Use of Retention Increments for Identification and Correlation of Saturated and Unsaturated Cyclopropane Hydrocarbons by Means of Kovats Indices G e r h a r d S c h o m b u r g and G e r d D i e l m a n n Max-Planck-lnstitut fur Kohlenforschung, Mulheim-Ruhr, Germany
Retention data of a large variety of substituted cyclopropane hydrocarbons are tabulated and used to derive retention increments for many isomers. The use of increments which are defined on the basis of molecular structural elements is mainly a pragmatical approach for a better understanding of retention behavior of isomeric compounds. The increments are related to the influences of partial molecular configurations on intermolecular solute-solvent interaction. The high standard and reliability of retention index predictions with such increments is demonstrated. The problem of the precision of retention index determination is discussed with regard to different modes of application for identification and correlation of separated species of mixtures.
For several reasons the retention index system of Kovats ( I ) seems to be t h e most suitable and reliable method for standardization of retention parameters. Collections of Kovats indices ( I a n d A n can be used directly for identification of components of mixtures by table search. Moreover a lot of rules governing retention behavior can be derived from such data. These rules are interpretable on the basis of incremental contributions of each functional group of the molecule to t h e I and AI values. Therefore. retention index increments are characteristic for certain structural elements as described in previous papers (2-7). They can be used for prediction of retention indices or a t least of elution sequences in mixtures, t h u s supporting t h e structural correlation of unknown components by means of spectroscopic, preferably mass spectrometric, methods (7-12). The range of validity and, therefore, applicability of such increments depends Qn certain assumptions. These include the structural and conformational features-for instance: chainlength, distance between functional groups. and other geometrical factors. In this paper, t h e retention d a t a of a large number of cyclopropane hydrocarbons will be presented, new retention index increments derived, a n d t h e validity of t h e latter checked for prediction of retention indices by comparison of calculated and found data.
Such increments can be derived only in a logarithmic system like t h a t of Kovats, because additivity of structural increments can be expected only for the enthalpy of SOlution which is proportional to log VK. The Kovats indices are enthalpies of solution of a compound relative to those of n-paraffins. Owing to a significant but weak curvature of the log VR-C-nUmber plot in the region of low C-numbers specification, whether interpolation between n-paraffin standards with n and n 4- 2 C atoms or n and n + 1 C atoms has been done, should be indicated. The documentation of Kovats indices or of any other standardized retention parameters is of practical use only if the d a t a are determined with adequate precision. This is extremely important when the compounds are identified by table search (13). Difficulties do not arise so much with the repeatability as with the reproducibility of retention indices. T h e main source of errors influencing reproducibility is dependent on the total system, including liquid phase plus support of t h e column used. Extensive measurements have been carried out in order to obtain a variance analysis of I-determination dependence on temperature, column polarity, sample load, carrier gas flow, etc. (13). All measurements in this paper were obtained with open tubular columns-i.e., with high chromatographic resolution in order to reveal even minor variations of molecular structure by means of corresponding changes in retention. The present study was also undertaken as a contribution to the investigation of complex mixtures of isomeric olefins including diolefins with isolated and conjugated double bonds. Olefins are easily converted into the corresponding cyclopropanes by copper- or copper-I-catalyzed reaction with CH2N2 (14) and C2HdN2 ( 1 5 ) or applying the Simmons-Smith reaction (16). The configuration of t h e double bond is preserved in this reaction but there is a difference between t h e rate of formation of cyclopropanes from the cis and trans isomers. T h e resulting substituted cyclopropanes exhibit a characteristic retention behavior. By means of t h e retention indices or the increments of the cyclopropanes, the identification of t h e original olefin isomers is supported (see Table V.)
EXPERIMENTAL (1) E. Kovats, Helv. Chim. Acta, 41. 1915 (1958). (2) G. Schomburg. J . Chromatogr.. 23, 1 (1966). ( 3 ) G . Schomburg, J . Chromatogr., 23,18 (1966) (4) G. Schomburg.Anal. Chim. Acta. 38,45 (1967) (5) G. Schomburg, Advan. Chromafogr.. 6,211 (1968) (6) G . Schomburg and D. Henneberg, Fresenius' Z.Anal. Chem., 236, 279 (1968). (7) G. Schomburg, Chromatographia. 4 , 286 (1971). (8) D. Henneberg and G. Schomburg, Advan. Mass Spectrom., 4, 333 (1966). (9) M . B Evans and J. F. Smith, J. Chromatogr., 8 , 303 (1962). (10) M. B. Evansand J. F. Smith, Nature ( L o n d o n ) , 190,905 (1961). (11) P. A . T. Swoboda in "Gas Chromatography." M. van Swaay, Ed., Butterworths. Washington, D. C., 1962. (12) C. Merritt and J T. Walsh. Ana/. Chem.. 34,903 (1962).
Origin of S u b s t a n c e s : S u b s t i t u t e d cyclopropanes h a v e b e e n p r e p a r e d by t h e copper- or copper-I-salt-catalyzed r e a c t i o n w i t h CHzNz o r CzH4Nz w i t h olefins of k n o w n structure, w h i c h in some cases h a v e b e e n a v a i l a b l e o n l y as m i x t u r e s . T h i s r e a c t i o n is c a l l e d t h e c y c l o p r o p a n i z a t i o n r e a c t i o n (CPR)
(13) G. Schomburg, lecture held at "10. Regionales GC-Kolloquium," Frankfurt, Germany, 1972. (14) compilation in W. Kirmse, "Carbene, Carbenoide und Carbenonalogue," W. Foerst and H. Grunewald, Ed., Verlag Chemie, Weinheim, Germany, 1969. (15) G. Schomburg and G. Dielmann. unpublished work. 80, 5323 (16) H. E. Simmons and R. D. Smith, J. Amer. Chem. SOC., (1958); 81, 4256 (1959).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
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Table I. Correction of Retention Indices for Changing Column Polarity by Means of Standard CompoundsaYb A / (March) Ala0
(March)
A180
(October)
A / (October)
14.52
15.99
0.908
18.66
20.93
0.891
16.37
17.90
0.914
22.8SC
25.56
0.893
23.42c
26.1 1
0.896
25.59
28.50
0.897
25.59
28.50
0.897
27.17
30.53
0.889
25.43
28.24
0.900
28.33
31.53
0.898
29.27
32.89
0.889
a Columns: polypropylenglycoi 100-mstainless steel, 0.25 i.d.; squaiane 100-m stainless steel, 0.25 i.d. bThe correction was done with standard compounds with the same or at least similar fu,nctional groups. In the latter case the accuracy of manipulation decreases. If any of the compounds of the table is chosen as the standard compound for correction, the deviation of A / does not exceed 2% relative. Diastereoisomers.
Methyl-substituted cylopropanes were made either by the methylene insertion reaction (MIR) using pure cyclopropane hydrocarbons or by the CPR with methyl-branched olefins. The MIR (17-19) can be applied in all cases where methyl-substituted derivatives of known hydrocarbons are to be made (20). The resulting number of isomeric species and to some extent also their quantitative ratios depend on the number of different types of CH bonds contained in the original hydrocarbon. The MIR has been used in several works for generation and identification of methyl derivatives of hydrocarbon isomers and fatty acid esters (5, 6, 21). The correlation of separated species in the chromatograms of CPR or MIR mixtures was carried out using the following sources of information: comparison of qualitative and quantitative composition of original olefin sample and of cyclopropanization products; “statistics” of methylene insertion reaction; comparison of retention indices and mass spectra of the olefinic compounds with those of the corresponding hydrogenated samples (7); mass spectra obtained by on-line GC-MS measurements (6) (GC = gas chromatography; MS = mass spectrometry); NMR and IR spectra of purified species (prep. GC); test substances prepared by other chemical reactions; retention index increments known before or derived from the data of related compounds. (17) W. v. E. Doehring. R. G . Buttery, R. G . J . Amer. Chem. Sac., 70 3224 (1 956).
Laughlin. and
M.
Chaudhuri,
Simmons, D. B. Richardson, and I . Dvoretzky, “Gas Chromatography.” R . P. W . Scott, Ed., Butterworths, London, 1960. (19) I. Dvoretzky, D. B. Richardson, and C. R. Durett, Anal. Chem., 35, 545 (1963) (20) P. Heimbach and G . Schomburg, to be published. (21) G . Schomburg,J. Chrornatogr., 1 4 , 157 (1964). (18) M . C.
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In some cases the complete separation of isomers was impossible even after changing or optimizing resolution, column temperature, or polarity of the stationary phase. Gas Chromatographic Separations. Columns. The separation power of the open tubular columns of stainless steel or glass was between 100 and 200,OOO effective theoretical plates (22) for 50- to 100-m columns with 0.25-mm i.d. Most of the data have been measured with squalane as the stationary phase. During the period of collection of the data of the cyclopropanes (about 3 years), three different squalane columns with slightly differing “polarity” were used. With the classes of compounds investigated, no greater differences in Is values than about 0.5 IU (IU = index units) were found with the different columns. In cases where higher precision of P values was necessary, all measurements for comparison or identification were made in the same column. The reproducibility of index values was controlled by measuring some standard compounds from time to time. Temperatures. The average temperature dependency of Is of the investigated classes of compounds was about 0.1 IU/”C. Column temperatures were measured with a mercury thermometer, at a defined position inside the oven, with a precision better than 0.1 “C. The gradients of oven temperatures measured in the used column ovens (Perkin-Elmer F 20 and Varian 1400) have been determined to be about l “C along one axis of the oven (at average oven temperatures of 80 “Cj (23). They are in accordance with the specifications given by the manufacturers. Carrier Gas Flows. The initial pressure of the columns was regulated by Fisher regulators (type 67/18 special). Sample Load. Indices are determined only within chromatograms with symmetric peaks of components and standard n-paraffins. The latter were added to the sample to improve precision. In all cases, sufficiently low sample loads were introduced to avoid “leading.” No “tailing” was observed for the investigated classes of compounds. An investigation concerned with the dependency of I values on sample loads will be published elsewhere (24, 25). Determination of Kovats Indices. Summarizing, the following
points are of general importance for index determination: “ideal” symmetry of peak profiles (e.g., no tailing or leading); the standards (n-paraffins) are contained within the sample; interpolation is exclusively done between n and n + 2 n-paraffins if not particularly specified, differences of 0.1-0.3 IU are observed by interpolation with n and n + 1 or n and n + 2 paraffins, compare reference 13; the repeatibility of index calculation by computer is 0.05-0.1 IU and with a rule, i.e., by hand, 0.20 IU (26) (for nonpolar substances and nonpolar stationary phases). The determination of accurate peak positions is of course impossible in the case of overlapped peaks. The index values of these species have been marked by asterisks within the data tables. Documentation of Kovats Indices. Assuming that the temperature of the column is measured and regulated properly and provided the sample load and the concentrations of the n-paraffin standards are chosen correctly, the only difficulty for the extended practical application of Kovats indices is the “polarity” of the columns. There are two ways of overcoming this problem. First, columns with reproducible polarity of the liquid phase plus the support could be made stable and retain stability a long time with regard to polarity. Second, the polarity of columns is characterized by Kovats indices of a selected set of similar compounds with regard to the functional group and the carbon skeleton and checked every time before measurement of retention parameters for timedependent changes in the polarity of the system. In this case, corrections of the I values with the data of the test compounds can be made with suitable accuracy. See Table I (correcting “manipulation” of retention indices by means of the data of standard compounds). The ratio of P(March) and P(0ctober) is constant at about 0.90 and shows deviations no greater than 2% relative at the given variety of different cyclopropane compounds. Using capillary columns with different liquid phases over a longer period, we found the following. It is difficult to produce (22) D. H . Desty, A. Goldup, and W . T. Swanton in “Gas Chromatography.” N. Brenner, E . Calien, and D. Weiss, E d . . Academic Press, London, 1962. (23) G . Schomburgand H . Husmann, unpublished work. (24) L. J . Lorenzand L. 6 .Rogers, Anal. Chem.. 43. 1593 (1971). (25) G. Schomburgeta/..to be published. (26) G . Schomburg. F. Weeke, B. Weimann, and E. Ziegler, “Gas Chro-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
matography,” 1970.
R.
Stock.
Ed.,
The Institute of Petroleum, London,
columns with reproducible polarity for the precision desired in this type of work, even with the liquid phase, the support, and the tubing material (glass or stainless steel) of the same origin. One of the reasons for this is probably an inhomogeneous film on the tubing wall. Others may be the progressive oxidation of the liquid phase or the accumulation of impurities. In judging the extent of deviations of retention indices, the dependency on the polarity of both the solute and the stationary liquid phase has to be considered. Nevertheless, most of the columns can be used for long periods without considerable increase or decrease of retention data. This above-mentioned case of changing polarity of a column is an exceptional one and was therefore chosen for demonstration of the correction procedure. Changes of polarity to such an extent are not common. Columns with nonpolar stationary phases (Siliconoil DC 200, squalane) are more stable than those with polar phases (for example: PPG (polypropylenglycol), E M 0 (Emulphor 0),CW (Carbowax 20 M). Glass capillary columns exhibit a “polarity” different from those of stainless steel ( 7 ) and they give better resolution and show less “tailing” than stainless steel columns even for polar solutes in nonpolar solvents. In the case of gas chromatography of hydrocarbons (branched, cyclic, unsaturated), with weak or medium polarity, retention indices can be measured with a reproducibility (from laboratory to laboratory) of about 1 IU. This experiment was made by comparison of our data with those of Hively (27), Matukuma (28), and other authors. Errors of index reproducibility of this size can be explained partially on the basis of the experiment made by a French group (29) of chromatographers having done index measurements in several laboratories for purposes of comparison. Inaccurate measurement and poor constancy of column temperature were the main reasons for poor reproducibility of retention indices. Retention data files stored in computer data banks for more sophisticated research on hydrocarbon isomers are set up with data determined within the .same column (compare Table IV). If any other columns with the same liquid phase are applied for identification, the polarity of the used column has to be characterized by related standard compounds and corrections have to be done as described before.
RESULTS Definition of Retention Increments and Their Dependence on the Size of the Molecule. Incremental identification uia retention parameter can be done in two ways ( 5 ) : either by using structural influences on retention in t h e I* values (retention index of a substance in t h e nonpolar stationary phase, in this work squalane) or uia t h e AZ values. T h e increments derived by the first method are called HA ( H A = homomorphic factor in nonpolar phases [in this work squalane]). They result from t h e comparison of retention behavior of a compound with t h a t of a standard compound in a certain nonpolar stationary phase. Increments are also obtained by comparing t h e difference in retention A I of a certain compound in two stationary phases with that of a standard compound having a homomorphous structure (Equation 2 ) . T h e two types of increments are defined by t h e following equations
HA = Z*(compound) - Z*(homomorphous compound)
(1)
(AI)= AZ(compound) - AZ(homomorphous compound) where
A I = Ip
- ZA
(2) (3)
For all equations T is a constant a n d Ip is the index in polar phases (in this work polypropyleneglycol). T h e homomorphous compound must be a n n-paraffin of t h e same carbon number or a compound with t h e same (27) R . A . Hively‘and R. E . Hinton,J. Gas Chromatogr.. 6, 203 (1968). (28) A . M a t u k u m a , “Gas Chromatography 1968,” C. L . A . Harbourn, E d . . T h e Institute of Petroleum, London, 1969. (29) J. C. Loewenguth. 5 t h International Symposium on Separation Methods: Column Chromatography (Lausanne 1969). E. sz. Kovats. E d . , Sauerlander AG. Aarau, Suisse.
Table II. HS and AI Data of 2- and 3-Methylalkanes 2-Methylal kanes
No. of C atoms
HgoSa
A/goPPG-Sb
3-Methylalkanes ~
$
a
AlgOPPG-Sb
-32.3 -2.9 -22.2 -0.7 -2.7 -26.5 -0.2c -34.3 -3.0 -28.7 -0.4 c9 -34.9 ClO -35.6 -3.1 -29.6 -0.6 c11 - 36.0 -3.1 -30.4 -0.7 c12 -36.1 -3.3 -30.9 -0.7 a H g o S : homomorphic factor in squalane, column temperature 80 “ C . A l g ~ ~ AI ~ ~value - ~ :for t h e system polypropyleneglycol-squa. lane, column temperature 80 “C. Accurate measurement was disturbed by overlapping. c7
C8
carbon skeleton as t h e compound for which t h e HA is to be calculated. For example, t h e homomorph of a n olefin is t h e corresponding saturated hydrocarbon. Dependency of Increments on Size of Molecules-e.g., Length of Carbon Chain. In Table 11, t h e Hs values are given of t h e 2- a n d 3-monomethyl-branched alkanes from C7 to C13. Increments valid within a large range of C numbers can be defined if the molecules < C g are not considered. I n these molecules, t h e functional group is too large in relation to t h e alkyl group bonded to t h e branching C atom for a consistent definition. Average values of Hs for 2- and 3-methylalkanes with C numbers >Cg are 35.5 and 29.5, respectively. T h e A I values of t h e methylbranched n-paraffins are not to be neglected if Z values are measured with a precision of about 0.5 IU. Schulze et al. (30) found t h a t methyl-branched hydrocarbons have characteristic values whereas Kovats assumed these A I to be zero in the early publications on his index system. Increments “averaged” across a n extended range of chain length should not be used for small molecules
