Electrophilic and Free Radical Nitration of Benzene and Toluene with Various Nitrating Agents* (Aromatic Compounds/Selectivity) GEORGE A

Home , Nitrosonium, Onium ion, Tetranitromethane

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* (aromatic compounds/selectivity) 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 RESULTS AND DISCUSSION was studied under various conditions with several nitrating systems. It was found that high ortlopara regioselectivity is With Nitronium Salts. Although we had previously exam- prevalent in all reactions and is independent of the reactivity ined competitive nitration using high-speed mixing (7), it was of the nitrating agent. The methyl group of toluene is predom- considered of interest to extend the studies by using more ad- inantly ortho-para directing under all reaction conditions. Steric vanced methods such as the mixing chamber of an efficient factors are considered to be important but not the sole reason Durrum-Gibson stopped-flow apparatus. Competitive nitra- for the variation in the ortho/para ratio. The results reinforce our earlier views that, in electrophilic aromatic nitrations with tions, with nitronium hexafluorophosphate in nitromethane, reactive nitrating agents, substrate and positional selectivities provided the data in Table 1. Whereas mixing still can be in- are determined in two separate steps. The first step involves a complete before reaction, with the nitration rates being very ir-aromatic-NO2 ion complex or encounter pair, whereas the fast (or reaching the encounter-controlled limit), the data seem subsequent step is of arenium ion nature (separate for the oftho, to indicate that, in the present system, both toluene and benzene meta, and para positions). The former determines substrate react by the same mechanism. In other words, if the reactions selectivity, whereas the latter determines regioselectivity. Thermal free radical nitration of benzene and toluene with indeed reach encounter-controlled limiting rates, this must be tetranitromethane in sharp contrast gave nearly statistical the case in the studied system not only for toluene but also for product distributions. benzene, accounting for the diminishing substrate selectivity. Stable nitronium salts were introduced as new nitrating agents Transfer Nitrations with Nitro and Nitrito Onium Salts. by Olah and coworkers (1) in 1956. In the course of these studies Zollinger and coworkers (8) showed that addition of 2 equiva- (2-5), the competitive nitration of benzene and toluene, as well lents of water changes the substrate reactivities observed in as other aromatics, was carried out in organic solvents. nitronium salt nitrations to those conventionally observed in Under usual conditions of electrophilic nitration, toluene nitric acid solutions. A more detailed study of the competitive reacts about 20 times more rapidly than benzene whereas, with nitration of toluene and benzene in the presence of a series of nitronium salts, toluene was found to react only 1.7 times faster nucleophiles was undertaken. The results, summarized in Table than benzene (2). The practical disappearance of intermolecular 2, show that the ktoluene/kbenzene rate ratios are in the range of (substrate) selectivity was accompanied by no significant al- 2-5 when 1 equivalent of alcohol, ether, or thioether is added teration of isomer distribution (regioselectivity). This obser- but are 25-66 when 2 equivalents of the nucleophile are used. vation led to the suggestion that the transition state of highest The relative reactivity of the nitrating agent in the presence of energy (which determines substrate selectivity) is of starting added nucleophiles is in the decreasing order ROH > ROR > aromatic (i.e., 7r-complex) nature, which is then followed by RSR. The isomer distributions, however, stay similar. The data separate a-complex formation (for the individual positions), are best interpreted in terms of the nitronium ion reacting with determining positional selectivity. the n-donor nucleophile forming an 0- or S-nitronium ion in- In a series of studies, we have found that, in electrophilic termediate, which can either reverse, or transfer nitrate, or form aromatic substitutions, the position of the transition state of a covalent intermediate.

R-X-R NO2Y + R-X- Y- RXNO2 + RF + PF5(BF3)

Y = PF6-, BFJ-; X =0, S; R = alkyl, H. highest energy is not rigidly fixed (6) but can shift from "early" An isomer of the dimethylnitrosulfonium ion (r-complex-like) to "late" (u-complex-like) nature, depending upon the reactivity of the electrophiles and the basicity of the CH3 aromatic substrates. zS+-NO2 In order to further explore electrophilic nitration, we carried CH3 out a comprehensive study of nitration of benzene and toluene i.e., the corresponding nitrito complex, was also prepared from under various conditions. dimethyl sulfoxide and NO+. A similar nitrito complex was obtained from 4-nitropyridine-N-oxide. Both of these new ni- The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked * This paper is no. 42 in the series, "Aromatic Substitution." Paper no. "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate 41 is Olah, G. A., Lin, H. C., Olah, J. A. & Narang, S. C. (1978) Proc. this fact. Nati. Acad. Sci. USA 75,545-548. 1045 Downloaded by guest on October 2, 2021 1046 Chemistry: Olah et al. Proc. Nati. Acad. Sci. USA 75 (1978)

Table 1. Competitive nitration of benzene and toluene at 250 Table 3. Lewis acid halide-catalyzed Friedel-Crafts nitration of with NO'PF- in nitromethane solution (Durrum-Gibson stopped- benzene and toluene with nitryl chloride at 250 flow mixing chamber) in excess of aromatics Toluene/ Lewis acid Isomer distribution, % benzene, Isomer distribution, % halide kT/kB ortho meta para o/p mol ratio kT/kB,* ortho meta para o/p AlCl3 11.2 53 2 45 1.18 10:1 1.4 62 4 34 1.82 TiC14 17.6 53 2 45 1.18 5:1 1.4 62 3 35 1.77 BF3 25.1 57 2 41 1.39 1:1 1.7 64 3 33 1.94 SbCl5 26.7 56 2 42 1.33 1:5 2.4 64 3 33 1.94 PF5 39.3 57 2 41 1.39 1:10 2.5 63 3 34 1.85 * kT, k for toluene; kB, k for benzene. This explains the lower ortho/para' ratio observed in the Friedel-Crafts nitrations in aromatics. However, these factors trito onium ion reagents act as weak nitrating agents, requiring can decrease when the reactions are carried out in ionizing polar reaction temperatures of 550°60°. solvents such as nitromethane (Table 4). According to Ingold (9), the reactivity of a nitrating agent, Nitration with Acyl Nitrates. We have studied nitrations X-NO2, is proportional to the electron affinity of X. As a con- of toluene and benzene with a series of acyl and aroyl nitrates sequence, it is obvious that differences in species such as (11). The results summarized in Table 5 indicate some changes R2XNOZ R2tXONO, and R-X-NO2 play an important role in in substrate selectivity with only minor variations in positional these reactions. selectivity, but there is no common relationship between sub- Lewis Acid-Catalyzed Nitration with Nitryl Chloride. We strate and positional selectivities. The ktoiuene/kbenzene values have extended the study of Friedel-Crafts nitrations to an ad- increase with increasing pKa values of the corresponding acids. ditional number of Lewis acid halide catalysts (10). The data Present studies thus do not give a firm indication of the nature are shown in Table 3. With an excess of the aromatics as solvent, of the nitrating agent involved. the substrate selectivity varied from 11 to 39, accompanied by Nitration with Chloropicrin and Tetranliromethane. In slight changes in regioselectivity. Generally, the ortho/para earlier studies (7), one of us and Overchuck compared elec- ratio is lower than in nitrations with nitronium salts. trophilic with free radical nitrations-and found the latter to give In general, the kwue./k. ratio decreases with increasing nearly statistical product distributions, reflected in both sub- acidity of the catalyst. The stronger catalyst forms a more po- strate selectivity and regioselectivity. larized complex, thereby generating an early transition state. In the present studies, when tetranitromethane (12) was The complex is a bulkier nitrating agent than the nitronium mixed with benzene and toluene in ether, ethanol, nitrometh- salts, which are highly polarized in the generally used solvents ane, or pyridine/ethanol solutions, no nitration occurs up to 60°. of high dielectric constant and show no effects of ion pairing. However, when an ethereal solution of benzene/toluene (1:1) containing tetranitromethane was injected into a gas chroma- Table 2. Competitive nitration of benzene and toluene with tograph with injection block temperature of 3000, a significant NO'PF and NO+PF- in the presence of alcohofs, ethers, amount of nitro products was detected (Table 6). thioethers (sulfoxide), and N-oxide in CH3NO2 at 250 Nitroarene product composition clearly indicates that, under Isomer distribution, thermal decomposition conditions, free radical nitration is fa- Nitrating __ % vored. Low substrate selectivity (kto1uene/kbe.nzene = 0.7) is ac- agent kT/kB ortho meta para o/p companied by low positional selectivity, giving nearly statistical 40% ortho, 40% meta, and 20% para isomer distribution. NO'PF6/methanol (1:1) 3.3 63 3 34 1.85 Friedel-Crafts type nitration by tetranitromethane NO+PFj/methanol (1:2) 26.1 62- 3 35 1.77 and NOMPFe/neopentyl chloropicrin was also studied in the presence of BF3 and PF5 alcohol (1:1) 2.8 62 3 35 1.77 catalysts. The isomer distribution and .ktd.,./kb, rate ratio NO+PF/neopentyl shows (Table 7) that nitration under these conditions proceeds alcohol (1:2) 25.4 62 3 35 1.77 via an electrophilic substitution. The Lewis acids polarise the NOfPFj/methyl ether (1:1) 4.0 62 4 34 1.82 nitrating agents, thereby weakening the GN bond and making NO+PFj/methyl ether (1:2) 31.3 62 4 34 1.82 it susceptible to heterolytic cleavage. NO+PFj/ethyl ether (1:1) 3.8 62 4 34 1.82 We have previously suggested that the reaction path leading NO+PF /ethyl ether (1:2) 32.8 62 4 34 1.82 to products in the case of nitration of reactive aromatics with NO PFi/tetrahydrofuran nitronium salts must involve two separate steps leading to the (1:1) 3.6 62 3 35 1.77 arenium ion type intermediate. NO+ Fi/tetrahydrofuran + (1:2) 28.9 62 4 34 1.82 ArH + NO2 [ArH -- NO+] HArNO2 NOrPFf/dimethyl sulfide (1:1)* 4.6 62 3 35 1.77 Table 4. Lewis acid halide-catalyzed Friedel-Crafts nitration of NO+F-/dimethyl benzene and toluene with nitryl chloride in nitromethane sulfide (1:2)* 65.7 62 3 35 1.77 solution at 250 NO+PFl/dimethyl sulfoxide (1:1)t 27.3 59 4 37 1.60 Lewis acid Isomer distribution, % NO +PFj/4-nitropyridine- halide kT/kn ortho meta para o/p N-oxide (1:1)t 33.4 51 8 41 1.24 AlCl3 26.8 61 4 35 1.74 *-In nitroethane at -78°. TiC14 27.8 61 4 35 1.74 tAt60O. PF5 28.5 62 3 35 1.77 Downloaded by guest on October 2, 2021 Chemistry: Olah et al. Proc. Nati. Acad. Sci. USA 75 (1978) 1047

Table 5. Competitive nitration of benzene and toluene with acyl Table 7. Lewis acid halide-catalyzed competitive nitration of nitrates (from AgNO3 and acyl chlorides) in acetonitile' benzene and toluene with chloropicrin and tetranitromethane in solution at 25° nitromethane solution at 250 Isomer distribution, % Nitrating Isomer distribution, % Acyl nitrate kT/kB ortho meta para olp agent Catalyst kT/kn ortho meta para o/p Trifluoroacetyl nitrate 29.8 63 4 33 1.91 Chloropicrin BF3 36.8 64 4 32 2.0 Propionyl nitrate 33.3 64 4 32 2.0 PF5 34.2 63 4 33 1.91 Methoxyacetyl nitrate 35.5 65 4 31 2.1 Acetyl nitrate 44.3 61 2 37 1.65 Tetranitro- BF3 40.0 64 2 34 1.88 Pentafluorobenzoyl nitrate 27.0 63 4 33 1.91 methane PF5 37.0 63 5 32 1.97 p-Nitrobenzoyl nitrate 28.0 61 4 35 1.74 Benzoyl nitrate 30.7 64 5 31 2.07 explain the decrease in ortho/para ratio in nitrations with nitryl p-Methylbenzoyl nitrate 34.0 61 4 35 1.74 chloride-Lewis acid halide complexes (Tables 3 and 4), when p-Methoxybenzoyl nitrate 37.5 63 6 31 2.03 the reaction medium is changed from nitromethane to excess aromatics. It is much more plausible to suggest that certain nitronium ion precursors can act as nitrating agents in their own With various NO2X type nitrating agents, interaction of right (11). aromatics with polarized NO2X must be considered prior to the To give a comprehensive review of the variation of isomer formation of the nitronium ion. In nonpolar solvents, there is distribution in nitration of-toluene, our own experimental data obviously greater steric interaction between the methyl group and pertinent results of nitration of toluene or benzene/toluene of the substrate and the bulky NO2X reagents, giving lower mixtures from the literature are listed in Table 8, in order of ortho/para ratios in the nitration of toluene. The degree of decreasing ortho/para ratios. It is concluded that there is no separation between NO' and X- is greater in polar solventsj common relationship between substrate selectivity and which allows the less bulky NO2 ion to attack the ortho positions regioselectivity. The ortho/para ratio changes between the more easily, thus increasing the ortho/para ratio. The reaction limits of 2.38 and 0.49. The low values observed, however, seem can therefore be considered as a nucleophilic displacement of to be always under heterogeneous reaction conditions. The X- by the aromatic hydrocarbon from NO2X. Not only the decrease of the ortho/para ratio is best explained by steric ef- electrophilicity of the reagent but also the nucleophilicity of fects, as reported earlier. Because the aromatic substrate studied the aromatic substrate can affect the relative position of the is always toluene, the steric inhibition of ortho substitution is transition state involved. This should be taken into consideration due to- the interaction between the nitrating agent and the when comparing highly deactivated aromatic substrates to methyl group. Thus, the nitrating agent can be expressed as benzene or toluene. NO' for the free or solvated nitronium ion and as NO2X for a In the studied electrophilic nitrations, the substrate selectivity solvated ion-pair or a complex with a catalyst. (ktoluene/kbenzene) varied from 1.4 to 65.7. The isomer distri- Because there is no-significant increase observed in the butions observed, are however, are only slightly different in the amount of meta substitution, irrelevant of the activity-of the various systems and solvents. The ortho/para ratio was found reagent system, the lack of a relationship between reactivity to when were nitrated under be smaller benzene and toluene and reinforces our held view-that there Friedel-Crafts conditions with excess of aromatics as sol- selectivity previously is no justification for a simple reactivity-selectivity relationship vent. substrate If all nitrations had type transition states in electrophilic aromatic substitution, when selectivity involved a-complex and are determined in separate steps. The of energy, then the decrease in substrate selectivity must regioselectivity highest second step of the nitration reaction is clearly the formation of have been accompanied by lowering of ortho/para ratios and arenium ions. increase in meta However, this is not observed. the corresponding ortho, meta, and para substitution. Whether the distinct first step of the reaction is the rapid but Thus, we must conclude that electrophilic aromatic nitration a must two separate steps, with irreversible initial formation of a wr-bonded complex (ie., of toluene and benzene involve two-electron transfer process), the formation a solvent-enclosed the transition state of highest energy of nature or 7r-complex encounter pair between the NO' ion and aromatics, or, as encounter pair (ionic or radical), followed by the formation of Perrin (15) recently suggested, a radical pair formed between isomeric a-complexes. The substrate and positional selectivity which is will thus be determined in separate steps (6). NO' and ArH+- (i.e., a one-electron transfer process If the nitronium ion is the only effective-nitrating agent in energetically improbable in the case of toluene and benzene but can be of significance for aromatics of higher electron all nitrations, as suggested by Ingold (13) and Ridd (14), then can be but our suggestion of two its activity still must be dependent on the medium. Changes in density) debated, original substrate selectivity and regioselectivity thus would reflect the separate steps determining substrate and positional selectivity is valid in all cases. in nitronium ion in of varying nu- change reactivity of media The studied free radical nitrations demonstrate the case for cleophilicity. The concept of a single nitrating agent can hardly a single-step process determining both substrate selectivity and Table 6. Competitive radical type nitration of benzene and regioselectivity. The energetic NOa radical reacts statistically toluene with tetranitromethane under thermal conditions and with all available positions, giving close to statistical product N204 under UV irradiation* distributions. Nitrating Isomer distribution, % EXPERIMENTAL agent kT/kB ortho meta para o/p All solvents, toluene, benzene, and their nitro derivatives were the highest purity materials commercially available, purified C(N02)4 0.7 42 39 19 2.21 by usual methods before use. Nitronium salts (Cationics) were N204 2.6 37 38 25 1.48 thoroughly purified from nitrosonium ion impurities or were * See ref. 8. prepared from methyl nitrate free of nitrite. Downloaded by guest on October 2, 2021 1048 Chemistry: Olah et al. Proc. Natl. Acad. Scd. USA 75 (1978)

Table 8. Nitration of benzene and toluene %Iaomer % Isomer Ref. Reagent/solvent OC kT/kB 0 m p o/p Ref. Reagent/solvent* 0C kT/kB a m p o/p 3 NO2C1, AgBFJ/CH3NO2 15 - 69 2 29 2.38 17 HNO&/CH3NO2 25 - 62 2 36 1.72 7 NO2PFe/CH3NO2 25 1.6 68 3 29 2.34 16 5-NO2-CoHWNNO2BF-/CH3CN 25 13.2 62 2 36 1.72 3 NO2PFwTMS 25 1.4 68 2 30 2.27 11 HNO/CFsCOOH 25 28 61 3 36 1.69 3 NO2AsFs/CH3NO2 25 1.0 67 2 31 2.16 22 CH3COONO2/CH3NO2 25 - 60 4 36 1.67 3 NO2BF/CH3NO2 25 1.2 66 3 31 2.13 t CH3COONO2/CH3CN 25 44.3 61 2 37 1.65 3 NO2CI04STMS 25 1.6 66 3 31 2.13 4 HNO3/Ac2O 25 27 61 2 37 1.65 t CHaOCH2ONO2/CHsCN 25 35.5 65 4 31 2.10 23 HNO0/Ac20 0 - 61 2 37 1.65 t CfH5ONO2/CH3CN 25 30.7 64 5 31 2.06 11 HNO3/68.3% H2SO4 25 17.2 60 5 3 5 374-5 1.62 3 NO2BF4 WMS 25 1.7 65 3 32 2.03 24 HNO3 (94%)/none 0 - 60 3 37 1.62 3 NOsAsFflMS 25 1.5 65 3 32 2.03 25 N205/CH3CN 0 - 60 3 37 1.62 2 N20,/TMS 25 - 65 3 32 2.03 26 CeH5COONO2/CH3CN 0 - 59 4 37 1.59 t p-CHsOC6H4ONO2/CH3CN 25 37.5 63 6 31 2.03 27 HNO/CH3NO2 30 21 59 4 37 1.59 t C13CNO2BF3/CHsNO2 25 36.8 64 4 32 2.00 28 HNO0/H2SO4 30 - 59 4 37 1.59 t C2HCOONOd/CHaCN 25 33.3 64 4 32 2.00 29 HNO3, HNO2/77% H2SO4 30 - 59 4 37 1.59 t C(NO2)4PF&/CH3NO2 25 37.0 63 5 34 1.97 30 HNO0/H2SO4 40 - 59 4 37 1.59 10 NO2C1 AIC13/CS2 0 - - - - 1.96 27 HNO0/Ac2O 30 23 58 5 37 1.57 16 2-CHS-C6H4N+NO2BF4/CHCN 25 36.5 64 3 33 1.94 26 C6H5COONO2/CC4 0 - 57 6 37 1.54 16 2,6-(CHs)2C6HN+NO2BFd/CH3CN 25 39.0 64 3 33 1.94 27 HNO3/Ac2O 0 27 58 4 38 1.53 16, 4-CHsO-2,6-(CH9)2- 4 HNO3, H2SOSArH 25 1.2 56 5 39 1.44 CsH2N+NO2BF4-/CHsCN 25 44.5 64 3 33 1.94 31 HNOs (d - 1.47)/ArH 30 - 57 3 40 1.43 t CF3COONO2/CH3CN 25 29.8 63 4 33 1.91 4 HNO3/AcOH 25 28.8 57 3 40 1.43 t C.FCOONO2/CH3CN 25 27.0 63 4 33 1.91 32 C5HulONO2/H2SO4 50 - -- - 1.42 t C13CNO2,PFB/CHSNO2 25 34.2 63 4 33 1.91 1i HN0O/90% AcOH 45 24 56 4 40 1.40 t C(NO2), BFs/CHsNO2 25 40.0 64 2 34 1.88 t NO2Cl, PFJ/ArH 25 39.3 57 2 41 1.39 12 HNO3/H2S04 25-50 - 62 5 33 1.87 t NO2CI BFJ/ArH 25 25.1 57 2 41 1.39 16 2,4,6-(CHs)sC6H NNO2BF4-/CHCN 25 41.4 63 3 34 1.85 4 HNO, H2SO4 (75%)/TMS 25 1.6 56 3 41 1.37 17 HNOs/AcgO 25 - 63 3 34 1.85 22 CHsCOONO2/Ac20 25 - 56 3 41 1.37 t NO2PF&, CHsOH(1:1)/CHsNO2 25 3.3 63 3 34 1.85 11 HNOs/Ac2O 25 - 56 3 41 1.37 t NO2PFe, (CHs)20(1.2)/CHsNO2 25 31.3 63 3 34 1.865 t NOCl, SnC4/ArH 25 30.5 57 1 42 1.36 t NO2PF6, (C2H,)20(1:2)/CH3NO2 25 32.8 62 4 34 1.82 t NOC1, SbClJ/ArH 25 26.7 56 2 42 1.33 t NO2PF&, THF (1:2)/CH3NO2 25 28.9 62 4 34 1.82 3 NO2CI04/ArH 25 - 55 3 42 1.31 t NO2PF.. (CHs),0 (1:1)/CHaNO2 25 4.0 62 4 34 1.82 3 NO2PF6/ArH 25 - 55 2 43 1.28 t NO2PF&, (C2H5)20 (1:1)/CH3NO2 25 3.8 62 4 34 1.82 22 CH3COONO2/CC4 25 - 55 2 43 1.28 18 CHsCONOJ/CH2C12 -25 89.3 63 2 35 1.80 3 NO2BFSArH 25 - 54 3 43 1.26 t NO2PF&, (CH,)2S (1:2)/EtN02 -78 65.7 62 3 35 1.77 22 CH3COONO2/AcOH 25 - 54 3 43 1.26 t NO2PFe, CH3OH (1:2)/CHsNO2 25 26.1 62 3 35 1.77 33 HNO&/CCI4 25 17. 53 3 44 1.20 t NO2PF, THF (1:1)/CH3NO2 25 3.6 62 3 35 1.77 t NO2C1, TiC4/ArH 25 17.6 53 2 45 1.18 4 Mixed acid (35%)TMS 25 28 62 3 38 1.77 32 HNOa, PPA:/CHCl3 24-40 - - - - 1.16 4 HNO&/TMS 25 17 62 3 35 1.77 t NO2CI, AlClArH 25 11.2 53 1 46 1.15 19 CHsONO2/Ac2O 25 38k 5 62 3 35 1.77 § CH30NO2, PPAS/CH3NO2 25 - 50 3 47 1.06 t NOXCI, PF&/CH3NO2 25 28.5 62 3 35 1.77 3 NO2HSAO7/ArH 25 - 49 4 47 1.04 t NO2PF&, (CHs)2S (1:1)/EtNO2 -78 4.6 62 3 35 1.77 § CH3ONO2, PPAI/ArH, CH3NO21 25 - 47 3 50 0.94 3 NO2HS,07fI'MS 25 1.5 62 3 35 1.77 3 HNO3, H2SOSArH -15 - 48 1 51 0.94 4 HNOs/CH3NO2 25 26.4 62 3 35 1.77 32 HNO, PPA/ArH 24-40 - -- - 0.86 20 HNOs, H2S04/TMS 25 37 62 3 35 1.77 32 HNO, P20/CHC13 25 - 44 4 52 0.85 t NOtPFsneo-CHuOH (1:2)/CH3NO2 25 25.4 62 3 35 1.77 32 EtONO2, PPAS/ArH 45 - 42 3 55 0.76 t NOtPFs,neo-C5H,,OH (1:1)/CH3NO2 25 2.8 62 3 35 1.77 3 NOCI, TiC4lArH 25 - 41 3 56 0.73 21 Fuming HNO,/Ac2O 40 - -- - 1.76 32 n-BuONO2, PPA:/ArH 26-35 - 39 3 58 0.67 t p-NOrCsH4COONO,/CHaCN 25 28.0 61 4 35 1.74 32 C6Hi1ONO2, PPAI/ArH 50 - - - - 0.64 t p-CHSCeHGCOONO2/CHaCN 25 34.0 61 4 35 1.74 § CH30NO2, PPAI/ArH 25 - 37 4 59 0.63 t NOC1 TiC/CH3NO2C 25 27.8 61 4 35 1.74 32 sec-BuONO2, PPAI/ArH 32-40 - 36 3 61 0.59 t NO2C, AlCWCHsNO2 25 26.8 61 4 35 1.74 32 t-BuONO2, PPAt/ArH 25-30 - - - - 0.50 3 NO2CI,TiClISMS 25 - 61 4 35 1.74 32 neo-CsHuONO2PPAt/ArH 30-40 - -- - 0.49 * TMS, tetramethylene sulfone; THF, tetrahydrofuran; PPA, polyphosphoric acid. t This work. S Heterogeneous nitration, § G. A. Olah and H. C. Lin, unpublished data. I ArH/CH3NO2, 1:1. With Nitronium Salts Using Mixing Chamber of was brought to 250. Ten milliliters of this solution was added Stopped-Flow Apparatus. Mixing studies were performed in dropwise to a well-stirred solution of 0.01 mol of benzene and a Durrum-Gibson stopped-flow apparatus capable of providing 0.01 mol of toluene in 40 ml of nitromethane and the temper- 99.5% mixing of two components within 2 insec and observation ature was kept at 250. The reaction was allowed to proceed of reaction time as short as 5 msec. About 0.02 M solutions of further for 30 min at 25°. The mixture was then poured into the reactant (nitronium salt and aromatics) were stored in two ice water and extracted, dried, concentrated, and analyzed by large reservoir syringes positioned at right angles to two driving gas/liquid chromatography. syringes. After the two components are drawn into the driving In the case of thioethers, nitroethane was used as a solvent syringes, they are forced rapidly forward by hydraulic pressure. and the reaction was run at -78°. The reaction components are then forced through a mixing jet In the case of dimethyl sulfoxide and 4-nitropyridine-N- and pass on to the "stop" block. The reaction products were oxide, the nitrosonium salt was added at 00, the contents were washed with aqueous NaHCO3, extracted with ether, and stirred for 30 min at that temperature, and the nitration was concentrated before gas chromatrophy analysis. performed at 600. Competitive Nitration of Benzene and Toluene with Ni- With Nitryl Chloride and Lewis Acid Halides. (i) Excess tronium Hexafluorophosphate in the Presence of Alcohols, of aromatics as solvent. To a mixture of benzene (0.1 mol), Ethers, and Thioethers and with Nitrosonium Hexafluoro- toluene (0.1 mol), and Lewis acid halide (0.01 mol), 0.01 mol phosphate in the Presence of Dimethyl Sulfoxide or 4-Ni- of NO2CI was added with stirring, while the temperature of the tropyridine-N-oxide. To a well-stirred solution of nitronium mixture was kept at 250 in a constant temperature bath. The hexafluorophosphate (0.01 mol) in nitromethane (40 ml) at reaction was allowed to proceed for 30 min and then was -10°, a solution of 0.01 mol (or 0.02 mol) of alcohol or ether in quenched with ice water, extracted, dried, concentrated, and nitromethane (10 ml) was added slowly. The resulting solution analyzed by gas chromatography. (ii) In nitromethane solu- Downloaded by guest on October 2, 2021 Chemistry: Olah et al. Proc. Natl. Acad. Sci. USA 75 (1978) 1049 tion. Benzene (0.05 mol), toluene (0.05 mol), and 0.0,1 0iol of 6. Olah, G. A. (1971) Acc. Chem. Res. 4,240-248. Lewis acid halide were dissolved in 80 ml of nitromethae. STo 7:' O1Ah, G. A. & Overchuck, N. A. (1965) Can. J. Chem. 43, the stirred solution, 0.01 mol of NO2CI dissolved in 20 ml of 3279-3293. 8. Hanna, S. B., Hunziker, E., Saito, T. & Zollinger, H. (1969) Helv. nitromethane was added dropwise, and the temperature of the Chim. Acta 52, 1537-1548. reaction mixture was maintained at 25°. After 30 min of re- 9. Benford, G. A. & Ingold, C. K. (1938) J. Chem. Soc. 929-955. action at the same temperature, the reaction mixture was 10. Price, C. C. & Sears, C. A. (1953) J. Am. Chem. Soc. 75,3276- treated as described above. 3277. With Acyl Nitrates. To a solution of benzene (0.05 mol), 11. Hoggett, J. G., Moodie, R. B., Penton, J. R. & Schofield, K. (1971) toluene (0.05 mol), and silver nitrate (0.01 mol) in 80 ml of Nitration and Aromatic Reactivity (Cambridge University Press, acetonitrile, 0.01 mol of acyl chloride in 20 ml of acetonitrile New York). was added dropwise with stirring, while the temperature of the 12. de la Mare, P. B. D. & Ridd, J. H. (1959) Aromatic Substitu- mixture was maintained at 250 throughout the addition of the tion-Nitration and Halogenation (Academic Press Inc., New York). acyl chloride solution. The reaction was allowed to proceed for 13. Ingold, C. K. (1969) Structure and Mechanism in Organic 2 hr. Precipitated silver chloride was filtered off and the filtrate Chemistry (Cornell University Press, Ithaca, NY), 2nd Ed. was then washed and analyzed as previously described. 14. Ridd, J. H. (1971) Acc. Chem. Res. 4,248-253. With Chloropicrin and Tetranitromethane. Ten milliliters 15. Perrin, C. L (1977) J. Am. Chem. Soc. 99,5516-518. of saturated solution of BF3 in nitromethane was added drop- 16. Cupas, C. A. & Pearson, R. L. (1968) J. Am. Chem. Soc. 90, wise with stirring at 250 to a solution of benzene (0.05 mol), 4742-4743. toluene (0.05 mol), and chloropicrin or tetranitromethane (0.01 17. Stock, L. M. (1961) J. Org. Chem. 26,4120-4122. mol) in 90 ml of nitromethane. The reaction was allowed to 18. Stock, L. M. & Young, P. E. (1972) J. Am. Chem. Soc. 94, proceed for 2 hr and was quenched, extracted, dried, concen- 4247-4255. trated, and analyzed as described. 19. Hartshorn, S. R., Moodie, R. B. & Schofield, K. (1971) J. Chem. Soc. B, 1256-1261. Analytical Procedure. Analyses of nitroaromatic products 20. Tolgyesi, W. S. (1965) Can. J. Chem. 43,343-55. were carried out by using a Perkin-Elmer model 226 gas 21. Ketcham, R., Cavestri, R. & Jambotkar, D. (1963) J. Org. Chem. chromatograph equipped with a hydrogen flame ionization 28,2139-2141. detector and a 150 ft X 0.01 inch open tubular column coated 22. Sparks, A. K. (1966) J. Org. Chem. 31, 2299-2302. with butanediol succinate, at 1600 with helium carrier gas at 23. Knowles, J. R., Norman, R. 0. C. & Radda, G. K. (1960) J. Chem. 20 psi (140 kPa). Peak areas were determined with an In- Soc. 4885-4896. fotronics model CRS-100 electronic printing integrator. 24. Gibson, W. H., Duckham, R. & Fairbairn, R L. (1922) J. Chem. Soc. 270-283. 25. Norman, R. 0. C. & Radda, G. K. (1961), J. Chem. Soc., Support of our work by the U.S. Army Office of Research is grate- 3030-3037. fully acknowledged. 26. Halvarson, K. & Melander, L. (1957) Ark. Kemi. 11, 77-88. 27. Ingold, C. K., Lapworth, A., Rothstein, E. & Ward, D. (1931) J. 1. Olah, G. A., Kuhn, S. & Mlinko, A. (1956) J. Chem. Soc. 4257- Chem. Soc. 1959-1982. 4258. 28. Nelson, K. L. & Brown, H. C. (1951) J. Am. Chem. Soc. 73, 2. Olah, G. A., Kuhn, S. J. & Flood, S. H. (1961) J. Am. Chem. Soc. 5605-5607. 83,4571-4580. 29. Jones, W. W. & Russel, M. (1947) J. Chem. Soc. 921-923. 3. Olah, G. A. & Kuhn, S. J. (1962) J. Am. Chem. Soc. 3684-3687 30. Brown, H. C. & Bonner, W. H. (1954) J. Am. Chem. Soc. 76, and references therein. 605-606. 4. Olah, G. A., Kuhn, S. J., Flood, S. H. & Evans, J. C. (1962) J. Am. 31. Holleman, M. A. F. (1914) Rec. Trav. Chim. 33, 11-34. Chem. Soc. 84,3687-3693. 32. Tsang, S. M., Paul, A. P. & DiGiaimo, M. P. (1964) J. Org. Chem. 5. Olah, G. A. & Kuhn, S. J. (1964) J. Am. Chem. Soc. 86,1067- 29,3387-390. 1070. 33. Coombes, R. B. (1969) J. Chem. Soc. B 1256-1260. Downloaded by guest on October 2, 2021