1306
J. Phys. Chern. 1980, 84, 1306-1309
Gas-Phase Alkylbenzene Dealkylation E. Lilla and 0. Perez’ Laboratorlo Chlmlca Nucleare, C.N.R., Area della Rlcerca dl Roma, Monterotondo Stazlone 000 IS, C.P. 10, Rome, Italy (Recelved June 6, 1979; Revised Manuscript Received February 11, 1980) Publication costs assisted by Laboratorio Chimlca Nucleare de/ C.N.R.
The gas-phase electrophilic attack of radiolytically formed D3+ions on toluene, ethylbenzene, isopropylbenzene, and tert-butylbenzene was investigated. With the exception of toluene, the analysis of the products showed the formation of benzene for the other alkylbenzenes. The first step of the reaction leads to the formation of an excited arenium ion that, if not collisionally stabilized,can decompose to give benzene and the correspondent alkyl cation. Dialkylarenes are not formed under reaction conditions. The extent of dealkylation, that increases in the order ethylbenzene < isopropylbenzene < tert-butylbenzene, shows a dependence on the substrate concentration. The occurrence of competitive reactions is suggested. Comparison with solution chemistry data must take into account the different environmental conditions. Introduction It is well-known that in strong acidic solutions aromatic compounds undergo ring protonation to form arenium ions. The first direct spectroscopic evidence for the protonation of an aromatic hydrocarbon was reported by Gold and Tye,l who observed the electronic spectra of protonated anthracene. A few years later, spectra of protonated alkylbenzenes were reported by Reid.2 Definitive evidence in support of the arenium ions structures can be obtained from NMR spectraas The reactions which alkylbenzenes can undergo in the course of their protonation are of five main types: (a) isomerization (intramolecular alkyl transfer from one carbon of the ring to another), (b) intramolecular rearrangement of the side chains, (c) fragmentation of the alkyl side chains, (d) dealkylation, (e) disproportionation. There are many reports in the literature4 of protodealkylation processes which occur under Friedel-Crafts conditions. The rate of alkyl migration of monoalkylbenzenes has been reported5 to follow the order t-Bu > i-Pr > Et > Me. In a study of protonation of alkylbenzenes, tert-butylbenzene was found to cleave readily to benzene and tert-butyl cation in superacid media even at low temperature! The first step of the reaction was the formation of tert-butylbenzenium ion, followed by an intramolecular proton migration, as shown by NMR ~ p e c t r a .When ~ the proton was attached to the ring carbon bearing the side chain, the tert-butyl cation was cleaved. During the determination of the heats of formation of protonated alkylbenzenes Arnelt and Larsens reported the occurrence of annoying dealkylation processes. Rates of protodetert-butylation of tert-butylbenzene were measured in aqueous sulfuric acid solution^,^ whereas for toluene, ethylbenzene, and isopropylbenzene dealkylation was shown not to take place under the same conditions.1° It was observed that the ease with which the reaction takes place depends upon the stability of the leaving carbonium ion;l’ consequently,the de-tert-butylation reaction is most frequently observed. In the gaseous phase the formation of protonated aromatic hydrocarbons has been observed by Volpi and his co-workers.12 Chemical ionization spectra with methane as a reactant gas have been reported for some alkylbenzenes.13 In a previous papert4 gas-phase ethyltoluene isomerization, promoted by D3+ions, was studied, and toluene was found among the products. Since its yield was very low, a clear mechanism for its formation could not be proposed. 0022-3654/80/20841306$01 .OO/O
I t might be formed either by a dealkylation process (eq 1) or by a transalkylation reaction (eq 2).
In order to elucidate the dealkylation mechanism and to compare the different dealkylation rates, the gas-phase reactions between D,+ ions and some alkylbenzenes (the “big four” series-methyl-, ethyl-, isopropyl-, and tertbutylbenzene) are studied in the present paper. Since the technique, introduced by Ausloos and his cow o r k e r ~ ,has ~ ~been * ~ ~discussed in detail p r e v i ~ u s l y , just ~~J~ a brief review will be given. A high pressure of Dz (500 torr) and a low pressure of additive (generally 1 torr) are mixed together. The mixture, with the addition of a small concentration of oxygen (3 torr) that ensures the scavenging of the thermal radicals, is irradiated with y rays. Because of the large excess of D2 and the small concentration of arene, almost exclusively D3+ions are formed.
Experimental Section Materials. Alkylarenes were commercial Research Grade products from C. Erba (Italy) and were purified by preparative gas chromatography on a 4-m didecyl phthalate column heated at 130 “C with a helium flow rate of 50 cm3 min-l. The purification of each compound was repeated until the impurity content was below 0.02% and benzene was absent. Deuterium, oxygen, and helium were commercial Research Grade products and were used without further purification. Procedure. The gaseous reaction mixtures were, prepared according to the standard vacuum techniques, introducing deuterium (or helium) and oxygen together with a fragile, weighed, glass ampule containing the aromatic substrate into outgassed 1.5-L Pyrex bulbs equipped with a break-seal tip. The irradiations were carried out with @)Co y rays in a 220 gamma cell (Atomic Energy of Canada) at a dose rate of 3.2 x lo5 rd h-l, as measured by a Fricke dosimeter. The total dose absorbed by each system varied to minimize the direct radiation damage on the substrates, 0 1980 American Chemlcal Society
The Journal of Physical Chemistty, Vol. 84, No. 11, 1980 1307
Gas-Phase Alkylbenzene Dealkylation
TABLIEI: Dealkylation of Arenes by D : --
Ions
ab-
concni
pmol
~ubstrate'~ L 1 -ethyl-
benzene
isopropylbenzene
tert-butylbenzene
4.80 4.99 6.41 7.52 18.41 45.31 2.52 6.32 11.51 22.15 34.03 49.27 58.15 12.06 3.09 6.12 11.76 25.03 35.79 17.93 18.54 16.99 17.54
sorbed
dose,b Mrd 4.4 4.4 4.4 4.4 4.4 4.4 3.2 3.2 3.2 3.2 3.2 3.2 3.2 1.6 1.28 1.28 1.28 1.28 1.28 0.32 0.64 0.96 1.28
benzene yield
abs G values
0.18 X 0.20 x 0.27 X 0.29 X 0.58 X 1.11 X 0.14 0.26 0.47 0.55 0.93 1.12 1.29 0.47 0.46 0.72 1.04 1,36 1,60 1.10 1.24 1.11 1.14
lo-'
IO-' lo-' lo-' lo-' lo-'
relC % 0.01 0.01 0.01 0.01 0.02 0.04 0.05 0.09 0.17 0.20 0.33 0.41 0.47 0.17 0.17 0.26 0.38 0.49 0.58 0.40 0.45 0.40 0.41
Except toluene; see text. Referred to deuterium. Referred to the D; ions formed. a
as measured in specific runs performed by substituting deuterium with helium. The selected doses in which benzene production by direct radiation damage was found below 0.02% are reported in Table I. In order to study the dependence of benzene formation on the absorbed dose, specific runs were carried out at different irradiation times. The analysis of the reaction products was performed by using a Model 2450 C. Erba gas chromatograph, equipped with H FID unit. The same conditions used to purify the starting compounds were employed. The yields of the benzene were determined from the area of the correspondent elution peaks, using appropriate calibration factors.
Results The yields of the products from the gas-phase atta.ck of D3+ ions on selected arenes at various concentrations in highly diluted Dz solution are listed in Table I. Since from toluene a formation of benzene exceeding its production by direct radiation damage was not observed, the relative data are not reported. It should be noted that only benzene is taken into account, omitting other species formed in very low yields by fragmentation of the side chain. Only in the case of isopropylbenzene was a formation of ethylbenzene (0.4-0.7 % ) observed, which significantly exceeded its production by direct radiation damage (
