Organic Mass Spectrometry. 1970. Vol. 3, pp. 639 to 646. Heyden & Son Limited. Printed in Northern Ireland

KINETIC STUDIES IN MASS SPECTROMETRY-IV THE [M - CI] REACTION IN SUBSTITUTED CHLOROBENZENES AND THE QUESTION OF MOLECULAR ION ISOMERIZATION. *

PETER BROWN Chemistry Department, Arizona State University, Tempe, Arizona 85281, USA

(Receiued 9 October 1969; accepted 4 December 1969)

Abstract-The [MI'' -+ [M - C1]+ reaction in a series of m- and p-X substituted chlorobenzenes has been studied, utilizing a simple kinetic approach, comparison of metastable ion relative abundan- ces, and by measurement of ionization and appearance potentials. All evidence obtained is consistent with rearrangement prior to cleavage in the molecular ions, in which substituent position becomes effectively randomized. These findings are related to known hydrogen randomization reactions occurring in either the molecular ion or [M - CI] ion of chlorobenzenes. Mechanisms involving car- bon scrambling via such species as ionized benzvalenes or prismanes, or ring-opening to isomeric acyclic molecular ions in which hydrogen randomization might occur can be entertained, but mecha- nisms involving simple hydrogen shifts in the intact benzene ring appear less likely.

I N T R O D U C T I O N

IN THE recent literature there have been intriguing reports of hydrogen/deuterium scrambling in the molecular ion of partially labelled benzene,l benz~ni t r i le ,~*~ phenyl isocyanide: pyridine? triphenylphosphine and related compounds,G some bicyclic aromatic systems,' furan: thiophenY8 diphenyl ethers and fluoroben~ene,~ and in either the molecular ion or M-halogen ion of chloro-, bromo- and iodobenzene~.~ In each case, relative metastable ion peak intensities for loss of some hydrogen-con- taining neutral species showed that statistical randomization of all hydrogen and deuterium atoms in the decomposing labeled molecules of low energy had occurred before fragmentation took place. Three of the most obvious explanations for such behavior are :

A. Simple H/D reciprocal shifts (Scheme 1.1), during the course of which the integrity of the ring carbon skeleton of the molecular ion remains intact.

B. Randomization of the ring carbon atoms (Scheme 1.2) possibly via ionized valence-bond isomers such as benzvalenes (a) and prismanes, as originally suggested by Jennings,l and later by Williams et al. ,6*8*s and leading to hydrogen randomization as a direct consequence. This type of skeletal rearrangement has photochemical analogy in substituted benzeneslO and thiophenes,ll where migrating groups such as hydrogen, methyl and aryl remain attached to their original ring carbon atom^.^^.'^

C. Opening of the benzene ring (Scheme 1.3), and hydrogen randomization effected via the intermediacy of such species as ionized hexadienynes (b)?4 Such a ring-opening reaction also has photochemical analogy,15 and hydrogen scrambling is known16 to occur in the molecular ion of simple alkenes such as 1-pentene.

It appeared that substituent effects on the energetics of a common reaction of initially isomeric molecular ions might indicate whether the rearrangements occur at

* Presented in part at the 8th National Meeting of the Society for Applied Spectroscopy, and the 5th Western Regional Meeting of the American Chemical Society at Anaheim, California, October, 1969.

639

PETER BROWN 640

1

Y = halogen

Y 3.

SCHEME 1

the molecular ion stage or later, and by which mechanism. For example, in the [MI+- --+ [M - C1]+ reaction of a series of m- and p-X substituted chlorobenzenes, mechanism A (Scheme 2.1) involves no loss of substituent positional identity, and different activation energies might be expected for meta- and para-compounds with the same substituent. On the other hand, mechanism B (Scheme 2.2) requires very similar activation energies (assuming very similar ionization potentia1s)l' for metu- and para-isomers, since complete substituent randomization will result in the same mix- ture of rearranged molecular ions, regardless of original substituent location. In order to differentiate between mechanisms A and B in this way, obviously the activated complex(es) for isomerization must be lower on the potential energy surface than that for the [M - Cl] cleavage reaction. For mechanism C (Scheme 1.3), substituent

Y = halogen i

2.

SCHEME 2

Kinetic studies in mass spectrometry-IV 641

position is again expected to be irrelevant, and operation of this pathway will be extremely difficult to distinguish from mechanism B by an energetic approach. Complete resolution of the problem will require at least double isotope labeling (e.g. 2H and 13C) and such experiments are in progress.g*

As methods of estimating activation energy differences between a comiiion reaction in isomer pairs of molecular ions (whose neutral precursors have essentially the same heats of formation), we have employed (i) a modified kinetic treatment,lsJ9 based on the simple kinetic approach developed by McLafferty and Bursey;20 (ii) metastable ion relative abundances21 of both [MI and [M - Cl] ions;19 and (iii) ionization and appearance potential determinations on both [MI and [M - Cl] ions. Since at the present time each technique separately has its shortcomings and limi- t a t i o n ~ , 2 ~ * ~ ~ , ~ ~ evidence obtained for a particular reaction pathway or mechanism by any one experimental approach listed above will not be conclusive, but only indicative. Taken together, however, a consistent picture may emerge.

RESULTS AND DISCUSSION In Table 1 are listed wide range electron energy k i n e t i c ~ l ~ ~ ~ ~ data. If Z-values in

this case are a valid reflection of reaction rate, and hence activation energy,? then the constancy of the Zp/Zm parameters suggests transition states for decomposition of the

TABLE 1 . Z,/Z, FOR THE [M - Cl] REACTION IN SUBSTITUTED CHLOROBENZENES

X* 70 50 35 30 25 20

NH2 1.03 1.02 0.99 0.99 0.95 0.98 F 1.02 1.00 1.02 1-00 1.00 0.99 C1 099 0.99 0.97 0.96 0.95 1.00 CN 1.04 1.02 1.05 1.04 1.05 1.00 CH, 1.25 1.30 1.31 1.34 1.34 1.09 CF, 072 0.68 0.72 0.72 0.70 0.70

18

1.03 1.03 1.00 1.00 1.08 0.71

- 16 15 14 13 12 1leV

1.00 0.97 0.96 0.96 0.94 - 1.00 1.03 1.02 1.10 1.00 - 0.98 0.99 0.99 1.00 1.00 - 0.97 0.98 0.95 - - - 1.16 1.11 1.16 1.23 1.22 1.4t 0.73 0.75 0.78 0.72 - -

~~

* X = OCH, and NO2 did not show [M - C1 peaks. The major reaction for X = OH was loss of

t From very low rates. co.

same e n e r g ~ l ~ . ~ ~ for m- andp-X pairs of substrates with the same substituent. This in turn would allow consideration of transition states of common structure for each pair of isomers. Recent ~ o r k ~ ~ , ~ ~ * ~ ~ has shown, however, that Zvalues are not measures of reaction rate, principally because [A]/ [MI measured at the collector is not necessarily the same as [A]/ [MI in the source ;%5 and because an appreciable fraction of molecular ions may have insufficient energy to decompose, so that [A]/[M] is dependent on competing reactions in [M].22*23*2s

Z-values for the [MI+. -+ [M - Cl]+reaction were measured at 20 eV and 8,6,4and 3 kV, and found to be almost independent of accelerating voltage. At 3 kV, the maximum increase observed for [A]/[M] relative to that at 8 kV was 6%. Thus for this particular reaction series, Z-values recorded at the collector will be quite closely representative of Z-values in the but they still provide no theoretically sound measure of the reaction rate.

* D. H. Williams, private communication. t Frequency factors for a common reaction of initially isomeric (closely related structurally such

as m- andp-X substituted benzene derivatives) molecular ions are assumed to be effectively the same.

642 PETER BROWN

In Table 2 are presented ionization (IF) and appearance potentials (AP) for the [MI+* -+ [M - Cl]+ reaction, and AP-IP as an approximate indication of activation energy. Although the parameters are given to two decimal places, reproducibility was kO.1 eV. Very similar IPS , AP’s and AP-IP values were secured for each m- andp-X isomer pair of compounds, the differences being almost entirely within the experi- mental error. Activation energies were in the range 3 to 4.4 eV for all substituents except X = CH,, where the anomalously low AP-IP values are undoubtedly due to prior ring expansion to an ionized chlorocy~loheptatriene.~~~~~ Such close cor- respondence in activation energies between isomers for all compounds examined is clearly consistent with rearranged, common molecular ions for each pair of isomers with the same substituent, and common transition states for the [MI+- -+ [M - C1]+ reaction, even at threshold energies.

TABLE 2. APPEARANCE POTENTIALS AND Z-VALUES FOR THE [M J -+ [M - a] REACTION OF SUBSTITUTED CHLOROBENZENES

x IP AP for

[M - CI] AP-IP

m-NH, p-N& rn-CH3 p-CH3 H m-F P-F m-CI p-c1 m-CF, p-CFz m-CN p-CN

8.09 8.18 8.93 8.85 9.19 9.35 9.21 9.46 9.27 9.16 9.82

10.00 9-96

12.25 12.50 11.32 11.14 13.30 13.35 13.26 13.29 13.24 12-99 12-89 13.83 13.97

4.16 4.32 2.39 2-29 4.11 4.00 4.05 3.83 3.91 3.23 3.07 3.83 401

Z,,

0-216 0.223 2-28 2.86 0.610 0.514 0.525 0.407 0.402 0.714 0.516 0.279 0.291

All IP and AP values given in eV.

In the last column of Table 2 are displayed Z-values at 70 eV for the [MIf. +

[M - Cl]+ reaction. It is interesting to note that although there is no direct correlation between [A]/[M] and activation energy for all substrates, nevertheless, the same close relationship between m- and p-X isomers with the same substituent is suggested.

In Table 3 are displayed metastable ion relative abundances for the [MIf. + [M - Cl]+ reaction (upper transition for each substituent) and for further decom- positions of the [M - CI] ion (lower transitions). These data were taken for ions in the first field-free region of a double-focusing mass spectrometer operating in the ‘defocused and are relative to the daughter ion abundance for the particular transition. For all compounds studied here, the [m*]/[A] ratios are essentially the same for each m- and p-X isomer pair. Despite the possibility of different energy distributions in the initially isomeric molecular it is felt that decomposing structurally common moIecular ions with very simiIar internal energies and energy djs- tributionsmost satisfactorily account forthe consistent equalitybetween isomers. If this picture is correct, then one would expect the [M - CI] ions for each substituent to be

Kinetic studies in mass spectrometry-IV

TABLE 3. METASTABLE ION RELATIVE ABUNDANCES FOR SUBSTITUTED CIILOROBENZENES

X Transition' m* nz-X p-X

127 + 92 NH2 92-65

126 + 91 CH3 91 +65

130 + 95 95 + 75

146 + 111 111 -75 180 -+ 145

CF, 145-125 145 -P 95 137 + 102

F

CN 1 0 2 4 7 5

66.7 11.1 10.8 45.9 14.5 14.9 65.7 5.5 5.3 46.4 1.5 1 *2 69.4 7.6 7.8 59.2 3.7 4.0 84.4 10.3 10.4 50.7 5.3 5.5

116.8 7.1 8.5 107.8 0.65 0.91 62.2 1.5 1.5 75.9 10.4 9.7 55.1 3.0 3.0

643

* m/e values for 35C1. Metastable intensities were also determined at 25, 20 and 15 eV, and m-X/p- X - 1.0 in every case.

effectively identical in energy and structure, and also to fragment further in identical manner.30 This expectation is realized (Table 2), at least as far as metastable ion relative abundances are concerned.

In the substituted chlorobenzenes X = NH2 and CN, possible competing reactions in the molecular ions were apparent ([M - C2H2], [M - HCN]), and metastable ion relative abundances were measured (Table 4). Again the close correspondence between m- and p-X isomers clearly suggests common molecular ions for each re- action, but not necessarily the same structure for all three reactions. Decomposition reactions involving the loss of C1, C2H2 and HCN from isolated electronic state^^^.^^ of the molecular ions can be eliminated, however, since the relative abundances of metastables for the three reactions cited when X = NH2 and CN were essentially the same (Table 5) over a wide range of electron energies.

TABLE 4. METASTABLE ION RELATIVE ABUNDANCES FOR COMPETING

REACTIONS OF SUBSTITUTED CHLOROBENZENES

[metastable] [parent]

x 103

X Transition* Metastable 70 25 20 15 eV

m-NH, p-NHz m-NH, p-NH2 m-CN

m-CN p-CN

p-CN

127 + 101 --CzHz

127 -+ 100

137 --+ 111

137 4 110

-HCN

-CzHz

-HCN

80.3 1.2 097 0.70 1.2 1.0 0-73

78.7 3.4 2.8 2.0 3.5 2.9 2.0

89.9 0.21 0.19 0.14 0.21 0.20 0.14

88.3 0.59 0.58 0.43 0.58 0.58 0.42

0.04 004 0.12 0.13

* m/e values for 35Cl.

644 PETER BROWN

TABLE 5. RELATIVE METASTABLE ION ABUNDANCE RATIOS FOR COMPETING REACTIONS IN SUBSTITUTED CHLOROBENZENES

[metastable] [parent]

Relative values

X Reaction Metastable* 70 25 2o 15 ev

667 1.0 087 0.62 -

[M - CzHz] 80.3 1.0 0.83 0.58 - [M -HCN] 78.7 1.0 0.83 0.57 -

[MI + [M - C11 NHz

[MI -+ [M - Cl] 75.9 1.0 1.0 0.75 0.18 CN [M - CZHZ] 89.9 1.0 095 0.67 0.19

[M - HCN] 88.3 1.0 0.98 0.71 020

* Values for s5CI.

TABLE 6. APPEARANCE POTENTIALS FOR COMPETITIVE REACTIONS IN SUBSTITUTED CHLOROBENZENES

x IP Reaction AeV AP-IP AP

normal m*

p-NHz 8.18 [M - CI]

rn-NH2 8.09 [M - CI]

p-CN 9.96 [M - CI]

rn-CN 10.00 [M - CI]

[M - HCN]

[M - HCN]

[M - HCN]

[M - HCN] Chlorobenzene 9.19 [M - CI] Aniline 7.85 [M - HCN] Benzonitrile 9.98 [M - HCN]

12.50 12.09 041 4.32 12.30 11.83 0.47 4.12 12.25 11.61 064 416 12.04 11.75 029 3.95 13.97 13.54 0.43 4.01 13.83 13.46 0.37 3.87 13.83 13.12 0.71 3.83 13.67 13.20 0.47 3.67 13.30 12.66 0.64 411 12.45 12.05 0.40 4.60 15.12 1444 0.68 5.14

All values in eV.

The constant ratios observed for three competing reactions also suggest that the activation energies involved will be quite similar.22 In Table 6 are collected TP’s, AP’s and AP-IP values for the [M - CI] and [M - HCN] reactions (the [M - C2H2] peak was insufficiently intense for accurate measurement). In order to get some idea of the kinetic shifts22*32.33 involved, the AP’s of the corresponding metastable ions were determined (APm,) in the first field-free region at > 8 kV, and A eV in Table 6 is APnorma,-APm.. Again, the activation energies for both reactions are very similar for m- andp-X isomer pairs of substrates, and within the limits of experimental error. Also noteworthy is the fact that although activation energies for loss of C1 from chloro- benzene and loss of HCN from aniline and benzonitrile (Table 6) are distinctly different, combining the two functionalities in one molecule has the effect of ap- proximately equalizing them.

SUMMARY A N D CONCLUSIONS

Within the experimental limits, the m- and p-X isomers of substituted chloroben- zenes used in this work could not be reliably distinguished by mass spectrometry. Thus in the absence of precise indications of the activation energy differences to be expected if no carbon or substituent scrambling were to intervene, the evidence presented cannot be absolutely conclusive. All the data reported here is consistent

Kinetic studies in mass spectrometry-IV 645

with (but does not demand) common, rearranged molecular ions for each isomer pair of compounds with the same substituent, for relatively high energy fragmenting ions (70 eV normal peaks), and for lower energy ions as well (70 to 15 eV metastable peaks, and IP’s at threshold energies). It is also consistent with common, rearranged [M - Cl] ions for each isomer pair with the same substituent, again for both high and low energy ions.

In terms of reaction pathway, mechanism B is preferred to mechanism A (although mechanism C cannot be ruled out), and hydrogen randomization at the molecular ion stage by a carbon scrambling process (or processes) invoked. * Concomitant operation of mechanism A with either B or C could not of course be detected in these experi- ments. Effective substituent randomization might also take place (i) by substituent migrations from carbon to carbon around the ring, but this is unprecedented; or (ii) by ring expansion/contraction cycles. This is almost certainly the case for X = CH,27*28 (Table 2), but again is without precedent for X = F, C1, CN and CF,, while for X = NH2 it has been shown not to occur significantly, at least for the elimination of HCN from a r ~ i l i n e ? ~ . ~ ~

E X P E R I M E N T A L (i) Mass Spectra

Full details of normal and metastable peak height determination have been given.’# Metastable ion relative intensities were also obtained in the first field-free region of a Varian Atlas SMlB double- focusing (Mattauch-Herzog) mass spectrometer. Conditions were as follows : accelerating voltage varied from 8 to 12 kV,t trap current 300 pA, source temp. 175”, high temperature inlet system and line 165”. Ionization and appearance potentials were also determined on the double focusing instru- ment, using accelerating voltage 8 kV, trap current 20 PA stabilized, pusher and drawout plates at ion house potential, resolution approx. 800, and argon as internal standard. IP’s and AP’s were evaluated from semi-logarithmic plots.36 AP’s of metastable ions were measured similarly,s’ but in the ‘defocused mode.29 Each IP and AP was determined at least twice, on different occasions, and reproducibility was always in the range 10 .1 eV. For the runs at different accelerating voltages, nominal fixed values of 8, 6, 4 and 3 kV were used.

(ii) Compounds All the substituted chlorobenzenes were commercial samples of as high initial purity as possible.

Each compound was further purified by repeated injection/collection cycles on a preparative g.1.c. fitted with a 10’ x &” column of 5 % SE-30 on Chromosorb P. The substituent X location was verified in the m- orp-position for each isomeric pair of substrates by 60 MHz n.m.r. spectrometry.

Acknowledgements-We are greatly indebted to the National Science Foundation for funds to pur- chase the Varian-Atlas CH4B (Grant No. GB-4939) and SMlB (Grant No. GP-6979) mass spectrom- eters, and also to the University Grants Committee at Arizona State University for a Faculty Research Grant. Acknowledgement is also made to the donors of the Petroleum Research Fund, for support of this research.

* After submission of this manuscript, it was learned at the 8th National Meeting of the Society for Applied Spectroscopy and the 5th Western Regional Meeting of the American Chemical Society at Anaheim, California, October 1969, that Prof R. G. Cooks of the University of Kansas has detec- ted statistical scrambling of hydrogen and deuterium in certain fragmentation modes of partially labelled bromobenzene molecular ion or [M - Br] ioas. It was also learned that Dr D. H. Williams of Cambridge has elegantly shown the operation of carbon randomization in 13C- and ZH- labeled benzene molecular ions, and also specifically that 110 cleavage of carbon-hydrogen bonds occurs.

t We thank Mr Edwin M. Bebee for skilful construction of the continuously variable (3 to 12 kV) accelerating voltage unit.

646 PETER BROWN

R E F E R E N C E S 1. K. R. Jennings, 2. Nuturforsch. 22a, 454 (1967). 2. R. G. Cooks, R. S. Ward and D. H. Williams, Chem. Commun. 850 (1967). 3. A. N. €I. Yeo, R. G. Cooks and D. H. Williams, J. Chem. SOC. (B) , 149 (1969). 4. A. N. H. Yeo, R. G . Cooks and D. H. Williams, Org. Muss Spectrom. 1, 910 (1968). 5. D. H. Williams and J. Ronayne, Chem. Commun. 1129 (1967). 6. D. H. Williams, R. S. Ward and R. G. Cooks, J. Am. Chem. SOC. 90,966 (1968). 7. R. G. Cooks, I. Howe, S. W. Tam and D. H. Williams, J. Am Chem. SOC. 90,4064 (1968). 8. D. H. Williams, R. G. Cooks, J. Ronayne and S. W. Tam, Tetrahedron Letters 1777 (1968). 9. D. H. Williams, S. W. Tam and R. G. Cooks, J. Am. Chem. SOC. 90,2150 (1968).

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(b) K. E. Wilzbach, A. L. Harkness and L. Kaplan, J. Am. Chem. SOC. 90, 1116 (1968). 13. H. Wynberg, R. M. Kellogg, H. van Driel and G. E. Beekhuis,J. Am. Chem. SOC. 89,3501 (1967). 14. J. Momigny, L. Brakier and L. D’Or, BUN. Classe Sci. Acad. Roy. Be&. 48, 1002 (1962). 15. L. Kaplan, S. P. Walch and K. E. Wilzbach, J. Am. Chem. SOC. 90, 5646 (1968). 16. B. J. Millard and D. F. Shaw, J. Chem. SOC. ( B ) 664 (1966). 17. G. F. Crable and G. L. Kearns, J. Phys. Chem. 66,436 (1962). 18. P. Brown, J. Am. Chem. SOC. 90,4459 (1968). 19. P. Brown, Org. Muss Spectrom. 2, 1085 (1969). 20. M. M. Bursey and F. W. McLafferty, J. Am. Chem. SOC. 88,529 (1966), and subsequent papers. 21. T. W. Shannon and F. W. McLafferty,J. Am. Chem. SOC. 88,5021 (1966), and subsequent papers. 22. R. G. Cooks, I. Howe and D. H. Williams, Org. Muss Spectrom. 2, 137 (1969). 23. I. Howe and D. H. Williams, J. Chem. SOC. (B) 1213 (1968). 24. F. W. McLafferty, Chem. Commun. 956 (1968). 25. I. Howe and D. H. Williams, Chem. Commun. 220 (1968). 26. R. G. Cooks, R. S. Ward, I. Howe and D. H. Williams, Chem. Commun. 837 (1968). 27. H. M. Grubb and S. Meycrson, in F. W. McLafferty (Ed.) Muss spectrometry of Organic Ions,

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Academic Press, New York 1963, Chap. 10.

trometry and Allied Topics, Montreal, June, 1964.