Determination of the Carbon Skeleton and Other Structural Features of Organic Compounds by Gas Chromatography M O R T O N BEROZA and RAFAEL SARMIENTO Entomology Research Division, Agricultural Research Service,
b Improved catalysts have increased from Cg to a t least C20 the length of carbon chain that ciin b e analyzed by the hydrogenolytic gas chromatographic technique previously advanced b y the senior author. The new catalysts are prepared b y adding an amount of alkali equivalent to or in slight excess of that needed to neutralize the anion of the catalyst. Palladium catalysts prepared in this manner may b e used to analyze amines or their precursors. The potential value of fragmenlation studies has been illustrated by showing that primary-, secondary-, and tertiaryalkyl amines; mono-, di-, and trialkyl amines; and isomers containing ketone or hydroxyl groups can b e readily distinguished and identified. Two new apparatJs designs permit the collection of sufficient hydrocarbon product for characterization b y another procedure. These innovations greatly extend the scope of the technique.
A
for tletcrriiining tl, e carbon skeleton and other structural features of micro amounts of organic compounds by gas chromatography has been described in an earlier issue of this journal ( 2 ) . The present paper presents several significant improvenients in the technique and the results of some additional studies that extend the usefulness of the procedure. The most important innovation is a catalyst that increases the length of carbon chain that can be analyzed from the previous limit of Cs to a t least Go. The new palladiur catalyst usually gives less tailing than the previous ones arid can be used for determinations on nitrogen-containing compounds. Studies in which tlie carbon chains of amines, ketones, and alcohols were partially fragmented showed that in certain instances the position of functional groups msy be determined from the identity of the fragments. Descriptions are given of several ap1)aratuj designs t h t permit collection of sufficient hydrocarbon product for ~ i ~ ~ ; c i i x ITLIP:I' ( ~ u ~IS USI:FUL
U. S.
Department o f Agriculture, Beltsville,
confirmation of identity by other means-e.g., infrared or mass spectrometry. APPARATUS AND MATERIALS
Apparatus. Three types were employed : A. The catalyst assembly and flameionization lras chromatoeTarJhic seturJ used previ&ly ( 2 ) . 13. An apparatus identical vith the preceding one except that the catalyst tube has a 3/s-inch i.d. and a 3/c-inch 0.d. C. A third design, shown schcmatically in Figure I, incorporates a trap, D , between the catalyst tubc, C, and the chromatographic column, E. 'l'lie trap, made from 2 feet of 3/16-inch 0.d. copper tubing, is partially filled nith the same packing used in the chromatographic column (4). In a variation of this design a valve that permits inclusion or exclusion of the trap is inserted between the catalyst tube and the chromatographic column. Catalysts. Three types, designated acid, neutral, and alkaline, have been employed. Platinum and palladium catalysts were made up to contain 1 to 37, metal salt (by weight :ts thc metal) on 60- to 80-mesh Gas-Chrom P (Applied Science Laboratories, State College, Pa.) or acid-washed Chromosorb W (Johns-AIanville, New York, N. Y.). They were activated just hefore use as previously described for 30 minutes a t 125" C., 30 minutes a t 200" C., and 20 minutes a t the test temperature. The catalysts were dis-
Figure 1. Schematic diagram of a p paratus with trap A.
Injection port Hydrogen inlet Catalyst tube D. Trap E. Chromatographic column F. Detector See text for explanation of valve arrangements a and b. B. C.
Md.
carded 2 or 3 days after activation regardless of amount of usage. The following directions for preparing catalysts are typical. A. ACID. A solution of palladium chloride in 57, aqueous acetic acid (or platinic chloride in water) is evaporated to dryness in contact with the catalyst support on a Rinco rotating evaporator and dried a t 110" C . B. NEUTRAL.Sufficient nonvolatile alkali-e.g., sodium or potassium hydroxide or carbonate-to neutralize the anion of tlie metallic salt is added to the mixture of the metallic salt solution and the support. For esample, to 88.8 mg. of PdClz add 40 mg. of sodium hydroxide. The product is then evaporated and processed as described under A. C. ALKALINE.The catalyst is prepared as described under B except that, in addition to the alkali required to make the catalyst neutral, excess alkali (usually a 0.2% excess based on the weight of the support) is added. Chromatographic Columns. For identification of CI to Cehydrocarbons, the 15-foot, 3/16-in~h0.d. copper column containing 5% squalane on 60- to 80mesh acid-washed Chroniowrb W a t room temperature was used. For identification of C4 to C20hydrocarbons, a &foot, 3/la-inch o.d. copper columii containing 57, silicone gum SE-30 on the same support was used a t room and elevated temperature>. PROCEDURE
The general procedure i- thc same as that described in the earlier report ( 2 ) . 'The catalyst tube was maintained a t 295" C. for palladium catalysts and a t about 230' C. for platinum catalysts; the hydrogen flow rate was 20 ml. per minute. The amount of sample in. 20 pg.), except jected was 0.02 ~ l (ca. hen othernise noted. Appropriate hJdrocarbon mixtures of k n o w com1)oYition were analyzed t o establish retention times. With apparatus II, the hvdrogen flow rate is maintained a t 100 ml. per minute and about 0.3 pl. of sample may be injected a t one time. When injecting a compound into the apparatus of Figure 1, the trap is immersed in a dry ice-acetone bath. The cold ti u1) retains the hydrocarboil 1'1 oduct. U y removing the cold bath and VOL. 35, NO. 10, SEPTEMBER 1963
1353
immersing the trap in a hot liquid bath (Nujol a t 100" to 150' (3.). the hvdrocarbon is released and starts through the chromatographic column. Retention times are calculated from the time the trap is immersed in the hot bath and are compared with those of conipounds run in the same way. With heavy hydrocarbons it is better technique to isolate trap D of Figure 1 (valve position a) while heating and to pas\ the contents when fully heated into the analytical column (valve position b ) . Retention times are measured from the time the valve is turned to position b. RESULTS
Many of the compounds listed in Table I of reference (2) were rerun with the neutral and alkaline catalysts; these data uill not be reported because, with but very few exceptions, previouq and present results agreed. [For a summary of previous results and typical elution patterns, see Table I1 and Figure 2 of reference ($.I Typical data obtained with the new catalysts are given in Table I. Compounds having as many as 20 connected carbon atoms produced the expected hydrocarbons. Xitrogen-containing compounds, such as amines, amides, and nitro compounds, which gave no response with the acid palladium catalysts previoudy used, gave the expected hydrocarbons with the nrutral or alkaline palladium catalysts. 81though there appeared to be no great difference between the neutral and alkaline catalysts, there may be some advantage in using an alkaline catalyst if most of the analyses are to be made with amines. The following observations refer to Irincipal products ot CIo-Cl0 compounds obtained with neutral (with sodium carbonate) 1% palladium on Gas-Chrom P a t 295' C., the catalyst used most. Halides, sulfides, and unsaturated compounds gave the parent compounds. Primary-attached etherq, the alcohol part of esters, and amines gave more of the parent hydrocarbon than the next lower homolog (except for formic acid esters which gave little or no parent hydrocarbon), the corresponding secondary- and tertiaryattached compounds gave only the parent. Aldehydes and amides gave a m a l l amount of parent hydrocarbon along 15 ith the next loiver homolog. Ketones (none above CSavailable) gave the parent compound. Secondary or tertiary alcohols gave the parent compound, primary alcohols the next lower homolog with little or no parent compound. Yields of product from secondary or tertiary alcohols mere much less than those obtained from the same amount of primary ~ I r o h o l ~ Acid, and the acid moiety of eders gave low yields of the hydrocarbon rorrcsponding to the decarboxylated 1354
ANALYTICAL CHEMISTRY
Table I.
Type .kid
Compound Undecanoic acid Lauric acid Myristic acid Palmitic acid Stearic acid Oleic acid L41cohol 1-Dodecanol 1-Tetradecanol 1-Octadecanol cis-9-Octadecen-1-01 9,12-Octadecadien-l-01 1,12-0ctadecanediol, 12-acet at e 1-Hexadecene-3-01, 3,7,11,13-tetramethylAldehyde 10-Undecenal 10-Undecenal Undecanal Dodecanal Dodecanal Amide Ilodecanamide Dodecanamide, N,,V-dimethylDodecanamide, N,N-diethylDodecanamide, N,N-diethylTetradecanamide, N,N-dimethyl-
Principal products" ClO c 1 1
c 1 3
cis
c 1 7 c 1 7
Cll c 1 3 c 1 7 c 1 7
C17
cii
Br.Cloc
Hydrocarbon
Catalysth and temp., C. Neut. 17, Pd, 295 Neut. 1% Pd, 295 Neut. 1%Pd, 305 Neut. 1% Pd, 305 Seut. lYOPd, 305 Neut. 1% Pd, 305 Keut. lY0 Pd, 295 Neut. ly0Pd, 295 Neut. 1% Pd, 295 Neut. 1% Pd, 295 Seut. 1% Pd, 295 Seut. 17; Pd, 295 Neut. 1% Pd, 295 O
Neut,. 1% Pd, 295 Alk. 3% Pt, 240 Xeut. 1% Pd, 295 Neut. 1% Pd, 295 Alk. 3Y0 Pt, 240 Seut. 1% Pd, 295 Neut. 1% Pd. 295 Neut. 1% Pd; 295 Xeut.3% dk. 1% Pt, Pd,240 295 Alk. 1% Pd, 295 Alk. 3% Pt, 225 C,,
Alk. 370 Pt, 240
("> C7
Alk. 3% Pt, 240 Alk. 3% Pt, 240 -4lk. 3% Pt, 240
C I
C4d-CZ
8
c, -cz Cf or C,, Car cs Ilexadecylamine, ,\r,S-dimeth~l9-Octadecenylamine
Alk. 3% Pt, 215
C7, C8
Alk. 3% Pt, 215 Alk. 3% Pd, 260 Alk. 3y0 Pd, 260
CI 1
Alk. 3 7 , Pt, 2-10
Ci,, c16 Cli, CIS
Seut. 1% Pd, 205 Neut. 170Pd, 295
to neutralize the anion of the metal (as well as a slight excess) mas added to the catalyst preparation. (It is apparent that the catalyst support should be neutral a t the outset.) The desired effect was obtained, since compounds DISCUSSION containing 20 connected carbon atoms. including amines, could be analyzed The composition of the origina! Tvith the new catalysts. Thr iiI)prr catalyst (acidic type) mas varied or limit in terms of chain length has not changed in an attempt to adapt the been determined because coinpound; method to analyze compounds of and corresponding hydrocarbon standgreater chain length. Alkali was added ards were not available. Thompin one of these experiments because it son d nl. ( e ) were successful in drnitro\vas recognized that during the cata!yst genatiori studies with platjiiiuni on activation period the metallic chlorides porous glass, but were uriatk to firid react with hydrogen to give not oniy the a palladium catalyst that would permit free met,al but also hydrogen chloride: hydrogenolysis of airlines 01' amine if the latter is retained by the catalyst. it Iiiight cause 1)reakdown of o o i n y ' ~ i i ~ i c k precursors The ~ i ( t \ v iieutrul- 01'ulkti1ine-tyl)e palladiuiii c:lt:ilysts acc'om:id in atlditiori prevent amines. from plished this objective. passing through the catalyst. AcThe neutral palladium catalyst insiy cordingly, sufficient nonvolatile alkali
acid; peaks dragged and mere slightly de!ayed. Low yields may be offset by increasing sample size and /or increasing sensitivity.
Products from Compounds T>.pc
1,strr
Ether
Halide Others
Compouiid 9-Octadecenylamine, S,-V-dimethyl;-Tetradecyne-6,9-diamine, W,N,.V,N,2,2,4,11,13,13-decamethyl.icetic acid, bromo-, 2-propylheptyl ester .icetic acid, bromo-, decyl ester 1-Decanol, formate Acetic acid, bromo-, dodecyl ester 1-Tetradeca 101, formate Acetic acid, bromo-, tetradecyl ester Acetic acid, bromo-, tetradecyl ester Palmitic acii, methyl ester .icetic acid, bromo-, hexadecyl ester Stearic acid, vinyl ester Octadecanoic acid, g,lO-epox)--, allyl ester Oleic acid, methyl ester Oleic acid, iciopropvl ester Oleic acid, allyl ester Oleic acid, butyl ester Linolenic ac d, methyl ester 1-Octadecanol, formate 1-Octadecanol, acetate 1, 12-Octadt canediol, diacetate Propionic acid, 1Zhvdroxyoctadecyl ester Sorbic acid, octadecyl ester Ether, allyl iodecyl Ether, allyl Letradecyl Ether, allyl :etradecyl Ether, allyl iexadecyl Cndecane, 1-iodoDodecane, 1-bromoOctadecane, l-bromoBenzene, nitroDodecane, l!2-epoxy1-Dodecanet hiol or-Naphthol
Principal product,s” Clj, CIR Br.Czoc
Catalyst: and temp., C. Teut. lcoPd, 293 Seut. 17: Pd, 295
c I
Cg, CS-C-C~
Alk. 370 Pt, 240
C9, ClO Cs, ClO
Neut. ICo Pd, 295 Alk. 3% Pt, 240 Neut. 1% Pd, 295 Yeut. 1% Pd, 295 Seut. lYCPd, 295 .41k. 373, Pt, 240 Neut. 1% Pd, 295 Neut. lyCPd, 295 Neut. 1% Pd, 295 Neut. 1% Pd, 295
CII, CIZ
c1,
C I ~(314 , CI,, C I ~ CIS CIS,C I ~ e 1 7 c 1 7
c 1 7
CI? c 1 7
CI? Cn c17,
CIS
CI?, C I S
CI?, CIS C1?, CIS CI?, ClU c 1 1 ,
Cl?
Cl?, VI4
c13, c 1 4
CIS, c 1 6
C11 ClZ
c1s
0
ClO, Cl,, ClZ e 1 2
Seut. Seut. Seut. Neut. Seut. Teut. Seut. Xeut. Neut.
ITc Pd, 295
1% Pd, 295 1% Pd, 295
lyGPd, 295 1% Pd. 295
1% Pd; 295 17~ Pd, 295
1y0Pd, 295
1% Pd, 295
Neut. 1% Pd, 295 Yeut. 1yoPd, 295 Seut. 1% Pd, 295 Alk. 370 Pt, 240 Xeut. 1% Pd, 295 Seut. ly0Pd, 295 Neut. lyoPd, 29.5 Teut. 1% Pd, 295 *ilk. 37, Pt, 240 Neut. 1% Pd, 295 Neut. 1% Pd, 295 Seut. 1% Pd, 295
Seiit. 1% Pd, 295 Tridecanenitrile Onlv hydrocarbons of identified peaks are listed nnless othem ise noted. K O attenipt n RR made to identify lon,er hydrocarbons-e.g., the C3of isopropyl oleate. Seut. = neutral, alk. = alkaline, Pd = palladium, Pt = platinum. Catalyst support was Gas-Chrom P. No hydrocarbon standard was available to checnk identity of single peak obtained. Peak retention time appears satisfactory for branched (Br.) C2,.
be the most useful on(?found to date, pince it can be used to analyze all types of compounds thus far undertaken. Unlike platinum catalysts. it is not poisoned by sulfides and it appears to be less likely to cause ring cleavage. (Cvclopentane compounds that are y l i t by platinum catalysts are not qdit hy palladium catalyst5 ) The I):illadium catalyst dop- require a higher trniperature than that used ith idatinurn to cause hydrogenolysis, b u t the higher temperature may be a n advantage in the analysis of compounds of high molecular weight. Palladium catalysts have succeeded in analyses of C M - C I compounds ~ for which platinum catalysts gave no procuct; palladium catalysts neutralized with sodium carbonate worked especially n ell. Apparatus Modification. IVhrn the identity of u hydrocarbon product is
in doubt, i t is desirable t o verify the structure by another means-e.g., infrared, mass spectrometiy. Since the 10 t o 20 pg. of sample normally injected yieids insufficient hydrocarbon for such characterization, several modifications of the basic apparatus were made. One of thew, apparatus 13, has a catalyst tube bore of s inch and hold, 9 time5 more catalyit thaii a t r h e (13oreY larger with a *-inch bore. than 3/8 inch can be used ) The greater catalyst capacity makes it possible to inject a t one time much larger amounts of compound. Excessive contact time of the substance with the catalyst i h avoided by increasing the hydrogen flow rate to 100 ml. per minute. Thus it was found that a single injection of 0.3 pl. of heptyl butyrate ga\ e the qamp reyolice ab 0 1 pl. of hel)tane, the e\pected major product. This amount
of substance is sufficient for an infrared ( I ) or mass spectrum analysis (6). Another means that may be used to irepare enough hydrocarbon product for independent characterization is shown in Figure 1. The product from repeated injections of a sample may be immobilized in trap D until released by immersion of the trap in hot Xujol; the product is collected at the exit end of the chromatographic column. Time of collection may be determined by: (a) using a t the exit end of the chromatographic column a stream splitter that passes most of the product to a trap but allows a small portion to flow through the detector and register the composition of the effluent gas, (b) diverting the effluent gas through a trap for an appropriate interval as soon as t,he desired product starts to emerge, or (c) collecting at the appropriate time without the detector, using a kiiown hydrocarbon or a previously run chromatogram of the sample to establish the collection period. A detector that does not destroy the exiting compound-e.g., thermal conductivity unit, which map be easily introduced in certain gas chromatographs-might be useful in monitoring the effluent for preparation purposes. It has already been demonstrated that the retention time of hydrocarbon products can be delayed by the adsorptive properties of the catalyst and excessive tailing niay result. See Figure 3 of reference ( 2 ) . Since most, analyses were made with catalyst excessive tailtemperatures of 295’ C., ing has caused no difficulty; but it i.: presumed that such tailing may generate some uncertainty \Then the retention times of two possible compounds arr close. The effect of the catal.yst on retention time may be eliminated through the use of the trap of Figure 1. The product, even though emitted very slovr-ly by the catalyst, is held in the trap until released by the, rapid heating of the trap. Retention times from the time of release can be expected to agree precisely with those of knowns similarly trapped and released. This same effect is accomplished by temperature programming of compounds that do not move much on a column at room temperature. The inclusion of the 2-way valve of Figure 1 makes a convenient arraugcment since the apparatus may be uecd with (Figure l h ) or without (Figure I u ) the trap. Fragmentation Experiments. It has been noted t h a t fragmentation of the carbon chain of certain compounds inay be induced b y increasing the catalyst temperature or activity ( g ) , and that these fragments may shed light on the structure of the injected compound. Several studies of thc fragmentation process have boriie out VOL. 35, NO. 10, SEPTEMBER 1963
1355
compounds may be readily distinguished. Each of the compounds gives mostly heptane but, in addition, cleavage occurs next to the carbonyl group ns follows:
c3
w
I
u)
c4
AL DL 150
C
',
0 II I)
c+cf,c-c-c-c-c 0
'>I/:
c-c+€-c-c
