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  • Proc. Natl. Acad. Sci. USAyol. 75, No. 3, pp. 1045-1049, March 1978Chemistry

    Electrophilic and free radical nitration of benzene and toluene withvarious nitrating agents*

    (aromatic compounds/selectivity)

    GEORGE A. OLAH, HENRY C. LIN, JUDITH A. OLAH, AND SUBHASH C. NARANGInstitute 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 benzenewas studied under various conditions with several nitratingsystems. It was found that high ortlopara regioselectivity isprevalent in all reactions and is independent of the reactivityof the nitrating agent. The methyl group of toluene is predom-inantly ortho-para directing under all reaction conditions. Stericfactors are considered to be important but not the sole reasonfor the variation in the ortho/para ratio. The results reinforceour earlier views that, in electrophilic aromatic nitrations withreactive nitrating agents, substrate and positional selectivitiesare determined in two separate steps. The first step involves air-aromatic-NO2 ion complex or encounter pair, whereas thesubsequent step is of arenium ion nature (separate for the oftho,meta, and para positions). The former determines substrateselectivity, whereas the latter determines regioselectivity.Thermal free radical nitration of benzene and toluene withtetranitromethane in sharp contrast gave nearly statisticalproduct distributions.

    Stable nitronium salts were introduced as new nitrating agentsby Olah and coworkers (1) in 1956. In the course of these studies(2-5), the competitive nitration of benzene and toluene, as wellas other aromatics, was carried out in organic solvents.Under usual conditions of electrophilic nitration, toluene

    reacts about 20 times more rapidly than benzene whereas, withnitronium salts, toluene was found to react only 1.7 times fasterthan 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 highestenergy (which determines substrate selectivity) is of startingaromatic (i.e., 7r-complex) nature, which is then followed byseparate a-complex formation (for the individual positions),determining positional selectivity.

    In a series of studies, we have found that, in electrophilicaromatic 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, dependingupon the reactivity of the electrophiles and the basicity of thearomatic substrates.

    In order to further explore electrophilic nitration, we carriedout a comprehensive study of nitration of benzene and tolueneunder various conditions.

    RESULTS AND DISCUSSIONWith Nitronium Salts. Although we had previously exam-

    ined competitive nitration using high-speed mixing (7), it wasconsidered of interest to extend the studies by using more ad-vanced methods such as the mixing chamber of an efficientDurrum-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 veryfast (or reaching the encounter-controlled limit), the data seemto indicate that, in the present system, both toluene and benzenereact by the same mechanism. In other words, if the reactionsindeed reach encounter-controlled limiting rates, this must bethe case in the studied system not only for toluene but also forbenzene, 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 innitronium salt nitrations to those conventionally observed innitric acid solutions. A more detailed study of the competitivenitration of toluene and benzene in the presence of a series ofnucleophiles was undertaken. The results, summarized in Table2, show that the ktoluene/kbenzene rate ratios are in the range of2-5 when 1 equivalent of alcohol, ether, or thioether is addedbut are 25-66 when 2 equivalents of the nucleophile are used.The relative reactivity of the nitrating agent in the presence ofadded nucleophiles is in the decreasing order ROH > ROR >RSR. The isomer distributions, however, stay similar. The dataare best interpreted in terms of the nitronium ion reacting withthe n-donor nucleophile forming an 0- or S-nitronium ion in-termediate, which can either reverse, or transfer nitrate, or forma covalent intermediate.

    R-X-RY- RXNO2 + RF + PF5(BF3)

    Y = PF6-, BFJ-; X =0, S; R = alkyl, H.

    An isomer of the dimethylnitrosulfonium ionCH3

    zS+-NO2CH3

    i.e., the corresponding nitrito complex, was also prepared fromdimethyl sulfoxide and NO+. A similar nitrito complex wasobtained 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.

    1045

    The costs of publication of this article were defrayed in part by thepayment of page charges. This article must therefore be hereby marked"advertisement" in accordance with 18 U. S. C. 1734 solely to indicatethis fact.

  • Proc. Nati. Acad. Sci. USA 75 (1978)

    Table 1. Competitive nitration of benzene and toluene at 250with 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 1.825:1 1.4 62 3 35 1.771:1 1.7 64 3 33 1.941:5 2.4 64 3 33 1.941:10 2.5 63 3 34 1.85

    * kT, k for toluene; kB, k for benzene.

    trito onium ion reagents act as weak nitrating agents, requiringreaction temperatures of 55060.

    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 asR2XNOZ R2tXONO, and R-X-NO2 play an important role inthese 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 dataare shown in Table 3. With an excess of the aromatics as solvent,the substrate selectivity varied from 11 to 39, accompanied byslight changes in regioselectivity. Generally, the ortho/pararatio is lower than in nitrations with nitronium salts.

    In general, thekwue./k. ratio decreases with increasingacidity 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 nitroniumsalts, which are highly polarized in the generally used solventsof high dielectric constant and show no effects of ion pairing.

    Table 2. Competitive nitration of benzene and toluene withNO'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 1.85NO+PFj/methanol (1:2) 26.1 62- 3 35 1.77NOMPFe/neopentyl

    alcohol (1:1) 2.8 62 3 35 1.77NO+PF/neopentyl

    alcohol (1:2) 25.4 62 3 35 1.77NOfPFj/methyl ether (1:1) 4.0 62 4 34 1.82NO+PFj/methyl ether (1:2) 31.3 62 4 34 1.82NO+PFj/ethyl ether (1:1) 3.8 62 4 34 1.82NO+PF /ethyl ether (1:2) 32.8 62 4 34 1.82NO PFi/tetrahydrofuran

    (1:1) 3.6 62 3 35 1.77NO+ Fi/tetrahydrofuran

    (1:2) 28.9 62 4 34 1.82NOrPFf/dimethyl

    sulfide (1:1)* 4.6 62 3 35 1.77NO+F-/dimethyl

    sulfide (1:2)* 65.7 62 3 35 1.77NO+PFl/dimethyl sulfoxide

    (1:1)t 27.3 59 4 37 1.60NO +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 ofbenzene and toluene with nitryl chloride at 250

    in excess of aromatics

    Lewis acid Isomer distribution, %halide kT/kB ortho meta para o/pAlCl3 11.2 53 2 45 1.18TiC14 17.6 53 2 45 1.18BF3 25.1 57 2 41 1.39SbCl5 26.7 56 2 42 1.33PF5 39.3 57 2 41 1.39

    This explains the lower ortho/para' ratio observed in theFriedel-Crafts nitrations in aromatics. However, these factorscan decrease when the reactions are carried out in ionizing polarsolvents such as nitromethane (Table 4).

    Nitration with Acyl Nitrates. We have studied nitrationsof toluene and benzene with a series of acyl and aroyl nitrates(11). The results summarized in Table 5 indicate some changesin substrate selectivity with only minor variations in positionalselectivity, but there is no common relationship between sub-strate and positional selectivities. The ktoiuene/kbenzene valuesincrease with increasing pKa values of the corresponding acids.Present studies thus do not give a firm indication of the natureof the nitrating agent involved.

    Nitration with Chloropicrin and Tetranliromethane. Inearlier studies (7), one of us and Overchuck compared elec-trophilic with free radical nitrations-and found the latter to givenearly statistical product distributions, reflected in both sub-strate selectivity and regioselectivity.

    In the present studies, when tetranitromethane (12) wasmixed with benzene and toluene in ether, ethanol, nitrometh-ane, or pyridine/ethanol solutions, no nitration occurs up to 60.However, when an ethereal solution of benzene/toluene (1:1)containing tetranitromethane was injected into a gas chroma-tograph with injection block temperature of 3000, a significantamount of nitro product

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