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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 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 zS+-NO2 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. 1045 The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.
Transcript
Page 1: Electrophilic free benzene toluene - PNAS · Under usual conditions of electrophilic nitration, ... agent Catalyst kT/kn ortho meta para o/p ... inelectrophilic aromaticsubstitution,

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.

Page 2: Electrophilic free benzene toluene - PNAS · Under usual conditions of electrophilic nitration, ... agent Catalyst kT/kn ortho meta para o/p ... inelectrophilic aromaticsubstitution,

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 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 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/p

AlCl3 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 products was detected (Table 6).

Nitroarene product composition clearly indicates that, underthermal decomposition conditions, free radical nitration is fa-vored. Low substrate selectivity (kto1uene/kbe.nzene = 0.7) is ac-companied by low positional selectivity, giving nearly statistical40% ortho, 40% meta, and 20% para isomer distribution.

Friedel-Crafts type nitration by tetranitromethane andchloropicrin was also studied in the presence of BF3 and PF5catalysts. The isomer distribution and .ktd.,./kb, rate ratioshows (Table 7) that nitration under these conditions proceedsvia an electrophilic substitution. The Lewis acids polarise thenitrating agents, thereby weakening the GN bond and makingit susceptible to heterolytic cleavage.We have previously suggested that the reaction path leading

to products in the case of nitration of reactive aromatics withnitronium salts must involve two separate steps leading to thearenium ion type intermediate.

ArH + NO2+ [ArH -- NO+] HArNO2

Table 4. Lewis acid halide-catalyzed Friedel-Crafts nitration ofbenzene and toluene with nitryl chloride in nitromethane

solution at 250Lewis acid Isomer distribution, %

halide kT/kn ortho meta para o/p

AlCl3 26.8 61 4 35 1.74TiC14 27.8 61 4 35 1.74PF5 28.5 62 3 35 1.77

1046 Chemistry: Olah et al.

Page 3: Electrophilic free benzene toluene - PNAS · Under usual conditions of electrophilic nitration, ... agent Catalyst kT/kn ortho meta para o/p ... inelectrophilic aromaticsubstitution,

Proc. Nati. Acad. Sci. USA 75 (1978) 1047

Table 5. Competitive nitration of benzene and toluene with acylnitrates (from AgNO3 and acyl chlorides) in acetonitile'

solution at 25°

Isomer distribution, %Acyl nitrate kT/kB ortho meta para olp

Trifluoroacetyl nitrate 29.8 63 4 33 1.91Propionyl nitrate 33.3 64 4 32 2.0Methoxyacetyl nitrate 35.5 65 4 31 2.1Acetyl nitrate 44.3 61 2 37 1.65Pentafluorobenzoyl nitrate 27.0 63 4 33 1.91p-Nitrobenzoyl nitrate 28.0 61 4 35 1.74Benzoyl nitrate 30.7 64 5 31 2.07p-Methylbenzoyl nitrate 34.0 61 4 35 1.74p-Methoxybenzoyl nitrate 37.5 63 6 31 2.03

With various NO2X type nitrating agents, interaction ofaromatics with polarized NO2X must be considered prior to theformation of the nitronium ion. In nonpolar solvents, there isobviously greater steric interaction between the methyl group

of the substrate and the bulky NO2X reagents, giving lowerortho/para ratios in the nitration of toluene. The degree ofseparation between NO' and X- is greater in polar solventsjwhich allows the less bulky NO2 ion to attack the ortho positionsmore easily, thus increasing the ortho/para ratio. The reactioncan therefore be considered as a nucleophilic displacement ofX- by the aromatic hydrocarbon from NO2X. Not only theelectrophilicity of the reagent but also the nucleophilicity ofthe aromatic substrate can affect the relative position of thetransition state involved. This should be taken into considerationwhen comparing highly deactivated aromatic substrates tobenzene or toluene.

In the studied electrophilic nitrations, the substrate selectivity(ktoluene/kbenzene) varied from 1.4 to 65.7. The isomer distri-butions observed, are however, are only slightly different in thevarious systems and solvents. The ortho/para ratio was foundto be smaller when benzene and toluene were nitrated underFriedel-Crafts conditions with excess of aromatics as sol-vent.

If all nitrations had involved a-complex type transition statesof highest energy, then the decrease in substrate selectivity musthave been accompanied by lowering of ortho/para ratios andincrease in meta substitution. However, this is not observed.Thus, we must conclude that electrophilic aromatic nitrationof toluene and benzene must involve two separate steps, withthe transition state of highest energy of 7r-complex nature or

encounter pair (ionic or radical), followed by the formation ofisomeric a-complexes. The substrate and positional selectivitywill thus be determined in separate steps (6).

If the nitronium ion is the only effective-nitrating agent inall nitrations, as suggested by Ingold (13) and Ridd (14), thenits activity still must be dependent on the medium. Changes insubstrate selectivity and regioselectivity thus would reflect thechange in reactivity of nitronium ion in media of varying nu-

cleophilicity. The concept of a single nitrating agent can hardly

Table 6. Competitive radical type nitration of benzene andtoluene with tetranitromethane under thermal conditions and

N204 under UV irradiation*

Nitrating Isomer distribution, %agent kT/kB ortho meta para o/p

C(N02)4 0.7 42 39 19 2.21N204 2.6 37 38 25 1.48

* See ref. 8.

Table 7. Lewis acid halide-catalyzed competitive nitration ofbenzene and toluene with chloropicrin and tetranitromethane in

nitromethane solution at 250

Nitrating Isomer distribution, %agent Catalyst kT/kn ortho meta para o/p

Chloropicrin BF3 36.8 64 4 32 2.0PF5 34.2 63 4 33 1.91

Tetranitro- BF3 40.0 64 2 34 1.88methane PF5 37.0 63 5 32 1.97

explain the decrease in ortho/para ratio in nitrations with nitrylchloride-Lewis acid halide complexes (Tables 3 and 4), whenthe reaction medium is changed from nitromethane to excessaromatics. It is much more plausible to suggest that certainnitronium ion precursors can act as nitrating agents in their ownright (11).To give a comprehensive review of the variation of isomer

distribution in nitration of-toluene, our own experimental dataand pertinent results of nitration of toluene or benzene/toluenemixtures from the literature are listed in Table 8, in order ofdecreasing ortho/para ratios. It is concluded that there is nocommon relationship between substrate selectivity andregioselectivity. The ortho/para ratio changes between thelimits of 2.38 and 0.49. The low values observed, however, seemto be always under heterogeneous reaction conditions. Thedecrease of the ortho/para ratio is best explained by steric ef-fects, as reported earlier. Because the aromatic substrate studiedis always toluene, the steric inhibition of ortho substitution isdue to- the interaction between the nitrating agent and themethyl group. Thus, the nitrating agent can be expressed asNO' for the free or solvated nitronium ion and as NO2X for asolvated ion-pair or a complex with a catalyst.

Because there is no -significant increase observed in theamount of meta substitution, irrelevant of the activity-of thereagent system, the lack of a relationship between reactivityand selectivity reinforces our previously held view-that thereis no justification for a simple reactivity-selectivity relationshipin electrophilic aromatic substitution, when substrate selectivityand regioselectivity are determined in separate steps. Thesecond step of the nitration reaction is clearly the formation ofthe corresponding ortho, meta, and para arenium ions.Whether the distinct first step of the reaction is the rapid butirreversible initial formation of a wr-bonded complex (ie., atwo-electron transfer process), the formation a solvent-enclosedencounter pair between the NO' ion and aromatics, or, asPerrin (15) recently suggested, a radical pair formed betweenNO' and ArH+- (i.e., a one-electron transfer process which isenergetically improbable in the case of toluene and benzenebut can be of significance for aromatics of higher electrondensity) can be debated, but our original suggestion of twoseparate steps determining substrate and positional selectivityis valid in all cases.The studied free radical nitrations demonstrate the case for

a single-step process determining both substrate selectivity andregioselectivity. The energetic NOa radical reacts statisticallywith all available positions, giving close to statistical productdistributions.

EXPERIMENTALAll solvents, toluene, benzene, and their nitro derivatives werethe highest purity materials commercially available, purifiedby usual methods before use. Nitronium salts (Cationics) werethoroughly purified from nitrosonium ion impurities or wereprepared from methyl nitrate free of nitrite.

Chemistry: Olah et al.

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1048 Chemistry: Olah et al. Proc. Natl. Acad. Scd. USA 75 (1978)

Table 8. Nitration of benzene and toluene%Iaomer

Ref. Reagent/solvent OC kT/kB 0 m p o/p

3 NO2C1, AgBFJ/CH3NO2 15 - 69 2 29 2.387 NO2PFe/CH3NO2 25 1.6 68 3 29 2.343 NO2PFwTMS 25 1.4 68 2 30 2.273 NO2AsFs/CH3NO2 25 1.0 67 2 31 2.163 NO2BF/CH3NO2 25 1.2 66 3 31 2.133 NO2CI04STMS 25 1.6 66 3 31 2.13t CHaOCH2ONO2/CHsCN 25 35.5 65 4 31 2.10t CfH5ONO2/CH3CN 25 30.7 64 5 31 2.063 NO2BF4 WMS 25 1.7 65 3 32 2.033 NOsAsFflMS 25 1.5 65 3 32 2.032 N20,/TMS 25 - 65 3 32 2.03t p-CHsOC6H4ONO2/CH3CN 25 37.5 63 6 31 2.03t C13CNO2BF3/CHsNO2 25 36.8 64 4 32 2.00t C2HCOONOd/CHaCN 25 33.3 64 4 32 2.00t C(NO2)4PF&/CH3NO2 25 37.0 63 5 34 1.9710 NO2C1 AIC13/CS2 0 - - - - 1.9616 2-CHS-C6H4N+NO2BF4/CHCN 25 36.5 64 3 33 1.9416 2,6-(CHs)2C6HN+NO2BFd/CH3CN 25 39.0 64 3 33 1.9416, 4-CHsO-2,6-(CH9)2-

CsH2N+NO2BF4-/CHsCN 25 44.5 64 3 33 1.94t CF3COONO2/CH3CN 25 29.8 63 4 33 1.91t C.FCOONO2/CH3CN 25 27.0 63 4 33 1.91t C13CNO2,PFB/CHSNO2 25 34.2 63 4 33 1.91t C(NO2), BFs/CHsNO2 25 40.0 64 2 34 1.8812 HNO3/H2S04 25-50 - 62 5 33 1.8716 2,4,6-(CHs)sC6H NNO2BF4-/CHCN 25 41.4 63 3 34 1.8517 HNOs/AcgO 25 - 63 3 34 1.85t NO2PF&, CHsOH(1:1)/CHsNO2 25 3.3 63 3 34 1.85t NO2PFe, (CHs)20(1.2)/CHsNO2 25 31.3 63 3 34 1.865t NO2PF6, (C2H,)20(1:2)/CH3NO2 25 32.8 62 4 34 1.82t NO2PF&, THF (1:2)/CH3NO2 25 28.9 62 4 34 1.82t NO2PF.. (CHs),0 (1:1)/CHaNO2 25 4.0 62 4 34 1.82t NO2PF&, (C2H5)20 (1:1)/CH3NO2 25 3.8 62 4 34 1.8218 CHsCONOJ/CH2C12 -25 89.3 63 2 35 1.80t NO2PF&, (CH,)2S (1:2)/EtN02 -78 65.7 62 3 35 1.77t NO2PFe, CH3OH (1:2)/CHsNO2 25 26.1 62 3 35 1.77t NO2PF, THF (1:1)/CH3NO2 25 3.6 62 3 35 1.774 Mixed acid (35%)TMS 25 28 62 3 38 1.774 HNO&/TMS 25 17 62 3 35 1.7719 CHsONO2/Ac2O 25 38k 5 62 3 35 1.77t NOXCI, PF&/CH3NO2 25 28.5 62 3 35 1.77t NO2PF&, (CHs)2S (1:1)/EtNO2 -78 4.6 62 3 35 1.773 NO2HS,07fI'MS 25 1.5 62 3 35 1.774 HNOs/CH3NO2 25 26.4 62 3 35 1.77

20 HNOs, H2S04/TMS 25 37 62 3 35 1.77t NOtPFsneo-CHuOH (1:2)/CH3NO2 25 25.4 62 3 35 1.77t NOtPFs,neo-C5H,,OH (1:1)/CH3NO2 25 2.8 62 3 35 1.7721 Fuming HNO,/Ac2O 40 - - - - 1.76t p-NOrCsH4COONO,/CHaCN 25 28.0 61 4 35 1.74t p-CHSCeHGCOONO2/CHaCN 25 34.0 61 4 35 1.74t NOC1 CTiC/CH3NO2 25 27.8 61 4 35 1.74t NO2C, AlCWCHsNO2 25 26.8 61 4 35 1.743 NO2CI,TiClISMS 25 - 61 4 35 1.74

Ref. Reagent/solvent*17 HNO&/CH3NO216 5-NO2-CoHWNNO2BF-/CH3CN11 HNO/CFsCOOH22 CH3COONO2/CH3NO2

t CH3COONO2/CH3CN4 HNO3/Ac2O

23 HNO0/Ac2011 HNO3/68.3% H2SO424 HNO3 (94%)/none25 N205/CH3CN26 CeH5COONO2/CH3CN27 HNO/CH3NO228 HNO0/H2SO429 HNO3, HNO2/77% H2SO430 HNO0/H2SO427 HNO0/Ac2O26 C6H5COONO2/CC427 HNO3/Ac2O4 HNO3, H2SOSArH

31 HNOs (d - 1.47)/ArH4 HNO3/AcOH

32 C5HulONO2/H2SO41i HN0O/90%AcOHt NO2Cl, PFJ/ArHt NO2CI BFJ/ArH4 HNO, H2SO4 (75%)/TMS

22 CHsCOONO2/Ac2011 HNOs/Ac2Ot NOCl, SnC4/ArHt NOC1, SbClJ/ArH3 NO2CI04/ArH3 NO2PF6/ArH

22 CH3COONO2/CC43 NO2BFSArH

22 CH3COONO2/AcOH33 HNO&/CCI4t NO2C1, TiC4/ArH32 HNOa, PPA:/CHCl3t NO2CI, AlClArH§ CH30NO2, PPAS/CH3NO23 NO2HSAO7/ArH§ CH3ONO2, PPAI/ArH, CH3NO213 HNO3, H2SOSArH

32 HNO, PPA/ArH32 HNO, P20/CHC1332 EtONO2, PPAS/ArH3 NOCI, TiC4lArH32 n-BuONO2, PPA:/ArH32 C6Hi1ONO2, PPAI/ArH§ CH30NO2, PPAI/ArH32 sec-BuONO2, PPAI/ArH32 t-BuONO2, PPAt/ArH32 neo-CsHuONO2PPAt/ArH

% Isomer0C kT/kB a m p o/p

25 - 62 2 36 1.7225 13.2 62 2 36 1.7225 28 61 3 36 1.6925 - 60 4 36 1.6725 44.3 61 2 37 1.6525 27 61 2 37 1.650 - 61 2 37 1.65

25 17.2 60 5 3 5 374-5 1.620 - 60 3 37 1.620 - 60 3 37 1.620 - 59 4 37 1.5930 21 59 4 37 1.5930 - 59 4 37 1.5930 - 59 4 37 1.5940 - 59 4 37 1.5930 23 58 5 37 1.570 - 57 6 37 1.540 27 58 4 38 1.53

25 1.2 56 5 39 1.4430 - 57 3 40 1.4325 28.8 57 3 40 1.4350 - - - - 1.4245 24 56 4 40 1.4025 39.3 57 2 41 1.3925 25.1 57 2 41 1.3925 1.6 56 3 41 1.3725 - 56 3 41 1.3725 - 56 3 41 1.3725 30.5 57 1 42 1.3625 26.7 56 2 42 1.3325 - 55 3 42 1.3125 - 55 2 43 1.2825 - 55 2 43 1.2825 - 54 3 43 1.2625 - 54 3 43 1.2625 17. 53 3 44 1.2025 17.6 53 2 45 1.18

24-40 - - - - 1.1625 11.2 53 1 46 1.1525 - 50 3 47 1.0625 - 49 4 47 1.0425 - 47 3 50 0.94

-15 - 48 1 51 0.9424-40 - - - - 0.8625 - 44 4 52 0.8545 - 42 3 55 0.7625 - 41 3 56 0.73

26-35 - 39 3 58 0.6750 - - - - 0.6425 - 37 4 59 0.63

32-40 - 36 3 61 0.5925-30 - - - - 0.5030-40 - - - - 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 ofStopped-Flow Apparatus. Mixing studies were performed ina Durrum-Gibson stopped-flow apparatus capable of providing99.5% mixing of two components within 2 insec and observationof reaction time as short as 5 msec. About 0.02 M solutions ofthe reactant (nitronium salt and aromatics) were stored in twolarge reservoir syringes positioned at right angles to two drivingsyringes. After the two components are drawn into the drivingsyringes, they are forced rapidly forward by hydraulic pressure.The reaction components are then forced through a mixing jetand pass on to the "stop" block. The reaction products werewashed with aqueous NaHCO3, extracted with ether, andconcentrated before gas chromatrophy analysis.Competitive Nitration of Benzene and Toluene with Ni-

tronium Hexafluorophosphate in the Presence of Alcohols,Ethers, and Thioethers and with Nitrosonium Hexafluoro-phosphate in the Presence of Dimethyl Sulfoxide or 4-Ni-tropyridine-N-oxide. To a well-stirred solution of nitroniumhexafluorophosphate (0.01 mol) in nitromethane (40 ml) at-10°, a solution of 0.01 mol (or 0.02 mol) of alcohol or ether innitromethane (10 ml) was added slowly. The resulting solution

was brought to 250. Ten milliliters of this solution was addeddropwise to a well-stirred solution of 0.01 mol of benzene and0.01 mol of toluene in 40 ml of nitromethane and the temper-ature was kept at 250. The reaction was allowed to proceedfurther for 30 min at 25°. The mixture was then poured intoice water and extracted, dried, concentrated, and analyzed bygas/liquid chromatography.

In the case of thioethers, nitroethane was used as a solventand the reaction was run at -78°.

In the case of dimethyl sulfoxide and 4-nitropyridine-N-oxide, the nitrosonium salt was added at 00, the contents werestirred for 30 min at that temperature, and the nitration wasperformed at 600.With Nitryl Chloride and Lewis Acid Halides. (i) Excess

of aromatics as solvent. To a mixture of benzene (0.1 mol),toluene (0.1 mol), and Lewis acid halide (0.01 mol), 0.01 molof NO2CI was added with stirring, while the temperature of themixture was kept at 250 in a constant temperature bath. Thereaction was allowed to proceed for 30 min and then wasquenched with ice water, extracted, dried, concentrated, andanalyzed by gas chromatography. (ii) In nitromethane solu-

Page 5: Electrophilic free benzene toluene - PNAS · Under usual conditions of electrophilic nitration, ... agent Catalyst kT/kn ortho meta para o/p ... inelectrophilic aromaticsubstitution,

Proc. Natl. Acad. Sci. USA 75 (1978) 1049

tion. Benzene (0.05 mol), toluene (0.05 mol), and 0.0,1 0iol ofLewis acid halide were dissolved in 80 ml of nitromethae. STothe stirred solution, 0.01 mol of NO2CI dissolved in 20 ml ofnitromethane was added dropwise, and the temperature of thereaction mixture was maintained at 25°. After 30 min of re-action at the same temperature, the reaction mixture wastreated as described above.With Acyl Nitrates. To a solution of benzene (0.05 mol),

toluene (0.05 mol), and silver nitrate (0.01 mol) in 80 ml ofacetonitrile, 0.01 mol of acyl chloride in 20 ml of acetonitrilewas added dropwise with stirring, while the temperature of themixture was maintained at 250 throughout the addition of theacyl chloride solution. The reaction was allowed to proceed for2 hr. Precipitated silver chloride was filtered off and the filtratewas then washed and analyzed as previously described.With Chloropicrin and Tetranitromethane. Ten milliliters

of saturated solution of BF3 in nitromethane was added drop-wise with stirring at 250 to a solution of benzene (0.05 mol),toluene (0.05 mol), and chloropicrin or tetranitromethane (0.01mol) in 90 ml of nitromethane. The reaction was allowed toproceed for 2 hr and was quenched, extracted, dried, concen-trated, and analyzed as described.

Analytical Procedure. Analyses of nitroaromatic productswere carried out by using a Perkin-Elmer model 226 gaschromatograph equipped with a hydrogen flame ionizationdetector and a 150 ft X 0.01 inch open tubular column coatedwith butanediol succinate, at 1600 with helium carrier gas at20 psi (140 kPa). Peak areas were determined with an In-fotronics model CRS-100 electronic printing integrator.

Support of our work by the U.S. Army Office of Research is grate-fully acknowledged.

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Chemistry: Olah et al.


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