Organic Mass Spectrometry, 1969, Vol. 2, pp. 1317 to 1324. Heyden & Son Limited. Printed in Northern Ireland

KINETIC STUDIES IN MASS SPECTROMETRY-VI:

THE [M - CHJ REACTION IN SUBSTITUTED ETHY LBENZENES"

PETER BROWN Chemistry Department, Arizona State University,

Tempe, Arizona 85281, USA

(Received 16 June 1969; accepted (revised) 14 August 1969)

Abstract-The [MI --* [M - CH,] reaction in a series of in- andp-X substituted ethylbenzenes has been studied by wide range electron energy kinetics and metastable ion characteristics techniques. By this approach, qualitative measures of activation energy differences between [XC,H,CH,]+ ions derived from m- andp-X isomer substrates have been secured, for both their formation and further decomposition. These. energy differences are consistent with (but do not prove) ion structures that have been suggested by previous work in this area, involving the use of isotope labeling, and ionization and appearance potential methods.

INTRODUCTION

THE EXPERIMENTAL methods currently avaikble for ion energylstructure studies have recently been reviewedl and some of their major limitations carefully pointed out. In view of the rather limited number of such techniques, it has appeared worthwhile to investigate2 alternative approaches to the estimation of relative activation energies for unimolecular ion decomposition. It has been notedZ that activation energies for both formation and decomposition of the ion in question are a minimum contribution to complete characterization. Since precise activation energies (e.g. AP-IP)3*4 are not universally accessible, activation energy differences have been obtained in a qualitative manner2 for closely related reactions of isomeric ionized substrates (e.g. m- andp-X isomers), presumably with very similar frequency factors. In order to correlate relative activation energy differences with ion structure, systems have been investigated2 that have been previously studied by the conventional procedures of ionization and appearance potential determinations, and isotope labeling techniques.

DISCUSSION

As a convenient measure of unimolecular ion decomposition rate, the pioneering steady-state treatment of McLafferty and Bursey5 initially appeared to offer great promise. Recent work, particularly by Williams et al.,l*4*6 has led to a more funda- mental reinterpretation of the kinetic approach in terms of the quasi-equilibrium theory of mass ~ p e c t r a , ~ . ~ and several oversimplifications in the originals derivation, applications and assumptions have come to light.1*6*9,10

It has been suggested2 that under certain carefully specified conditions, use of Z = [A]/[M] as a measure5 of the reaction rate [MI+- +- [A]+ is most likely to be a

* Presented in part at the ASTM Committee E-14, 17th Annual Conference on Mass Spectrom- etry and Allied Topics, Dallas, Texas, May, 1969.

1317

1318 PETER BROWN

good approximation.* These are: (i) by selecting model systems in which the molec- ular [MI+. and daughter [A]+ ions comprise the major portion (ideally > 90%) of the total ionization at all electron energies (neither [MI+* nor [A]+ should undergo any other significant decomposition);6.10*11 and (ii) by comparing only the effect on activation energy for the same reaction of the smallest feasible structural change in isomers., Thus (i) eliminates any competing reactions in either [MI+- or [A]+, and (ii) is intended to ensure effectively the same heat of formation for the initial neutral isomers, and the same frequency factor and numbers of degrees of freedom for their reaction.z In practice, the one major [M - Y] reaction of m- and p-X substituted benzyl compounds2 XC,H4CH2Y (Y = OC,H,, H) and phenyl compounds12 XC6H4Y (Y = C1) has been studied by wide range electron energy kinetics2 and metastable ion relative abundance2#I3 techniques, and measures obtained of the similarity or difference between activation energies for the decomposition of rn- and p-X derived isomeric [MI+. ions, and both formation and decomposition of isomeric [A]+ ions.

In wide range electron energy kinetics determinations,2*14 the parameters Zp and Zm for the one major reaction of an isomer pair of substrates are compared as a function of electron energy, and this energy dependence of Zp/Zm is taken to reflect activation energy differences or similaritie~~*~* (Fig. 1 in ref. 2). Alternatively, the situation may be represented by hypothetical log k us. E curve^^,^ (Fig. l), of the type calculated15 from quasi-equilibrium theory. Figure 1 shows curves for the one major reaction of a pair of isomeric ionized substrates, of differing activation energies El and E,, but closely similar frequency factors. If the isomeric molecular ions have very similar ionization potentials, then the situation is equivalent to the simplified energy diagram B in Fig. 1 , ref. 2, whereas different ionization potentials are exenipli- fied by energy diagram A in Fig. 1 , ref. 2. In the same way, energy diagrams C and D in Fig. 1, ref. 2, can be visualized in terms of identical activation energies El and E,, log k us. E curves of very similar form, and the same (C) or different (D) ionization potentials. For example, from the log k us. E curves in Fig. 1, it is apparent that when El # E, and v1 = v2, the relative rate ratios for the reaction of each isomer pair of compounds will increase as average internal energy (E) of the molecular ion decreases. This is the same result expected from a consideration of energy diagram B in Fig. 1 , ref. 2.

As a further consequence of the theoretical log k us. E curve shapes, it has been suggested1 that at higher internal energies frequency factor differences become in- creasingly important, whereas at lower internal energies activation energies are d~rn inan t .~ ,~ Thus measurement of normal peaks produced by higher energy molecu- lar ions fragmenting in the ion source (log k > 5.5)l are expected to be rather in- sensitive to activation energy differences. Normal peaks formed from ions of lower internal energies and especially peaks due to decompositions of metastable ions (log k 4.5 to 53)l should in principle be better measures of activation energies, although there may be difficulties1*16 in expressing metastable ion peak intensities relative to some other normal peak in the spectrum. It has also been notedl that the use of low electron energies enhances the proportion of molecular ions with insufficient

* This is currently viewed as a temporary expedient until more secure experimental methods are developed.

Kinetic studies in mass spectrometry-VI 1319

FIG. 1. Log k us. E curves for the same reaction of m- and p-X isomers, with different activation energies and the same frequency factors.

energy to decompose. Recently, Williams et aL4 have reported the interesting finding that normal peak relative intensities in the 20 eV spectra of substituted phenyl benzyl ethers could be qualitatively accounted for by assuming energy distribution curves of a certain form to represent 20 eV conditions, and considering simply what fraction of molecular ions did not have sufficient energy to decompose.

With these considerations in mind, we report here wide range electron energy kinetics and metastable ion relative abundance data for the [M - CH,] reaction of a series of in- and p - X substituted ethylbenzenes. Among the reasons for selecting ethylbenzenes were the existence of previous studies that utilized ionization and appearance p o t e n t i a l ~ , l ~ * l ~ , ~ ~ and isotope labeling,17*19 and which presented compelling evidence for prior ring expansion in those molecular ions which undergo the [M - CH,] reaction.

More exactly, the earlier work established that only the original methyl group of ethylbenzene is lost,17 and that in the [M - CH,] ions [C7H7]+ that decompose further by loss of C,H, all the hydrogens become equivalent, as required by a tropy- lium ion structure.17 Subsequently, the important findings of identical appearance potentials for the [M - CH,] ion derived from m- and p-hydroxyethylbenzenes18 and that -17 % of the total [M - CH,] reaction of p-methylethylben~ene~~ involved loss of the ring methyl group suggested essentially complete ring expansion in the decomposing molecular ions before CH, bond cleavage at the appearance potential threshold:* and substantially less ring expansion at 50 eV.19

A further reason for selection of the [M - CH,] reaction in the substituted ethylbenzene system was that the analogous processes had already been examined2 in substituted benzyl phenyl ethers and toluenes. For the [M - HI reaction of m- and

1320 PETER BROWN

p-X substituted toluenes, wide range electron energy kinetics and metastable ion relative abundance techniques were consistent2*22 with ring expansion in the molecular ions when X = CH,, F, C1, Br and CN (tropylium-like activated complexes), and retention of substituent identity when X = OCH, (benzylic transition states). Metastable ion data were consonant with decomposing [XC,H,CH,]+ ions of sub- stituted tropylium symmetry when X = CH,, F, C1, Br and CN, but suggested less symmetrical ions when X = OCH,. For the [M - OC,H,] reaction of m- andp-X substituted benzyl phenyl ethers, contrary to earlier interpretati~ns,l~*~~ when X = CH,, F, C1, Br and CF,, ring-expanded molecular ions of the same structure but different energy and/or energy distribution for each m- andp-X isomer seem to be indicated: but when X = OCH,1* and substitutent position is apparently retained in both the formation and subsequent decomposition of [XC,H,CH,]+

In addition, preliminary results have recently been published25 concerning sub- stituent effects in a closely related system, the [M - 431 reaction of substituted n-butylbenzenes. For electron-donating substituents benzyl structures are apparently favored25 for the decomposing molecular ions, while for electron-withdrawing sub- stituents no clear choice was possible from a consideration of Z / Z o values5 at 70 and 12 eV.

RESULTS

ions.2.14.23

In Table 1 are collected the wide range electron energy kinetics measurements for the [MI .+ [M - CH,] reaction* in substituted ethylbenzenes. Zp/Zm ratios vary with decreasing electron energy2 for all substituents X, but two types of behavior can be discerned.

TABLE 1. zp /Zm RATIOS FOR THE REACTION D(C6H4CH2CH3]+- -+ [XC,H,CH,]+ IN SUBSI'ITUTED ETHYLBENZENES

X 70 50 35 30 25 20 17 15 14 13 12 11

2.44 2.44 2.43 2.42 2.40 2.47 2.60 2.62 2.98 3.48 5.01 12.2 2.06 2.02 2.02 1-98 1.94 1.94 1.93 1.97 1.96 2.23 3.18 7.40 1.30 1.32 1.33 1.36 1.38 1.27 1.42 1.40 1.34 1.40 1.32 1.51 1.04 1.02 1.02 1.06 1.06 1.04 1.03 1-03 1.06 1.01 1-09 1.11 1.12 1.11 1-11 1.10 1.09 1.09 1.08 1.11 1.14 1-13 1.17 1.17 1.20 1.22 1.19 1.20 1.24 1.38 1.47 1.51 1.63 1.82 2.20 3.3* 1.47 1.48 1.53 1.57 1.56 1.67 1.80 2.00 2.18 2.66 3-60 7*3* 0.71 0.70 0.68 0.66 0.63 0.65 0.63 0.57 0.59 0.58 0.51 0.6*

10 eV.

50 * 24 *

__

1.8* 1*2* 1.2*

* Measurements made from low rates. t [M - 151 measurements made using d3-methoxy-ethylbenzenes.

(a) When X = NH, and OCH,, Zp/Zm varies a great deal (more than a factor of 10) as threshold is approached. Most simply, this finding can be interpreted in terms of very different transition state energies2 for m- and p-X isomers for this reaction (e.g. energy diagram B, Fig. 1, ref. 2) precisely as expected for benzylic structures. In support of this contention, appearance potentials differing by 1-15 eV have been reportedls for the [M - Cl] ions from m- and p-methylbenzyl chlorides,

* [MI and [M - CHJ [A] ion peaks accounted for >90% &" at 70eV for all substituents except X = C1 and Br, where competing [M - XI reactions reduced this to -80%.

Kinetic studies in mass spectrometry-VI 1321

and similar large rate factor differences for benzylic cleavage in m- and p-amino- bibenzyls have been observed.,,

(b) When X = OH, CH,, F, C1, Br and CF,, relatively small variations of Zp/Zm with electron energy (Table 1) were observed (less than a factor of 5) consistent with quite similar transition state energies, for m- andp-X isomers. This energy difference data between isomers cannot be directly translated into ion structure differences, since such small differences could be in principle due to either unrearranged molecular ions of similar ionization potentials,* benzylic transition states and immediately- formed benzyl [XC,H,CH,]+ ions of similar appearance potentials" for the same substituent; or rearranged molecular ions of similar but not identical energies and energy distributions leading directly to substituted tropylium ions via tropylium-like activated complexes. The latter picture seems the more attractive, however, in the light of similar conclusions for the same substituents X in the benzyl phenyl ethers,, and also taking into account the closely similar appearance potentials reportedls for the analogous ions in the spectra of m- and p-X isomeric benzyl compounds (actually toluenes, benzyl bromides and ethylbenzenes) when X = CH,, F and OH, and the isotope labeling results19 discussed above for methylethylbenzenes.

m.1 + m*r -CRs

[XCsH,CH&H,]t - + XCBHICHl __f fragments

[MI [A1 SCHEME 1

Metastable ion characteristic^,^^ expressed as metastable ion intensity relative to that of its precursor, are listed in Table 2. Since ions decomposing in the field-free region before the magnetic analyzer are the longest-lived and, therefore, lowest in energy to give an interpretable signal at the collector, metastable ion peak relative abundances are more sensitive to activation energies than are normal peaks produced by ions fragmenting in the ion source? Thus ions of different internal energies (E, Fig. 1) are being observed at 70 eV, the appearance potential threshold, and through metastable ion decompositions,l and it is, therefore, quite possible that structural conclusions drawn from mass spectral peaks corresponding to these distinct energetic conditions will not necessarily be identical.

The differing [m*J/[M] ratios, (Table 2, upper transitions for each substituent X) for each m- and p-X isomer suggest that for all substituents the major portion of decomposing molecular ions has different energies and/or energy distributions,l or structures.2 For X = NH, and OCH, different (benzylic) structures are preferred (but not demanded by the metastable data), in line with the kinetic conclusions. For X = OH, CH,, F, Cl, Br and CF,, the majority of the molecular ions of the same rearranged structures of slightly differing energies and/or energy distributions1 for each m- andp-X isomer are preferred (but not demanded).

The further decomposition of the [XC,H,CH,]+ ions is reflected by [mz*]/[A] ratios2 (Table 2, lower transitions for each substituent). For X = NH, and OCH,, differing ratios for each m- andp-X isomer indicate that, at least for the [XC,H,CH,]+ ions undergoing the reaction in question in the field-free region, the ions are not

* These substituents have in general small Hammett ~constants,2' and ionization potentials of substituted aromatic compounds are not greatly different whether they are in the metu- or puru- p o s i t i ~ n . ~ ~ * ~ ~

1322 PETER BROWN

TABLE 2. METASTABLE ION CHARACTERISTICS FOR DECOMPOSITION REACTIONS IN SUBSTITUTED ETHYLBENZENES XC6H4CHsCHs AT 70 eV

[metastable] x 103 x Transition Metastable [parent]

OH

OCH*

F

c1

Br

m-X

121 -+ 106 92.9 2.6

122 -+ 107 93.8 4.2 122 -+ 94 72.4 0.22 107 -+ 79 58.3 0.37 107 -+ 17 55.4 012

(106 -+ 79 58.9 0-90

110.6 9 2 68.4 1.4

120 -+ 105 91.9 2.5 120 -+ 91 69.0 0.30 105 -+ 103 101.0 0.64 105 -+ 79 59.4 0.69 105 -+ 78 57.9 0.14 105 -+ 77 56.5 0.15 124 -+ 109 95.8 2-0 109 -+ 89 72.7 0.054(0*15)t 109 -+ 83 63.2 0*68(2*0)t

63.48 0.70(1.3)t

!

111.6 026

(17111 -90 47.4 0.73 174 - 159 145-3 1-8 174 -+ 105 63.4 5.1 159 -+ 139 121.5 0.41(1.2)t 159 -+ 109 74.7 071(2*l)t

140: -+ 125 (125: -89 18611 -+ 171 1572 0 54

P-X

1.7 061 1.6 0.18 0.27 0.16 4.1 0.24 1.6 0.30 0.60 0.67 0.13 0.14 1 -6 0*048(0*15)t 061(1.9)? 0.39 0.58(1.3)t 1.6 0.71 0.57 3.9 0.58(1.2)7 1.0 (2-2)t

* m/e values for d3-methoxyethylbenzenes given. Relative to molecular ion.

$ m/e values for 35Cl given. Q Metastable ion peaks at m/e 78-7 (140 -+ 105) and 78.4 (125 -+ 99) were not resolved. /I m/e values for 'lBr given.

common in all the three parameters energy, energy distribution and structure. However, when X = CH,, Br and possibly OH, very similar [m,*]/[A] ratios2 (Table 2) are consistent with decomposing [XC,H,CH,]+ ions of common energy, energy distribution and structure.

When X = F, C1 and CF, the interesting observation was made that although [m2*]/[A] ratios are not the same for each m- and p-X isomer, the ratios [m,*]/M] are the same (Table 2), within experimental error. This situation, which we have also encountered12 in the further decomposition of [M - CI] ions derived from substituted chlorobenzenes when X = CH, and CF,, had previously been ascribedSo to the intervention of common rearranged molecular ions, e.g. as with m- and p- ~ y l e n e . ~ , ~ ~ Clearly this cannot be the situation either for ethylbenzenes (X = F, C1, CF,) or chlorobenzenes (X = CH,, CF,), since in both cases different Zp/Zm and [ml*]/[M] ratios for each m- andp-X pair were recorded. A possible explanation invokes the formation of mixtures of [A]+ ions [XC,H,CH,]+ of different energies,

Kinetic studies in mass spectrometry-VI 1323

energy distributions and/or structures from each [MI+. ion, in which the common components for each substituent X are formed at approximately the same rate, and only decomposition of the common components contributes to the relevant metastable ion peak (m2* in Scheme 1).

CH,CH, * x

9' (1)

r-

SCHEME 2

CH,CH,

@ x

- (11) x

(111)

To summarize, the wide range electron energy kinetics and metastable ion relative abundance data can be reasonably accommodated structurally in terms of Scheme 2. When X = NH, and OCH,, the major processes are believed to involve unrearranged molecular ions I1 undergoing benzylic cleavage to afford benzyl [M - CH,] ions 111, which do not rearrange to common substituted tropylium ions before further frag- mentation, at least at 70 eV.* When X = OH, CH,, F, C1, Brand CF,, at least partial rearrangement of the initially produced molecular ions I1 to such structures as substituted ionized methylcycloheptatrienes 1V is visualized, exactly as discussed previously for toluenes.2 Cleavage of CH, then yields the substituted tropylium ion V, which decomposes further independent of substituent position.

(i) Mass spectra

been fully described before.,

(ii) Compounds The substituted ethylbenzenes X = NH2, OH, CH, were commercial samples of as high initial

purity as possible. For X = OCH,, the hydroxyethylbenzenes were methylated using CHJ and Williamson ether synthesis conditions.2 For X = F, C1, Br, CF,, the corresponding commercially available acetophenones were reduced under Wolff-Kishner conditions.

In a typical run, p-fluoroacetophenone (1.38 g, 10 mmole) was added with stirring to diethylene glycol (15 ml) in a three-neck 200 ml flask equipped with magnetic stirrer, condenser and thermo- meter, followed by hydrazine hydrate (go%, 1 ml) and finally potassium hydroxide (1.12 g 20 mmole). The continuously stirred reaction mixture was warmed for approx. 30 mins., then heated to reflux for 4 hrs. The condenser was then replaced by a distilling head and collector, and distillation effected until the pot temp. was in the range 175 to 185". The distillate was dissolved in ether, washed with water, dried and solvent removed under reduced pressure. The crude p-fluoroethylbenzene was

* The difference between APm and APp for X = NH, is only 0.2 f 0.2 eV, however, indicating

E X P E R I M E N T A L

Experimental conditions for the determination of mass spectra and peak relative abundances have

common structures at threshold.31 A. G. Harrison, private communication.

1324 PETER BROWN

purified by repeated injection/collection cycles on a preparative g.1.c. fitted with a 14' x $" column of 5 % XE-60 on Chromosorb W. All substituted ethylbenzene samples were purified in this same way, and purity checked by i.r. and low resolution mass spectrometry. Attempted reduction of nitro- and cyanoacetophenones under Wolff-Kishner conditions led to reduction of the substituent.

Acknowledgements-We are greatly indebted to the National Science Foundation for funds (Grant No. GB-4939) to purchase the Varian-Atlas CH-4B mass spectrometer, and to Arizona State Univer- sity for a Faculty Research Grant. Acknowledgement is also made to the donors of the Petroleum Research Fund, for support of this research.

R E F E R E N C E S 1. I. Howe, D. H. Williams and R. G. Cooks, Org. Mass Spectrom. 2,137 (1969). 2. P. Brown, Org. Mass. Spectrom. 2,1085 (1969). 3. R. A. W. Johnstone and D. W. Payling, Chem. Commun. 601 (1968). 4. R. S. Ward, R. G. Cooks and D. H. Williams, J. Am. Chem. SOC. 91,2727 (1969). 5. M. M. Bursey and F. W. McLafferty, J. Am. Chem. Soc. 88,529 (1966), and subsequent papers in

6. I. Howe and D. H. Williams, J. Chem. Soc. (B) 1213 (1968). 7. H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig and H. Eyring, Proc. NatI. Acad. Sci.

8. H. M. Rosenstock and M. Krauss, in F. W. McLafferty (Ed.) Mass Spectrometry of Organic

9. M. S . Chin and A. G. Harrison, Org. Mass Spectrom. 2, 1073 (1969). 10. F. W. McLafferty, Chem. Commun. 956 (1968). 11 . M. M. Bursey, Org. Mass Spectrom. 1, 31 (1968). 12. P. Brown, Org. Mass Spectrom. Submitted. 13. T . W. Shannon and F. W. McLafferty, J. Am. Chem. Soc. 88,5021 (1966), and subsequent papers

14. P. Brown, J. Am. Chem. SOC. 90,4459 (1968). 15. M. L. Vestal, J. Chem. Phys. 43, 1356 (1965). 16. D. H. Williams, I. Howe and R. G. Cooks, J. Am. Chem. SOC. 90,6759 (1968). 17. P. N. Rylander, S. Meyerson and H. M. Grubb, J. Am. Chem. SOC. 79,842 (1957). 18. J. M. S. Tait, T. W. Shannon and A. G. Harrison, J. Am. Chem. SOC. 84,4 (1962). 19. F. Meyer and A. G. Harrison, J. Am. Chem. SOC. 86,4757 (1964). 20. R. G. Cooks, R. S. Ward and D. H . Williams, Chem. Commun. 850 (1967). 21. A. N. H. Yeo, R. G. Cooks and D. H. Williams, J. Chem. SOC. (B) 149 (1969) and refs. cited

22. P. Brown, J. Am. Chem. SOC. 90,4461 (1968). 23. P. Brown, J. Am. Chem. SOC. 90,2694 (1968). 24. R. H. Shapiro and J. W. Serum, Org. Mass Spectrom. 2, 533 (1969). 25. R. Nicoletti and D. A. Lightner, Tetrahedron Letters 4553 (1968). 26. F. W. McLafferty and M. M. Bursey, J. Am. Chem. SOC. 90,5299 (1968). 27. J. Hine, Physical Organic Chemistry, McGraw-Hill, New York, 1962, Chap. 4. 28. R. W. Kiser, Introduction to Mass Spectrometry and Its Applications, Prentice-Hall, Englewood

29. G. F. Crable and G. L. Kearns, J. Phys. Chem. 66,436 (1962). 30. F. W. McLafferty and T. A. Bryce, Chem. Commun. 1215 (1967). 31. F. Meyer, Doctoral Thesis, University of Amsterdam, 1965.

this series.

U.S. 38,667 (1952).

Zons, Academic Press, New York, 1963, Chap. 1.

in this series.

therein.

Cliffs, NJ, 1964, Appendix 4.