Proc. Natl. Acad. Sci. USA yol. 75, No. 3, pp. 1045-1049, March 1978 Chemistry
Electrophilic and free radical nitration of benzene and toluene with various nitrating agents*
GEORGE A. OLAH, HENRY C. LIN, JUDITH A. OLAH, AND SUBHASH C. NARANG Institute of Hydrocarbon Chemistry, Department of Chemistry, University of Southern California, Los Angeles, California 90007
Contributed by George A. Olah, September 29, 1977
ABSTRACT Electrophilic nitration of toluene and benzene was studied under various conditions with several nitrating systems. It was found that high ortlopara regioselectivity is prevalent in all reactions and is independent of the reactivity of the nitrating agent. The methyl group of toluene is predom- inantly ortho-para directing under all reaction conditions. Steric factors are considered to be important but not the sole reason for the variation in the ortho/para ratio. The results reinforce our earlier views that, in electrophilic aromatic nitrations with reactive nitrating agents, substrate and positional selectivities are determined in two separate steps. The first step involves a ir-aromatic-NO2 ion complex or encounter pair, whereas the subsequent step is of arenium ion nature (separate for the oftho, meta, and para positions). The former determines substrate selectivity, whereas the latter determines regioselectivity. Thermal free radical nitration of benzene and toluene with tetranitromethane in sharp contrast gave nearly statistical product distributions.
Stable nitronium salts were introduced as new nitrating agents by Olah and coworkers (1) in 1956. In the course of these studies (2-5), the competitive nitration of benzene and toluene, as well as other aromatics, was carried out in organic solvents. Under usual conditions of electrophilic nitration, toluene
reacts about 20 times more rapidly than benzene whereas, with nitronium salts, toluene was found to react only 1.7 times faster than benzene (2). The practical disappearance of intermolecular (substrate) selectivity was accompanied by no significant al- teration of isomer distribution (regioselectivity). This obser- vation led to the suggestion that the transition state of highest energy (which determines substrate selectivity) is of starting aromatic (i.e., 7r-complex) nature, which is then followed by separate a-complex formation (for the individual positions), determining positional selectivity.
In a series of studies, we have found that, in electrophilic aromatic substitutions, the position of the transition state of
NO2Y + R-X-
highest energy is not rigidly fixed (6) but can shift from "early" (r-complex-like) to "late" (u-complex-like) nature, depending upon the reactivity of the electrophiles and the basicity of the aromatic substrates.
In order to further explore electrophilic nitration, we carried out a comprehensive study of nitration of benzene and toluene under various conditions.
RESULTS AND DISCUSSION With Nitronium Salts. Although we had previously exam-
ined competitive nitration using high-speed mixing (7), it was considered of interest to extend the studies by using more ad- vanced methods such as the mixing chamber of an efficient Durrum-Gibson stopped-flow apparatus. Competitive nitra- tions, with nitronium hexafluorophosphate in nitromethane, provided the data in Table 1. Whereas mixing still can be in- complete before reaction, with the nitration rates being very fast (or reaching the encounter-controlled limit), the data seem to indicate that, in the present system, both toluene and benzene react by the same mechanism. In other words, if the reactions indeed reach encounter-controlled limiting rates, this must be the case in the studied system not only for toluene but also for benzene, accounting for the diminishing substrate selectivi- ty.
Transfer Nitrations with Nitro and Nitrito Onium Salts. Zollinger and coworkers (8) showed that addition of 2 equiva- lents of water changes the substrate reactivities observed in nitronium salt nitrations to those conventionally observed in nitric acid solutions. A more detailed study of the competitive nitration of toluene and benzene in the presence of a series of nucleophiles was undertaken. The results, summarized in Table 2, show that the ktoluene/kbenzene rate ratios are in the range of 2-5 when 1 equivalent of alcohol, ether, or thioether is added but are 25-66 when 2 equivalents of the nucleophile are used. The relative reactivity of the nitrating agent in the presence of added nucleophiles is in the decreasing order ROH > ROR > RSR. The isomer distributions, however, stay similar. The data are best interpreted in terms of the nitronium ion reacting with the n-donor nucleophile forming an 0- or S-nitronium ion in- termediate, which can either reverse, or transfer nitrate, or form a covalent intermediate.
R-X-R Y- RXNO2 + RF + PF5(BF3)
Y = PF6-, BFJ-; X =0, S; R = alkyl, H.
An isomer of the dimethylnitrosulfonium ion CH3
i.e., the corresponding nitrito complex, was also prepared from dimethyl sulfoxide and NO+. A similar nitrito complex was obtained from 4-nitropyridine-N-oxide. Both of these new ni- * This paper is no. 42 in the series, "Aromatic Substitution." Paper no. 41 is Olah, G. A., Lin, H. C., Olah, J. A. & Narang, S. C. (1978) Proc. Nati. Acad. Sci. USA 75,545-548.
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Proc. Nati. Acad. Sci. USA 75 (1978)
Table 1. Competitive nitration of benzene and toluene at 250 with NO'PF- in nitromethane solution (Durrum-Gibson stopped-
flow mixing chamber)
Toluene/ benzene, Isomer distribution, % mol ratio kT/kB,* ortho meta para o/p
10:1 1.4 62 4 34 1.82 5:1 1.4 62 3 35 1.77 1:1 1.7 64 3 33 1.94 1:5 2.4 64 3 33 1.94 1:10 2.5 63 3 34 1.85
* kT, k for toluene; kB, k for benzene.
trito onium ion reagents act as weak nitrating agents, requiring reaction temperatures of 550°60°.
According to Ingold (9), the reactivity of a nitrating agent, X-NO2, is proportional to the electron affinity of X. As a con- sequence, it is obvious that differences in species such as R2XNOZ R2tXONO, and R-X-NO2 play an important role in these reactions. Lewis Acid-Catalyzed Nitration with Nitryl Chloride. We
have extended the study of Friedel-Crafts nitrations to an ad- ditional number of Lewis acid halide catalysts (10). The data are shown in Table 3. With an excess of the aromatics as solvent, the substrate selectivity varied from 11 to 39, accompanied by slight changes in regioselectivity. Generally, the ortho/para ratio is lower than in nitrations with nitronium salts.
In general, thekwue./k. ratio decreases with increasing acidity of the catalyst. The stronger catalyst forms a more po- larized complex, thereby generating an early transition state. The complex is a bulkier nitrating agent than the nitronium salts, which are highly polarized in the generally used solvents of high dielectric constant and show no effects of ion pairing.
Table 2. Competitive nitration of benzene and toluene with NO'PF and NO+PF- in the presence of alcohofs, ethers, thioethers (sulfoxide), and N-oxide in CH3NO2 at 250
Isomer distribution, Nitrating __ % agent kT/kB ortho meta para o/p
NO'PF6/methanol (1:1) 3.3 63 3 34 1.85 NO+PFj/methanol (1:2) 26.1 62- 3 35 1.77 NOMPFe/neopentyl
alcohol (1:1) 2.8 62 3 35 1.77 NO+PF/neopentyl
alcohol (1:2) 25.4 62 3 35 1.77 NOfPFj/methyl ether (1:1) 4.0 62 4 34 1.82 NO+PFj/methyl ether (1:2) 31.3 62 4 34 1.82 NO+PFj/ethyl ether (1:1) 3.8 62 4 34 1.82 NO+PF /ethyl ether (1:2) 32.8 62 4 34 1.82 NO PFi/tetrahydrofuran
(1:1) 3.6 62 3 35 1.77 NO+ Fi/tetrahydrofuran
(1:2) 28.9 62 4 34 1.82 NOrPFf/dimethyl
sulfide (1:1)* 4.6 62 3 35 1.77 NO+F-/dimethyl
sulfide (1:2)* 65.7 62 3 35 1.77 NO+PFl/dimethyl sulfoxide
(1:1)t 27.3 59 4 37 1.60 NO +PFj/4-nitropyridine- N-oxide (1:1)t 33.4 51 8 41 1.24
*-In nitroethane at -78°. tAt60O.
Table 3. Lewis acid halide-catalyzed Friedel-Crafts nitration of benzene and toluene with nitryl chloride at 250
in excess of aromatics
Lewis acid Isomer distribution, % halide kT/kB ortho meta para o/p AlCl3 11.2 53 2 45 1.18 TiC14 17.6 53 2 45 1.18 BF3 25.1 57 2 41 1.39 SbCl5 26.7 56 2 42 1.33 PF5 39.3 57 2 41 1.39
This explains the lower ortho/para' ratio observed in the Friedel-Crafts nitrations in aromatics. However, these factors can decrease when the reactions are carried out in ionizing polar solvents such as nitromethane (Table 4).
Nitration with Acyl Nitrates. We have studied nitrations of toluene and benzene with a series of acyl and aroyl nitrates (11). The results summarized in Table 5 indicate some changes in substrate selectivity with only minor variations in positional selectivity, but there is no common relationship between sub- strate and positional selectivities. The ktoiuene/kbenzene values increase with increasing pKa values of the corresponding acids. Present studies thus do not give a firm indication of the nature of the nitrating agent involved.
Nitration with Chloropicrin and Tetranliromethane. In earlier studies (7), one of us and Overchuck compared elec- trophilic with free radical nitrations-and found the latter to give nearly statistical product distributions, reflected in both sub- strate selectivity and r
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