using the steady-state approximation and related the electron capture response to the concentration of a capturing species in terms of the electron capture coefficient K . According to Wentworth's theory, the molecular electron affinity or the activation energy for an electron attachment accompanying a bond dissociation is obtained from K . A free electron in an electron capture detector can be regarded as the simplest nucleophile or radical, and then it is possible to consider that the electron attachment is a kind of organic reaction between the simplest reagent and a reactive substrate, both suspended in an inert gas. Since the electron affinity or the activation energy is the potential energy difference between the neutral state and the anion radical of a sample molecule or the electron-attached transition state of
EXPERIMENTAL
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The gas chromatograph used in this investigation was a modified Shimadzu GC-2C model. The electron capture detector employed was a concentric type with 15 mCi nickel-63 as a radioactive source. Applied voltage was supplied as a pulse through a pulse generator with an amplitude of 28 V, a pulse width of 3.2 ps and a pulse time of 3200 ps, and then electron capture reaction proceeded under field-free conditions. Glass columns (0.4 cm X 240 cm) were packed with Shimalite W 80/100 mesh, coated with 15%Apiezon Lor 20% TCP for separation of the chlorides and the bromides and their temperatures were maintained at 80 "C or 100 "C. For the iodides, a glass column (0.4 cm X 100 cm) was packed with Durapak (Carbowax 400Porasil C) and its temperature was maintained at 70 O C . Nitrogen was used as a carrier gas. In most cases, methane at a concentration of 5-10% is added to argon as a quenching gas for thermalization of fast electrons. Using methane, Wentworth et al. ( 6 )estimated that an electron with an energy of 10 keV was cooled to 10%above thermal energies (2-5 X eV) in 0.076 ws. Although the activation energies obtained in this study were somewhat smaller than those with argon-methane by Wentworth, it would not be unreasonable to assume that in the time of 3200 ps with no voltage applied, a near-thermal distribution could be obtained. Since van de Wiel and Tommassen have described that an oxygen molecule as an impurity in nitrogen gas captures a free electron (7), the oxygen in the carrier gas was removed by passing it through an absorption tube packed with cuprous chromate pellets. When this tube is inserted between the cylinder of the carrier gas and the GC inlet, an increase in the detector current is observed. Another tube packed with Molecular Sieve 5A was used for the removal of moisture.
The temperature of the detector cell was measured by insertion of a thermocouple in the detector bath. The reagents used were all commercially available and some of them were redistilled before use. The sample solution was prepared by dilution with pentane, hexane, or benzene to a concentration such that the detector current decreases to about a half of the background current. The electron capture coefficient K was calculated from the following equation derived by Wentworth (1,2):
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
20
15 2\
X
10
[I -J
E
2.0
1.5
1 x T
lo3
2.5 (deg-')
Figure 1. Arrhenius plots of electron capture coefficients for alkyl chlorides
Table I. Activation Energies E* for Alkyl Halides Compound E* 10.9 k 0.3" Isopropyl chloride n-Butyl chloride 12.1 t 0.3 11.5 f 0.3 sec-Butyl chloride 10.0 t 0.3 tert-Butyl chloride 10.8 t 0.3 n-Pentyl chloride 9.1 t 0.3 tertPenty1 chloride 10.7 f 0.3 n-Hexyl chloride 9.6 ?r 0.3 Cyclohexyl chloride 6.4 k 0.3 n-Propyl bromide 5.6 k 0.3 Isopropyl bromide 5.7 f 0.3 n-Butyl bromide 4.6 k 0.3 sec-Butyl bromide 2.4 ?: 0.3 tert-Butyl bromide 5.8 * 0.3 nPenty1 bromide 3.6 f 0.3 sec-Pentyl bromide 1.6 f 0.3 tert-Pentyl bromide 1.8 f 0.3 Ethyl iodide 2.0 t 0.3 n-Propyl iodide 2.3 k 0.3 n-Butyl iodide 3.6 ?: 0.1 Benzyl chloride 3.4 f 0.2 m -Methylbenzyl chloride p-Methylbenzyl chloride 3.2 t 0.2 in kcal/mol 0 Standard deviation,
(1) Isopropyl chloride, (2) n-butyl chloride, (3) sec-butyl chloride, (4) teffbutyl chloride, (5) n-pentyl chloride, (6) ted-pentyl chloride, (7) +hexyl chloride, (8) cyclohexyl chloride
sample molecule and a dissociative reaction where an anion and a radical are produced ( 1 , 2 ) . 2(
AB
+ e-
= AB- (nondissociative)
AB
+ e-
=A
+ B-
(dissociative)
(1) (2)
Wentworth and co-workers have correlated the electron capture coefficient K to the electron affinity of a sample molecule in the nondissociative reaction or the activation energy for the dissociative reaction as expressed in the following equations: In KT3/2 = Z In K = Z
51
20
1.5 L
T
x lo3
2.5 (de secondary > tertiary and in the order of chloride > bromide > iodide. This order is the same as that of the dissociation energy for the carbon-halogen bond and is contrary to the order of electron affinity for the halogen atom. This fact shows that the activation energy is predominantly determined by the bond dissociation energy. Since the magnitude of the activation energy is the measure of the sensitivity of an electron capture detector to the sample molecule, it will be concluded that if the dissociation energy
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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I
Figure 3.
-
II
Resonance structure in transition state
of the A-B bond is low, a sample molecule AB containing an electrophilic group B may be detected by an electron capture detector with a high sensitivity even if the electron affinity of group B is low. Wentworth et al. have described a linear relationship between the activation energy and the change in internal energy with a slope of unity for some alkyl halides as expressed in Equation 5 (8).
E* = A E
+ 15.3
(5)
where AE is the change in internal energy (the difference between the dissociation energy of the C-X bond and the electron affinity of group X). In this study, the relationship between E* and A E obtained is expressed by the following equation.
E* = 0.89 AE
+ 14
(6)
Since a standard error of about 2 kcal/mol is involved in the C-X bond dissociation energy, it seems that this empirical relationship is essentially identical with that obtained by Wentworth. This is a very convenient relation for estimating E* from AE or vice versa. As the activation energy is concerned with the difference of the potential energy between the neutral state and the transition state of a sample molecule, it is interesting to consider the structure of the transition state and to correlate the stability of the transition state to the value of the activation energy shown in Table I. The dissociative electron capture reaction of the alkyl halide could be assumed to be similar to a bimolecular nucleophilic substitution in solution. According to this assumption, the sp3 orbital on the a-carbon atom should change to an sp2 orbital as a free electron is approaching. Therefore, the stability of the transition state depends upon the extent to which the accepted odd-electron in the p orbital on the a-carbon atom is delocalized by resonance. In such a case, it can be considered that the resonance effect is based on the hyperconjugation in which the hydrogen atoms attached to the @-carbonatom participate (Figure 3). From this model, it can be explained that the activation energies for alkyl halides decrease in the order of primary > secondary > tertiary. In addition, it is expected that the delocalization of the odd electron on the a-carbon atom through the phenyl ring in benzyl halide gives an additional resonance energy and the activation energies for benzyl halides will be lower than those for alkyl halides. This assumption, in fact, was confirmed by comparison of the activation energies for alkyl halides and benzyl chloride as shown in Table I. Because of the contribution of the p-methyl group to the hypercon-
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jugation through the phenyl ring, it is also shown in Table I that p -methylbenzyl chloride has greater reactivity in the electron capture reaction than benzyl chloride or m-methylbenzyl chloride. While the reactivity of alkyl halides with a free electron decreases in the order of tertiary > secondary > primary, the reverse order has been found to hold in s N 2 reactions in solution. This difference may be explained by considering the steric hindrance between the alkyl groups around the a-carbon atom and a nucleophile. Since such an effect is negligible when a free electron is a nucleophile, the reactivity in the electron capture reaction is influenced only by the electronic effect. On the other hand, the steric hindrance between the alkyl groups around the a-carbon atom and the leaving group X contributes to weakening the C-X bond in analogy with an SN2 reaction. In the dissociative electron capture reaction, the hyperconjugation also contributes to the stabilization of the resulting radical. Then the factor contributing to the stabilization of the product parallels that found in the transition state. This fact is an explanation for the linear relationship between the activation energy and the change in internal energy. Our proposal on the mechanism of the electron capture reaction will be helpful for understanding the relationship between the activation energy measured and the chemical structure of the sample molecule and then the identification of a peak on the chromatogram in the GC-ECD system.
CONCLUSION An extensive study has been made of the structural effects on the electron capture reaction of alkyl halides. We concluded that the electron capture reaction of alkyl halides is a kind of SN2 reaction. The reactivity in the electron capture reaction of alkyl halides is fully interpreted on the basis of the mechanism for the SN2 reaction. The validity of the activation energy for the estimation of the chemical structure was suggested. If an electron capture detector is used in gas chromatagraphic analysis of alkyl halides, the activation energy E* and the retention index I for each compound are measured simultaneously. These values could conceivably aid in elucidating the structure of an unknown component. For example, the carbon number could be estimated from I , the kind of halogen atom from A I and E*, the number and the kinds of substituents on the a-carbon atom from AI and E*.
LITERATURE CITED (1) W. E. Wentworth, E. Chen, and J. E. Lovelock, J. Phys. Chern., 71, 1652 (1 967). (2) W. E. Wentworth and E. Chen, J. Gas Chromatogr., 5, 170 (1967). (3) M. Satouchi and T. Kojima, Anal. Left.,5 , 931 (1972). (4) T. Kojima and M. Satouchi, Jpn Anal., 22, 1428 (1973). (5)T. Kojima and M. Satouchi, Jpn Anal., 23, 79 (1974). (6) W. E. Wentworth, E. Chen, and J. E. Lovelock, J. Phys. Chern., 70, 445 (1966). (7) H. J. van de Wiel and P. Tommassen, J. Chromatogr., 71, 1 (1972). (8) W. E. Wentworth, R. George, and H. Keith, J. Chern. Phys., 51, 1791 (1969).
RECEIVEDfor review April 5, 1976. Accepted June 24, 1976.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
