June 5 , 1961 [CONTRIBUTION PROM THE
ELECTRON IMPACT
ON
BENZYL -ALCOHOL
2481
RESEARCH AKD DEVELOPXENT DEPARTMENT, AMERICANO I L CO.,
Organic Ions in the Gas Phase.
WHITING, IND.]
IX. Dissociation of Benzyl Alcohol by Electron Impact
BY ERNEST L. ELIEL,’JOHN D. MCCOLLUM, SEYMOUR MEYERSON AND PAULN. RYLANDER~ RECEIVED JASUARY 26, 1961 The mass spectra of deuterated benzyl alcohols reveal several ionic dissociation paths besides the previously studied formation of tropylium ion by loss of a hydroxyl radical. These competing paths seem generally directed by the oxygen atom. Metastable peaks and label retention in fragments allow the relative importance of a number of competing paths is formed by two different paths-loss of a formyl radical, and t o be assessed. The most abundant ion in the spectra, successive loss of a hydrogen atom and carbon monoxide. Although benzaldehyde (or an isomeric C~HBO’)ion comprises only 0.5% of the total fragment-ion intensity, as much as 6.8% of the observed fragment ions seem best accounted for by a primary dissociation t o form C7H6O +.
IYhen gaseous molecules a.re subjected to electron bombardment in a mass spectrometer, they react by unimolecular processes to yield a variety of ionized products. I n a recent studyP3evidence was presented that benzyl chloride and benzyl alcohol, like 1-phenylalkanes, undergo ring-expansion and cleavage of a bond originally beta to the ring to form C7H7+ with the tropylium structure. From benzyl chlorides, as from the lower l-phenylalkanes, C7H7+ is the most-abundant ion formed3; however, i t was found to be only a minor decomposition product of benzyl alcohol. The mostabundant ion in the mass spectrum of this compound is instead C6H7+, and oxygen-containing ions are prominent. The sharp contrast between the major mass-spectral features of benzyl alcohol and other benzyl compounds that have been examined suggested that the oxygen atom exerts an important influence on decomposition and prompted a more-extensive study of the alcohol. The approach selected was isotope-labeling, which has contributed heavily to existing knowledge of the structures and reactions of gaseous ions. Label retention in many fragment ions can be simply calculated from the mass spectra of labeled corn pound^.^ Such retentions necessarily reflect, and can therefore help clarify, the structures of the precursors and the nature of the decomposition proce~ses.~Independent evidence on some reaction paths was provided by metastable peaks, which stem from decompositions that occur near the slit between the accelerating region and the analyzer of the mass ~pectrometer.~The mass of a metastable peak nearly always determines a unique pair of values for the masses of the parent and daughter ion, and thus defines one step in a decomposition path.6 The mass spectra of benzyl alcohol unlabeled and deuterated in four different ways-ortho-d, meta-d, alpha-dl and alpha-dz-were recorded in the earlier work.3 Table I’ shows the three mass regions ( I ) Research participant from Department of Chemistry, University of Notre Dame, summer, 1956. ( 2 ) Engelhard Industries, 113 Aster St., Newark, N. J. (3) S. Meyerson, P. N. Rylander, E. L. Eliel and J. D. McCollum, J . A m . Chem. SOC.,81, 2606 (1959); see also A. G. Harrison, L. R . Honnen, H. J. Dauben and F. P. Lossing, i b i d . , 89, 5593 (1960). (4) S. Meyerson and P. N.Rylander. J . Phys. Chem., 6 2 , 2 (1958). ( 5 ) H . M. Rosenstock, A. L. R’ahrhaftig and H. Eyring, “The Mass Spectra of Large Molecules. 11. T h e Application of Absolute Rate Theory,” University of Utah, Salt Lake City, Utah, 1952, pp, 9 5
studied-105 to 110, corresponding to the composition C7H,0+; 75 to 83, C6H,+; and 29 to 33, CH,O+. Table I1 lists metastable peaks corresponding to the formation of C7H70+, mass 107; C6HS+,mass 77; C6H7+, mass 79; and C&+, TABLE I PARTIALSPECTRA OF BENZYL ALCOHOLS Relative intensities a-di m-d
7--
m/e
do
o-d
110 109 108 107 106 105 83 82 81 80 79 78 77 76 75 33 32 31 30 29
...
...
...
...
100.0 73.0 7.4 5.1 0.4
100.0 71.5 7.3 4.7 0.4
.1
.1
.3 5.1 111.9 21.4 54.1 16.9 1.9 3.6 0.0 0.6 10.8 2.2 14.2
.4 5.0 111.3 20.1 53.5 15.8 1.7 3.3
100.0 70.3 9.1 0.6 4.1 0.1 .3 4.8 94.4 32.0 35.2 36.5 2.0 2.8 0.1 10.4 2.0 8.7 7.3
... 100.0 80.7 2.7 5.4 0.1 .1 .4 4.4 117.4 11.o 65.5 2.3 2.7 0.0 0.2 9.8 1.2 14.1
0.0 0.6 11.6 1.7 14.3
a-dz
100.0 59.3 18.2 0.7 0.7 2.5 0.2 4.7 65.9 57.3 17.2 33.2 21.4 2.5 2.6 11.0 1.2 1.3 12.8 4.1
TABLE I1 SELECTED METASTABLE PEAKS IN THE SPECTRA OF BENZYL ALCOHOLS APparent Transition
m/e
108.0 107.0 106.0
(110+) (log+) (110+)
-+ -+ -+
(1OS+) 76.1 75.1 74.1 60.5 59.6
-.
(79+) -+ (80+) -+ ( l o g + ) --r (110+) -+
(108 +) 58.9 58.6 58.2 57.8 57.3 3 3 7
-+
+1 (108+) + 1 (108+) + 2 (107+) + 1 (78+) + 3 (77+) + 2 (77+) + 3 (811)+ 28 (81+)+ 29 (80+) + 28 (80+)+ 29 (79 +) + 28 (80+) + 30 (79+) + 29 ( 7 9 9 + 30 (78+) + 30 (log+)
--+
( l o g + ) -+ (107 +) (110+) --r (l08+) 4 (log+)
-+
-
(108+)
-+
do
..
.. ..
1.52
..
..
2.53
..
.. .. ..
..
0.35
..
.86
..
.03
Relative intensities o-d m-d a-di
a-dr
. . .. .. 1.38 1 . 5 9 1.77 1 . 8 6 .. .. .. .. 0.20 .. . . . . ..
2.05
..
2.35
..
0 . 1 4 0.14 .GO .65
.. ..
.. ..
.40 .64
.51 .69
.. .. ..
0.05
..
.. .. ..
0.04
..
2.34
..
0.64 .42
.. ..
.37 .58
..
..
.. ..
0.06
1.08 0.75
..
.46 .32 .58
..
,.
..
0.14
.. . ..
fi. ( 6 ) P. N. R y l ~ o d e rmad 6 . Meyerson, J. A m , C h c m . SOC., 78, 67R0 (1966).
(7) Spectra are corrected for isotopic impurities, for naturally curring C”, and, in the parsat-mmse region, for 0’’ and
0 ‘ 8 ,
OC.
2482
E.
r,.
ELIEL,
j. D.
MCCOLLUM,
S.MEYERSON
AND
P. N. RYIANDER
Vol. 83
mass 78, from the unlabeled alcohol and the analogous ions from the deuterated alcohols. Ions in the C7H,0+ Region TWO ions in the C;€I,O + region were of particular interest: C7H70+ because of its high relative intensity, and C7H60+ because of its well-defined deuterium retention despite its low intensity. The C7H70+ Ion.-The normal peaks a t the parent mass (molecular weight) less one and the metastable peaks a t 108.0, 107.0 and 106.0 establish the reaction C?HsO'
---f
C7H70C f H
The observed deuterium retention in the C7H70+ ion corresponds closely to random loss of one out of the eight hydrogen atoms, or somewhat less closely to random loss of one out of the seven non-hydroxyl hydrogen atoms, with a slight preference for loss from the side chain rather than fram the ring: Deuterium retention, yo
Calcd. for random loss: One out of eight One out of seven Observed
o-d
m-J
87.5 85.7 90.5
87.5 85.7 88.6
a-dl
a-dz
87.5 75.0 85.7 71.4 87.1 73.5
Whether hydrogen loss merely occurs in a random fashion among all the available atoms, or a randomizing rearrangement precedes the loss, cannot be determined conclusively from these data. Differences as large as 1 e.v. might be expected among the dissociation energies of the three types of bonds by which hydrogen atoms are held in the molecule. However, such differences evidently have little influence on the order of loss of hydrogen in forming the C1H70+ ion. Thus the data seem to favor a randomizing rearrangement, although details are not clear. The C7HSO+Ion.-The C7H60+ ion of mass 105 is a minor product in the spectrum of benzyl alcohol; it is formed only to the extent of 5% of the parent-ion yield. However, the 105-106 profile in the spectra of the labeled alcohols reveals a curious result. Intensities of the ring-d alcohols a t 106 are nearly equal to that of the unlabeled alcohol a t 105 and the ring-d species show almost no 105 peak. The a-dl alcohol, on the other hand, shows little a t 106 but gives a 105 peak of intensity approaching that of the unlabeled compound. Evidently the C7H50f ion is produced by preferential loss of the a-hydrogens from the moleculeion. The same preference for loss of alpha over ring hydrogen is found in the a-dz alcohol, although the occurrence of an isotope effect makes interpretation here less clear. No randomizing migration of hydrogens can have occurred in the formation of C7H6Of, unlike C7H70+ and CKHsf.' Hence, formation of C,HbO+ cannot involve a consecutive reaction via C7H70+ unless two different C7H70+species occur, of which only one remains intact long enough to be detected and the other gives rise to C?H50+. A more probable sequence mould seem to be -Ha
-H C7H80' ---P C ~ H G O+ ' CrH50'
A likely structure for the CtHaO + ion is that of benzaldehyde; for C7H50+,the benzoylium ion
The latter structure might be expected to have enhanced stability from the resonance forms shown and from conjugation of the carbonyl group with the ring. An ion with this probable structure is formed in high yield by electron impact from phenyl alkyl ketones* and from benzaldehydeg; in the mass spectrum of benzaldehyde, loss of hydrogen is solely from the a - p o ~ i t i o n . ~ Ions in the C6H,+Region The C8Hn+region contains two of the most abundant ions produced from benzyl alcohol by electron impact-C6H7+ and Cd&+-and significant quan tities of C6&+ and C&f+3+. Label retention in Cd&+ and CeH7+ is fairly well defined, but any calculation for CsH+3+is subject to large cumulative error^.^ Uields of unlabeled C6H5+ ions can be calculated if one neglects possible small contributions a t mass 77 of deuterated C6H4+ and C6H3+ ions. Total ion intensity in the mass range 77 to 83 is nearly constant for the five isotopic species: dl
o-d
m-d
a-di
a-dr
198.9
209.8
206.2
203.3
199.9
Thus, the over-all production of 6-carbon ions is attended by no more than (perhaps) a small isotope effect, in accord with the usual simplifying assumption4 that no appreciable isotope effect occurs in the formation of any one of the ions in the group. The CJ&+ Ion.-Relative intensities of the unlabeled, singly labeled and doubly labeled alcohols a t masses 80, 81 and 82 are nearly enough equal to leave little doubt that the ion involved has retained all the original hydrogen atoms. The mass of this ion corresponds to the loss from the moleculeion of 28 units-evidently as CO rather than CzHa, because some deuterium loss from the labeled species would surely be observed if ethylene were split out. The reaction may be summarized as le-
+ C B H ~ C H ~ O+ H C B H ~4+ CO f 2e-
The CeH8+ fragment must arise from some molecular rearrangement, although details of the reaction are not clear from the data thus far obtained. The evident loss of CO from the benzyl alcohol ion is in accord with the suggestion of Beynon lo that loss of CO is a favored reaction of gaseous ions even when it may entail extensive prior rearrangement. The C6H7+Ion.-The most abundant ion in the mass spectrum of benzyl alcohol is CeH7+, formed through loss of the elements of CHO. Metastable peaks a t 5S.6 and 57.8 in the spectrum of the unlabeled species indicate a t least two paths for formation of this ion, one from the C7H70+ ion and the other from the parent ion Path I:
C?H?O+--+ CBH7' f CO 107 79 28
Path 11: C7H80' 108
--
+CsHi' 79
f CEIO 29
(8) S. Meyerson and P. N Rylander, J. A m . Chcm. S O ~ .7,9 , 10% (1957). (Q) J. D. McCollum aud 5. Meyerson, in prepardtiou.
Corresponding metastable peaks occur in the spectra of the labeled species. Deuterium retention in CeH7+--95%, 95% and SOYo from the o-d, m-d and a-dl alcohols and 56% (both atoms) from the a-d%alcohol-indicates a marked preference for loss of a-hydrogen in forming the CCH7+ ion. Inasmuch as formation of the C7&0+ ion was shown to involve nearly random loss of hydrogen, C6H7+ formed from it by path I should reflect the same random loss and show no preference for retention of ring over a-deuterium atoms in the spectra of the labeled species. Discrimination in formation of the C&17+ ion must then be attributed to path 11, loss of CHO from the parent molecule-ion. If all the hydrogen lost by path I1 came, with equal probability, from the two a-positions and the hydroxyl group, one would expect 100% retention of deuterium in the C&+ fragment from the ringd species, 67y0 retention from the a-dl species, and 33Y0 retention of both deuterium atoms from the a-dz. On this basis one may approximate the contribution of each of the two identified paths to formation of the CeH7+ ion. The values of deuterium retention so calculated are Deuterium retention,
2483
EJ~ECTRON IMPACT ON BENZYLALCOHOL
June 5, 1961
70
m-d
o-d
Calculated : Path I alone Path I1 alone 55% path I, 45% path I1 Observed
90.5 100
a-di
88.6 87.1 67 78 95 80
100 94
95 95
a-di
73.5 33 55 56
The 55~45ratio of the contributions of the two paths is necessarily approximate. From metastable peaks, C6H7+ ions are known to decompose further. The ratio was arrived a t by assuming either that the C6H7+ ions produced by the two paths have the same structure and energy content or that, in any case, the two groups decompose a t about the same rate. Further, more than the two identified modes of formation of C6H7+ may exist. Nonetheless, good agreement of calculated with observed values suggests that the two paths contribute about equally to CsH7+ formation. To account for the observed labeling of the CsH7-t ions from labeled benzyl alcohols, some rearrangement must have occurred. Again, as in formation of the C?H60+ion, the data preclude random cominingling of the hydrogel1 atoms in all the parent ions decomposing to form C6H,+ ions. The C a , + Ion.-Although label retention in the C6H6+iori cannot be deduced with any coiifidence, one path for its formation CsHsCHzOH'
--+
CeHe,+
+ CIIzO
is established by the metastable peaks a t 56.7, 37.3 and 58.2 in the spectra of the unlabeled, singly labeled and doubly labeled alcohols. l0,11
+
CsHsCHDOIl+ +C&+ CDO, and Ce,HaCD20H++ C&D+ CDO
+
intensities a t mass 77 measure the yields of unlabeled C & , + ion. Furthermore, if the mass-77 intensity in the spectrum of the unlabeled alcohol is taken as a measure of total GH6+ yield, the percentage of unlabeled CsH5+ formed from the labeled alcohols then can be computed: o-d, 26%; m-d, 24%; a-dl, 56y0; a-dz, 33%. The only mode of formation for which metastable peaks were detected-at 76.1, 75.1 and 7 4 . 1 4 s loss of HZ from CsH7+.l2 This reaction alone-coupled with the assumption that hydrogen loss occurs in a random fashion-annot account for the observed statistics. These can be accounted for by assuming that 78% of the C6H6+ions are so derived and that the other 22% consists of the elements of the original phenyl group : Deuterium retention, yo
o-d
n2-d
a-di
a-d:
Calculated : (a) Random loss of HZfrom
caH7
+
(b) Originalphenylgroup 78% (a), 22% (b) Observed
32 0
25 26
32 0 25 24
43 100 55 56
15 100 34
33
A C6Hs+ ion derived from the original phenyl group might be formed either by direct dissociation of the C6H5-CHzOH+ bond in the molecule-ion or by a secondary reaction of a benzaldehyde-ion intermediate, as was suggested also for the C7H60+ ion. The lack of scrambling of hydrogens in these processes is consistent with the observed labeling of the complementary CH30+ and CHO+ ions discussed below. In the mass spectrum of benzaldehyde, CeH5+ is the most abundant ion and it is derived almost solely from the original phenyl group.9 Ions in the CH,O+ Region In the CH,O+region, two ions, CH30fand CHO+, attract special attention both because they give rise to prominent peaks and because their intensities in the spectra of the variously deuterated alcohols are simply related numerically, The CH30+ Ion.-Intensities of the unlabeled and ring-d alcohols a t mass 31, a-dl a t 32, and a-d2 at 33 are nearly enough equal to leave little doubt that they are due to CH,O+ ion containing the elements of the original side-chain. The carboncarbon bond to the phenyl group is cleaved with no detectable exchange of hydrogen atoms between the fragments. Such behavior contrasts sharply with that of toluene, in which neither the CeH5' nor the CH3+ ion contains solely the elements of the corresponding group in the original molecule. l a The reaction of benzyl alcohol seems to reflect the influence of the hydroxyl group and is in keeping with the observed cleavage in aliphatic alc o h o l of ~ ~the ~ C-CH20H bond-also with no accompanying hydrogen exchange.
The C&+ Ion.-If possible slight interference from labeled C.&+ and, in the spectrum of the adz alcohol, from doubly labeled C&3+ is neglected,
(12) This reaction has been reported in the dissociation of several other benzene derivatives (see refs. 4 and 6). It seems to be characteristic of CsH7 + ions regardless of the parent molecule. (13) P. N. Rylander and S. Meyerson, J . Chon. Phys., 87, 1116
(10) J. H. Beynon, G. R. Lester and A. E. Williams, J . Phys. Chcm., 63. 1861 (1959). . . (11) In the spectra of the a - d i end a 4 species, these metastable peaks may be due in part to the reaction8
(14) J. G. Burr, J . Phys. Chcm., 61,1477 (1957); L. Friedman, F. A. Long and M. Wolfsberg, J . Chcm. P h y s . , B7, 613 (1957); W. H. McPadden, M. Lounsbury and A. L. Wahrhaftig, Can. 1.Chcm., 86, QRIl (1958).
(1957).
2484
ROBERT 0. LINDBLOM, RICHARD M. EEMMON AND MELVIN C.ILVIN
C'ol. 83
The CHO + Ion.-Intensities a t masses 29 and 30 are accounted for by CHOf ions of which none is labeled in the ring-d alcohol, half are labeled in the a-dl, and all are labeled in the a-dz. These statistics suggest the reaction sequence
Intensity of the C7HsO + ion-benzaldehyde or 211 isomeric structure-comprises only 0.5yoof the total fragment-ion intensity in the mass spectrum of benzyl alcohol. However, if, as suggested, the entire observed yields of C7H60+and CHOi and 229; that of CGH5+ result from further reactions of benzc I r 7 aldehyde ions, the fraction of the total fragment,H ion yield resulting from a primary dissociation to form benzaldehyde ion rises to 6.8%. The intensities attributed to further decomposition products of the CiH60f ion, relative to the intensity of CiThe second step of the path shown is known from H60+itself, are much greater than are those of the study of the mass spectra of labeled benzaldehyde^.^ corresponding ions in the spectrum of Denzaldehydc. An alternative possible route-loss of Hz from CH2- Thus, the C7H60+ion derived from benzyl alcohol OH+-cannot be ruled out, but such a reaction is not appears to be formed in a different distribution of prominent in the spectra of aliphatic alcohols.14 excitation states from those formed directly by Moreover, one would expect further dissociation electron impact on benzaldehyde. The mass spectrum of benzyl alcohol presents a of CHzOH+ to be preceded or accompanied by exmore complex network of competing and consecutensive loss of identity of hydrogen atoms. tive reactions than do those of toluene, ethylbenConclusion zene and higher alkylbenzenes. 4 , f i ~ 8 , 1 3 , 1 b The added Any appreciable benzaldehyde impurities that complexity evidently results from the presence in might have escaped detection would compromise the the molecule of a second functional group that can data for C7H50+,CsHS+and CHO+. However, the also serve as a center of reactivity. The factors evident consistency among the spectra of the five that make interpretation of the spectra of such differently labeled benzyl alcohols seems to rule out molecules more difficult can, a t the same time, serve such impurities. Moreover, even if benzaldehyde as probes to explore aspects of competition and inwere solely responsible for the observed C7H60+ in- teraction between functional groups within a moletensities, it could account for no more than half the cule. observed C7H&+ intensities, 5% of the C6&+ and (15) S. Meyerson, A p g l . Specfi.., 9 , 120 (19.73); 1' S . Rylander, S , 3% of the CHO+. Thus, the characteristic features hleyerson and H. M , Grubh, J . Am. Chem. SOL.,79, 842 (1957); S. of the spectrum resulting from these ions must be Meyerson and P. N. Rylander, J . Chein. Phys., 27, 901 (1957); J . D. McCollum and S. Meyerson, J . Ani. C h e w SOL.,81,4116 (1959). attributed to benzyl alcohol itself. I -
[CONTRIBUTION FROM THE
DEPARTMENT OF
CHEMISTRY 4SL, 'I'HI) IA.4\VKENCE I?.ADI.1TI
