Mass Spectrometric Analysis Aliphatic Aldehydes J. A. GlLPlN and F. W. McLAFFERTY Spectroscopy laboratory, The Dow Chemical Co., Midland, Mich.
examination of the mass spectra saturated aliphatic aldehydes showed certain typical cleavages and rearrangements. In most cases the aldehydes gave a maior cleavage at the bond beta to the aldehyde group, accompanied by the rearrangement of one hydrogen to the oxygencontaining fragment. This cleavage was increased by chain branching adjacent to the beta bond. The straight-chain aldehydes of higher molecular weight (above C,) gave similar spectra at lower masses. Losses of 18 and 28 were typical for these molecules. ,An
of
NUMBER OF CARBON ATOMS
Figure 1 .
T
broad applicability of the mass spectrometer to qualitative and quantitative analysis has been shown (10). Certain generalizations have been made correlating molecular structure with favored bond cleavages under electron impact. I n the present report the bond cleavage for a particular cheniical class of compounds are examined. It is hoped in this LTay to predict the effect of a particular functional group on the mass spectrum of a molecule. A general classification of such effects for the organic functional groups can be made on the basis of their ability to attract electrons from or give up electrons to the molecule ( I O ) . Thus the electronegative aldehyde group should behave in a fashion generally similar to that of such functional groups as halogen (11), nitro, ketone (12), and ester (6). Here a marked tendency for cleavage of the bond alpha to the functional group is found.
Molecular ions of straight-chain aldehydes
Hh
R-CI
R-NO,
-
-
CHO
z
c]C 2 H 5
BO
1,
W
20
3
Figure 2. aldehydes
0
H I
5
Alpha cleavage in mass
0
/I
HZCCHZ
- CH2CNHZ
11 + + CH3CNH2
R*H
ANALYTICAL CHEMISTRY
4
NUMBER OF CARBON ATOMS
d-
Another niajor distinguishing spectral characteristic of unsaturated electron-withdrawing substituents is beta bond cleavage, accompanied by the rearrangement of hydrogen.
1 6
2
I
c
990
=
100
H
0 ,
~
H
~ C-H 2 ~ C H Z - C H p C ~ C H 3
-
0
0 c,H&H;
+
c&H;
For butyric acid, beta cleavage and hydrogen rearrangement give the stable acetic acid ion as the largest peak in the mass spectrum (9), and result in formation of the stable ethylene molecule. The primary amides give a similar clearage and rearrangement. Dibutyl ke-
29
straight-chain
tone shon s the rearrangement fragments resulting from cleavage of one beta bond to give the methyl butyl ketone ion and also cleavage of both beta bonds n-ith the rearrangement of two hydrogens to give the stable acetone ion (8). Cleavage of a single beta bond can be accompanied by rearrangement of more than one hydrogen atom for esters and other types of molecules v i t h two or more atoms like oxygen that contain unshared electron pairs (6, IO). The lower homologs of compounds of this type that contain only beta hydrogen atonis do not undergo the hydrogen rearrangement. This is in line with
100
90
sc
80
>- 70
c g
60
W
50
$
40
I-I
30
[L
20 IO
0 0
I
2
3
4
5
6
7
8
9
IO
II
12
1 3 1 4
NUMBER OF CARBON ATOMS
Figure 3. Straight-chain aldehydes of mass 44 (acetaldehyde)
the formation of the stable ethylene molecule as part of the driving force of the rearrangement. EXPERIMENTAL
.4 total of 20 aldehydes were obtained for the correlation-13 straiglitchain and seven liranched-chain structures. I n several cases the aldehydes were further purified by vapor phase chromatography techniques ( 7 ) . The spectra n'ere obtained on t n o 90" sector mass spectrometers ( 2 ) having 100" and 200' C. heated inlet systems ( 1 ) . These instruments gave spectra similar to those obtained on a Consolidated 21-103 mas'. spectrometer in this laboratory. The spectra of n-heptanal and the aldehydes Containing more than ten carbon atoms were ohtained a t 200' C.; all other aldehyde spectra n-ere obtained a t 100" C. (Table I). RESULTS
Table I summarizes the spectra 01,tained. The relative abundance of ions at each ina4s ( 1 7 1 e ) is based on the largest, or base. peak as 100%. Both branched and straight-chain aldehydes of Ion er molecular weight exhibit large peaks a t their molecular weights (also listed separately as "parent" ions, Table I). These molecular ions were detected for all the aldehydes examined 17 ith molecular TI eights ac: high as 212. Honever, this peak drops off rapidly above Cd (Figure 1). being 3 to 0.3% for n-pentanal and straightchain aldehydes of higher molecular weight and 4 to 0.3% for branchedchain aldehydes containing six or more carbon atoms. The molecular ion provides the largest peak in the mass range from the molecular weight to the loss of 18. Alpha cleavage predominates only for the aldehydes of lower molecular n eight and usually becomes minor nhere bets
cleavage can occur without involving the loss of a methyl group. The mass 29 peak can be an indication of alpha cleavage (Figure 2). This peak can be caused only by the aldehyde group (-CHO) in the cases of methanal and ethanal. Mass 29 is the highest peak in the spectra of these two aldehydes. Propanal likenise gives m/e 29 as the highest peak in its spectrum. The spectrum of oxygen-18-labeled propanal, prepared hy exchange with H2018 as described by Cohn and Urey ( S ) , indicates that the mass 29 peak is caused bv the aldehyde group. Mass 29 for the straight-chain aldehydes of higher molecular 11 eight fluctuates around 40% of the highest peak. The mass 29 peak of (ouygen-&labeled) n-butanal appears to be primarily a hydrocarbon fragment, marking the change in identity of this mass from the aldehyde group to the ethyl ion. All of the branched-chain aldehydes studied having more than five carbon atoms give 29 peaks of l e v than 35%. The alkyl fragment resulting from alpha cleavage is insignificant for most of the compounds observed. A significant and, in several cases, the highest peak in the straight-chain aldehyde series occurs a t m/e 44 (Figure 3). This ion must arise from rearrangement of the molecule during ionization and fragmentation, as no mode of simple bond cleavages will yield this mass. Because this peak can conceivably be accounted for by either a hydrocarbon (C3Hs) or an oxygencontaining fragment (CSH40) , definite evidence is needed to show whether the hydrogen is migrating to the alkyl or to the carbonyl-containing fragment. This n a s undertaken by an attempted resolution of the large mass 43 and mass 44 peaks of a mixture of propane and n-hexanal on the Consolidated 21-103 mass spectrometer. The metastable suppressor voltage was increased, thereby narrowing the virtual ion exit slit. Slow scanning of the mass 43
and mass 44 peaks of pure n-hexanal showed that both peaks are symmetrically shaped and that each peak isat least primarily-one type of fragment. A mixture of n-hexanal and propane run under these same conditions of high resolution gives a symmetrical mass 43 peak (Figure 4). Because a major part of this must be C3H71from propane it indicates that the main mass 43 fragment of n-hexanal is also the propyl ion and not C2H30+. The mass 44 peak is observed to be partially resolved. This peak results from the ionized propane (C3Hs+)and to a significant extent from the C2H40+fragnient of n-hexanal. The mass 44 peak of n-hexanal is therefore primarily a result of beta cleavage, accompanied by the rearrangement of a hydrogen to the ouygen-containing fragment. This beta cleavage n ith hydrogen rearrangement to the oxygen-containing fragment to give a mass 44 is a very prominent feature of straight-chain aldehyde spectra (Figure 3). It hecomes important only for aldehydes with 11hich the loss of a methyl group is not involved.
Mass 4 3
Figure 4. n-hexanal
Mass 4 4
Spectra of propane and
Propanal s h o w very little mass 44 (t,he loss of methyl is involved), but mass 44 is the highest peak in the spectra of straight-chain aldehydes having four to seven carbon atoms. I t gradually decreases n-ith increasing molecular weight to about 30% of the highest peak for n-tetradecanal, the aldehyde of highest molecular n-eight examined. Nass 44 is typical of the n-aldehydes. The straight-chain aldehydes having more than five carhon atonis give niajor losses of 44 mass units (listed separately in Table I as P-44). Oxygen-& labeled aldehydes shoiv that the fragments resulting from this loss are alkyl fragments of the general formula C,H2,,. The loss of 44 mass unit3 decreases n-ith increasing molecular weight, but is still 67, of the highest peak for n-tetradecanal. This beta cleavage with the charge going to the alkyl fragment minus a hydrogen its significant for the identification of VOL. 29, NO. 7 , JULY 1957
991
straight-chain aldehydes with more than five carbon atoms. All examples of branched aldehydes except one are alpha substituted. These show marked beta cleavage accompanied by the rearrangement of a hydrogen, g,iving a mass 58 carbonylcontaining ion for alpha-methyl substitution and a mass 72 for alpha-ethyl substitution. This beta cleavage is almost exclusively the loss of the larger alkyl fragment in the compounds studied. For branched aldehydes having the same alpha substitution, the relative abundance of the peak resulting from beta cleavage with hydrogen rearrangement increases as the number of carbon atoms in the larger alkyl branch increases. As is seen for the alphamethyl substituted aldehydes (Table I), the mass 58 peak rapidly becomes the highest in the spectrum as the number of carbon atoms increases. A similar observation is made for the two alpha-ethyl substituted aldehydes. The 2-methylpentanal spectrum (Table I) shows two major fragmentsthe mass 43 alkyl fragment and the mass 58 rearrangement fragment. Both are results of beta cleavage. These peaks are contrasted to the significantly smaller mass 71 fragment which results from alpha cleavage and/or from the loss of only a portion of the longer alkyl branch. The masses resulting from beta cleavage compose a larger part of the total number of ionized fragments than the fragments resulting from beta cleavage of the straight-chain aldehyde of the same molecular weight. The branched aldehydes give more exclusive cleavage a t bonds adjacent t o branching and less cleavage all along the carbon-carbon chain than the straight-chain aldehydes. This is in agreement ITith the increased fragmentation found a t chain branches in hydrocarbons. I n contrast to the behavior of the straight-chain aldehydes, the 42 peak (from which a hydrogen has rearranged) is much smaller than the 43 peak. 2-Ethylhexanal also shows two major peaks, which are results of beta cleavage-the mass 57 alkyl fragment and the mass 72 rearrangement fragment. These two fragments compose a larger portion of the total spectrum than fragments which result from beta cleavage of n-octanal. Only a very small mass 99 peak is observed, which shows that alpha cleavage, the loss of the smaller alpha-alkyl group, and the loss of only a portion of the larger alkyl group are insignificant. 3-Methylbutanal is the only example of beta substitution obtained. The two major peaks (Table I) are again results of beta cleavage as in the case of alpha substitution. The large mass 43 alkyl fragment and the major mass 44 rearrangement fragment result from beta cleavage. The loss of 29 to give
992
ANALYTICAL CHEMISTRY
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mass 57 and the loss of methyl to give mass 71 are small in comparison to the fragments given by beta cleavage. Ions resulting from the loss of 28 mass units are significant for identification of the straight-chain aldehydes (Figure 5 ) . Evidence from spectra of oxygen-18-labeled aldehydes indicates that the loss of 28 mass units is the loss of C2H4. This loss therefore results from beta, gamma, delta cleavage, etc., accompanied by the rearrangement of a hydrogen atom. The molecular ion minus 28 mass units is small for propanal; it is the largest peak in the spectrum of n-butanal (n-here beta cleavage and rearrangement are involved), and then slopes off n i t h increasing molecular weight. The lo-s of 28 is still a significant peak of IC/, relative abundance for tetradecanal. For the n-aldehydes with four or more carbon atoms the ion resulting from the loss of 28 mass units is larger than the molecular ion and one of the two major peaks observed in the mass range from the molecular ion to the loss of 44. This loss presents a difficulty in unknown mixtures, for the loss of 28 fall? a t the same mass as the molecular ion of an aldehyde with two less carbon atoms. I n the branched-chain series, the parent peak minus 28 varies ratlically, depending upon the type and position of substitution. Ions resulting from the loss of 18 mass units are also significant. For n-aldehydes with six or more carbon atoms this loss is larger than the molecular ion. The molecular ion minus 18 mass units is not typical for the branched-chain structures. With an increasing molecular weight in the straight-chain series, the hydrocarbon portion of the molecule becomes a predominant part of the spectrum. Thus the mass 43 or the mass 57 peak is the highest in the spectrum of a n-aldehyde with eight or more carbon atoms. Oxygen-18-labeling has shown that these peaks are results of alkyl fragments. Fragments resulting from carbon-carbon cleavage and the loss of two hydrogen (C,H2,-1) similar to that found in hydrocarbon spectra are also observed throughout the aldehyde spectra.
-?
E % N P - N ~
rnm
STRUCTURE DETERMINATION ri
Pahla
h*
o
An opportunity for structure elucidation from these correlations arose when a mixture of two aldehydes was obtained for identification. Some of the important peaks found in the mass spectrum were:
Mass 43 44
Relative Peak Height 64
7
Mass
Relative Peak Height
86 98
3 2T
VOL. 2 9 , NO. 7,JULY 1957
993
\Iars
Relative Peak Height
Mass
57
100
114
58 55
12
124
8
142
Relative Peak Height 0.3 0.4 0.5
The peak a t highest m/e given by the spectrum was 142-the molecular \veight of aldehydes with nine carbon atoms. The loss of 18 niass units ( ) i t ; e 124) and tlie loss of 28 mass units (m,!e 114) were both smaller than the molecular ion, showing that the aldehydes were not straight'-chain compounds. A very small mass 44 peak confirmed branching. The mass 58 and i 2 peaks were not much larger than could be accounted for by carbon isotope cont,ributioiis, which revealed that the aldehydes were not alpha-methyl or alpha-C2 substituted. There \\-as a significant peak ut mass 98-the loss of 44 niass units from the molecular ion. These compounds were therefore substituted farther from the aldehyde group tlian t h e alpha position. This loss of 44 mass units also slion-ed that beta cleuvage accompanied by hydro,wen rearrangeineiit was still an important spectral feature for substitutions other than alpha. The large mass 43 and 57 fragments pointed out that, the structures were sucli that cleavage adjacent to points of branching ivould result in these masses. The rather small mass SO fragment indicated that cleavage adjacent to a point of branching would not result in the inass. Because the sample was a mixture and primarily because standards with similar substitutions were Iacbking, no definite structures were claimed; however, many possible aldehydes ivere definitely eliminated. Two structures possible from the niass spectrum arid consist'eiit with cheniical indications are CH3CH(CH3)CH2CHrCH(CH,)CH,CHO anti CH3C(CH& C,H,CH(CH,)CII,CHO. MECHANISM
Initial ionization probably takes place by loss of one of the nonbonding electrons of the oxygen atom. Similar charge localization has been postulated for electron bombardment of alcohols (4, S). Tlie carbonyl group loners the electron density in the bond to the alkyl substituent, enhancing the probabilitjof rupture of the alpha bond to give the stnble HCO ion and tlie alkj 1 radical.
R
:c
: 0:'
R
+
H : C : '0.'
H
994
ANALYTICAL CHEMISTRY
NUMBER OF CARBON ATOMS
Figure 5.
Loss of 28 mass units in straight-chain aldehydes
The HCO ion is not as stable as the RCO ion formed in esters, acids, ketones, etc., so that as the alkyl group of the aldehyde gets larger, other cleavages predominate. TVhen rupture takes place at the beta bond, the rearrangement of a hydrogen atoni may be caused by an electron deficiency on the cmhonyl group.
the probahility that the single electron will go on the ethylenic fragment, so that tlie oxygen-containing fragnient is the niairi ion observed. Similar mechanisms have been proposed for other electronegative fiuictional groups (8, 5'). ACKNOWLEDGMENT
The authors n ish to thank Roland H H
R c c c,%
!4
H H H ' H
0'
- R
C C C
C 0 H'
H H H
H
S. Gohlke for the aldehydes purified by the use of vapor phase c~hromatopraphy. LITERATURE CITED
Formation of an enolic--t\-pe doulile bond as shon n from the ion n ould leave only sly electrons around the oxygen atom. Transfer of a gamma hydrogen with its elertron could tie acconiplishetl through a qterically favorable six-meinbered ring intermediate. Both the ethylenic molecule and the osvgencontaining ion formed are relatively stable, which contributes to the probability of their formation. Rearrangenient of the beta hydrogen ma\- also be a factor, but this would result in tlie unstahle R--CH,-CH radical. Hydrogen rearrangement n ith cleavage of a bond beta to an electronegative functional group is typical of t h k type of molecule containing a gamma-hydrogen atom (10). The ion structure CH3CHO' seemq less likely froni the geometry of formation involved. Preliminary reiults of appearance potential studies of Sharkey and Hickam (13) support this honding of the rearranged hydrogen to the oxygen in the resulting ion for aliphatic. esters. Cleavage of the beta bond in the last step to leave tlie single electron on tlie ouygen-containing fragment iyould yield the positive ethylenic ion nhich is observed in the spectra of the straightchain aldehydes. K i t h hranching on the alpha-carbon atom, the electrondonating alkyl substituent increases
(1) Cnldecourt, V. J . , +lx.u,. CHEN.27, 1070 (1955). ( 2 ) Caldecourt, T'. J., ASTLI Conimittee E-14 Conference on 11s~::
Specti,ometry, S e w Orlems, IIsy 19.X
(4) Cummings, C.' S.L Bleakney, \T-,. Phiis. li'ec. 58. T X r 11940) --). ( 5 ) Friedman, I,.; T&kevich, J . , J . A t i ) , Chem. SOC.74, 1666 (1952). (6) Gohlke, R . S., McLafferty, F. W,, XST1I Committee E-14 Conference on Mass Spectrometry, Sail Francisco, I I a v 1955, ( 7 ) Gohllte, R. S.,IlcLafferty, F. \I-,, Division of .inalytical Chemistry, \ - -
Symposium on Vapor Phasti Chromatography, 129th Ifeeting, ACS, Ilallas, Tek., 1956. (8) Gohlke, R . S.,McLafferty, F. IT-,, Division of Gas and Fuel Cheniistry, 127th LIeeting, .1CS,Cincinnati, Ohio, 1955, Happ, G. I'., Stewart, D. \I-.,J . .A v?. Chem. SOC. 74, 4404 (1952).
1IcLaffertv. F. IT-., 28, 306 "(1956).
-ISAI.. CHEX
'
JlcI,sffei,ty, F. K.,-4STLI Conimittee 1;-14 Conferrnce on ;\Tas* Spectromrtry, San Fraricisco, 11a)1055.
Sharkey, .%. G., Jr., Phnltz, J . I,.. Fi,iedel, R. A , ) ANAL. CHEAI.28, 93-1 (1'356).
Sharkej-, X. G., Jr., Shdtz, J . L., Friedel, R.A., .%ST11 Committec IC-14 Conference on l l a s s Spec.tromet>ry,Cincinnati, AIay 1956. RECEIVED for review October 5 , 1056. Accepted March 29, 1057. AST1I E-14 Comm,ittee on I I a s s Sl)ectronirti~y,(:incinnnti, 1 f a v 1956.
