Nitration of Naphthalene and Remarks on the Mechanism of Electrophilic Aromatic Nitration* (Two-Step Mechanism) GEORGE A

Proc. Nati. Acad. Sci. USA Vol. 78, No. 6, pp. 3298-3300, June 1981 Chemistry

Nitration of naphthalene and remarks on the mechanism of electrophilic aromatic nitration* (two-step mechanism) GEORGE A. OLAH, SUBHASH C. NARANG, AND JUDITH A. OLAH Institute of Hydrocarbon Chemistry, Department of Chemistry, University of Southern California, Los Angeles, California 90007 Contributed by George A. Olah, March 2, 1981

ABSTRACT Naphthalene was nitrated with a variety of ni- Table 1. Nitration of naphthalene with various nitrating agents trating agents. Comparison of data with Perrin's electrochemical nitration [Perrin, C. L. (1977)J. Am. Chem. Soc. 99, 5516-5518] a/p shows that nitration of naphthalene gives an a-nitronaphthalene Temp, isomer to fi-nitronaphthalene ratio that varies between 9 and 29 and is Reagent Solvent OC ratio Ref. thus not constant. Perrin's data, therefore, are considered to be NO2BF4 Sulfolane 25 10 * inconclusive evidence for the proposed one-electron transfer NO2BF4 Nitromethane 25 12 mechanism for the nitration of naphthalene and other reactive HNO3 Nitromethane 25 29 1 aromatics. Moodie and Schoefield [Hoggett, J. G., Moodie, R. B., HNO3 Acetic acid 25 21 1 Penton, J. R. & Schoefield, K. (1971) Nitration andAromatic Reac- HNO3 Acetic acid 50 16 1 tivity (Cambridge Univ. Press, London)], as well as Perrin, in- HNO3 Sulfuric acid 70 22 1 dependently concluded that, in the general scheme of nitration of HNO3 Acetic 25 9 reactive aromatics, there is the necessity to introduce into the clas- anhydride sical Ingold mechanism an additional step involving a distinct in- CH30NOjCH3OSO2F Acetonitrile 25 13 * termediate preceding the formation ofthe Wheland intermediate AgNO3/CH3COCI Acetonitrile 25 12 * (o complexes). This view coincides with our two-step mechanistic AgNO3/C6H5COCl Acetonitrile 25 12 * picture [Kuhn, S. J. & Olah, G. A. (1961) J. Am. Chem. Soc. 83, N204 Acetonitrile 25 24 * 4564-4571] of the nitronium salt nitration of aromatic hydrocar- N204/Ce(NO3)4 2NH4NO3 Acetonitrile 65 16 * bons (including benzene and toluene), in which low substrate se- HNO3/H2SO4/urea Acetonitrile - 11 2 lectivity but high positional selectivity was found, indicating the Electrochemical oxidation Acetonitrile 9 2 independence of substrate from positional selectivity. + N204 C(NO2)4 Gascphase 300 1 * The nitration of naphthalene, compared with that ofbenzene, AgNO3/BF3 Acetonitrile 25 19 * toluene, and alkylbenzenes, is considerably less investigated. Naphthalene has been nitrated with a series of conventional NO2 nitrating agents. Wells et al. (1) have discussed the mechanistic Acetonitrile 25 10 * aspects of nitration of naphthalene and methylnaphthalenes. BF4 Perrin (2) recently suggested that the nitration of naphthalene (and ofaromatics more reactive than toluene) with nitron- N O ium ion proceeds via an initial one-electron transfer followed by the collapse of the radical pair. °' N°2 encounter ± 1. NO + Ar-H - (NO; + ArH * Isomer ratios are accurate to controlled * This work. support Perrin's claim and we felt that it is of interest to report -H+ them. HArNO2 - - ArNO RESULTS AND DISCUSSION Perrin's experimental prooffor the suggested radical ion pair The nitration of naphthalene was carried out with various ni- mechanism was based on the preparation of the naphthalene trating agents in different solvents. The results, as well as rel- cation radical by controlled potential electrolysis in acetonitrile evant literature data for comparison, are summarized in Table solution and its subsequent quenching with NO0. He found the 1. They show that the a-/,3-nitronaphthalene ratio varies be- ratio of a-nitronaphthalene to ,B-nitronaphthalene in the elec- tween 9 and 29. We have also carried out the nitration of naph- trochemical nitration to be 9 as compared to 11 in nitration with thalene with NO; in the presence of ceric ammonium nitrate nitric acid/sulfuric acid. He concluded that ". . . the observation in acetonitrile and obtained an a-/,f-nitronaphthalene ratio of that the same product mixture is formed both electrochemically 16 (Table 1). Under these conditions, it can be reasonably as- and via NO is strong evidence that the radical pair is involved sumed that the naphthalene radical cation is first formed and not only in the electrochemical synthesis but also in aromatic is then captured by NO;. However, NOby itselfnitrates naph- nitration." Our results on the nitration of naphthalene do not thalene in the same solvent, thereby complicating the data. On The publication costs ofthis article were defrayed in part by page charge Abbreviation: GLC, gas/liquid chromatography. payment. This article must therefore be hereby marked "advertise- * This is paper no. 49 in the series "Aromatic Substitution." Paper no. ment" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 48 is ref. 9. 3298 Downloaded by guest on September 26, 2021 Chemistry: Olah et al. Proc. Natl. Acad. Sci. USA 78 (1981) 3299 the basis ofthese observations, it is improbable that no nitration a meta isomer content of3-4% (or less), with a more significant occurred by an alternative pathway in Perrin's (2) controlled variation of the ortho/para isomer ratio. High regioselectivity potential electrolysis. Due to the absence of experimental de- in the nitration of toluene, thus, is considered to be predomi- tails in Perrin's communication, it is further not clear ifhe car- nant ortho-para nitration, in other words, there is a high ortho- ried out necessary blank experiments and thus whether his re- para/meta isomer ratio. This ratio can be further increased in sults can be assigned to a purely radical cation nitration some experiments by carrying out the nitration oftoluene at low pathway. In fact, Eberson et al. (3, 4) have recently repeated temperatures, such as at -900C, in methylene chloride solution Perrin's experiments and concluded that the nitration ofnaph- with CH30NO2/BF3. The obtained isomer distribution of63% thalene observed in such an electrochemical experiment is pre- ortho-, 1% meta-, and 36% para-nitrotoluenes is the highest dominantly, if not exclusively, due to the homogeneous nitra- ortho + para/meta selectivity (of 99) observed so far in elec- tion of naphthalene by dinitrogen tetroxide, catalyzed by trophilic nitration of toluene in homogeneous solution. anodically generated acid. Furthermore, Eberson et al. (3, 4) have examined in detail the diffusion controlled radical cation MECHANISTIC CONCLUSIONS -N0 coupling by allowing solid naphthalenetPF- to react with N204. The results showed that nitronaphthalenes are The generally accepted mechanism for electrophilic aromatic formed with an a//3 ratio of'-40. On the other hand, the naph- nitration is based on the work of Ingold and Hughes (6): thalene/NO' reaction under similar conditions gives an a/,8 ratio of =15. Thus, it is not very likely that the radical cation H+ + HNO3 + H20NO2 -NO2 coupling is involved in the electrophilic nitration of s naphthalene. H20NO + NO+ + H20 Ridd's (5) previous criticism of Perrin's radical cation mechanism was based in part on experiments on the generation of NO+ + ArH HArNO+ fast a radical cation from mesitylene with ceric ammonium nitrate HArNO+ > ArNO2 + H+ in the presence of N204. Eberson et al. (3, 4) have, however, also shown that mesitylene does react with dinitrogen tetroxide In the course of our work on the nitration of aromatics by in acetonitrile at a reasonable rate, in contrast with a statement nitronium salts (7), we reported in 1961 the observation of low to the contrary by Ridd et al. (5), who claimed that the rate of substrate but high positional selectivity in the competitive ni- reaction between dinitrogen tetroxide and mesitylene is neg- tration of benzene and toluene (and other alkylbenzenes), ligible. Thus, whereas, it is improbable that the electrophilic thereby indicating the independence of substrate from posi- nitration of mesitylene proceeds through the radical cation tional selectivity. The experimental data led to the inevitable pathway, the effect of N204 nitration cannot be neglected. conclusion that substrate and positional selectivities must be The data of Table 1 further clearly negate Perrin's claim (2) determined in two distinct, separate steps, corresponding to that electrophilic nitration ofnaphthalene and the electrochem- intermediates separated by an energy barrierofsufficient height ical (radical cation) nitration give a constant a-/13-nitronaphtha- as to prevent reversal of the first step. To account for the low lene ratio of 9. Various nitrating systems for naphthalene show substrate selectivity, it was suggested that the first intermediate differing regioselectivity, although the differences in isomer is of i-complex nature (i.e., a molecularly ir-bonded species), distribution are obviously not excessively large, as is also the whereas the second step consists of formation of individual uf case in the different nitrations of toluene under homogeneous complexes (i.e., nitroarenium ions) corresponding to the regio- electrophilic conditions. It should be pointed out that naphtha- selective formation of isomers. lene, as well as other aromatics, can be nitrated by a variety of Subsequently and independently, Moodie and Schoefield electrophilic as well as radical nitrating agents. (8), also concluded that in the general scheme of nitration of Perrin's radical ion pair mechanism (2) certainly would seem reactive aromatics, which were shown in their studies to reach to have merit when considering specific cases and, particularly, the encounter-controlled limit, there is the necessity to intro- nitration ofhigher polycyclic aromatic hydrocarbons, for which duce into the classical Ingold mechanism an additional step in- one-electron transfer to the nitronium ion becomes increasingly volving a distinct intermediate preceding the formation of the exothermic. It cannot, however, be considered, as claimed, as of complexes. the universal nitration path for reactive aromatics. Further, in The differences between the conclusions of Kuhn and Olah some instances, such as the nitration of benzene and toluene, (7) and those of Moodie and Schoefield (8) is in the nature of the radical ion path seems to be energetically prohibitively un- thefirst intermediate. Moodie and Schoefield suggested that, favorable (even when one considers that different solvent media for aromatics more reactive than toluene, the first intermediate may affect the energetics of electron transfer). is an encounter pair. The nitronium ion and the aromatic sub- It is necessary to point out that, as found in our studies on strate diffuse together to form a solvent-caged molecule-ion pair nitration with nitronium salts in organic solvents, toluene and with no particular bonding interaction or structure. benzene, as well as naphthalene, behave similarly to xylenes Perrin (2) on the other hand, suggested that, in the case of and higher methylbenzenes in showing low substrate selectiv- aromatics more reactive than toluene, the initial interaction is ity. Thus, regardless whether the first reaction step is of en- a one-electron transfer process forming a radical ion pair, as a counter pair, of radical ion pair, or, as suggested by us, of mo- distinct intermediate, which subsequently collapses in a second lecularly bound ir-complex nature, this also must be the case step into the specific nitroarenium ion intermediate. Despite for benzene and toluene. The high regioselectivity retained in differences in the nature of the first intermediate, all mecha- nitration of all reactive aromatics, including naphthalene, with nisms agree on the necessity oftwo separate steps determining nitronium salts clearly necessitates a second separate step of independently substrate and positional (regio) selectivity. Thus nitroarenium ion nature. the original Ingold mechanism must be amended. Nitronium salt nitration ofalkylbenzenes, including toluene, and ofnaphthalene, shows uniformly high regioselectivity. The H+ + HNO3 a± H2ONO+ nitration of naphthalene gives 91-92% a- and 8-9% /-nitro- slow + naphthalene, whereas the nitration of toluene generally gives H ONO-t' - NO2 + Ho Downloaded by guest on September 26, 2021 3300 Chemistry: Olah et al. Proc. Natl. Acad. Sci. USA 78 (1981) NO+ + ArH . "First intermediate" mixture was quenched with 10% aqueous sodium bicarbonate solution and extracted with ether. The ethereal extract was "First intermediate" ± HArNO+2 washed with brine, dried over anhydrous sodium sulfate, and (Nitroarenium ion analyzed by GLC. intermediate) Nitration with Dinitrogen Tetroxide and Ceric Ammonium Nitrate. Dinitrogen tetroxide was directly bubbled into a de- +fast HArNO2 ArNO2r + H+ oxygenated solution of naphthalene (0.01 mol) and ceric ammonium nitrate (0.02 mol) in acetonitrile (50 ml) maintained at Electrophilic aromatic nitration is a typical aromatic substi- 65TC under a dry argon atmosphere. Aliquots (5 ml) were with- tution reaction and thus should not be considered as unique and drawn every 15 min, quenched with 10% aqueous sodium bi- necessitating a single, uniform reaction mechanism for all ni- carbonate solution, and extracted with ether. The ethereal ex- trations. Radical cation (i.e., one-electron transfer) nitration of tracts were dried over anhydrous sodium sulfate and analyzed certain reactive aromatics should be considered in this context by GLC. and not as replacing the well-established electrophilic (i.e., two- Gas-Phase Nitration with Tetranitromethane. A solution of electron transfer) path ofnitration. The involvement ofseparate naphthalene (100 mg) in tetranitromethane (1 ml) was injected overall rate- and regioselectivity-determining steps in the ni- into the injection port of a Varian 3700 gas chromatograph tration of reactive aromatics seems now well established, in equipped with a glass capillary column. The injection port was spite of the differing views relating to the nature of the first maintained at 300 'C. The product a- and 3-nitronaphthalenes intermediate. obtained under these pyrolytic conditions were analyzed directly by GLC. EXPERIMENTAL Nitration with Silver Nitrate/Boron Trifluoride Etherate. All solvents, aromatics, and reagents were commercially avail- To a stirred solution ofsilver nitrate (0.01 mol) and naphthalene able, highest-purity materials, purified by the usual methods (0.02 mol) in acetonitrile (25 ml) at 25TC, boron trifluoride eth- before use. Nitronium salts were thoroughly purified from ni- erate (2 ml) was added. The reaction mixture was stirred under trosonium ion impurities. a dry atmosphere at 25TC for 1 hr and poured into water. The Nitration with Nitronium Salts. A solution of naphthalene reaction mixture was extracted with ether and the ethereal ex- (0.01 mol) in sulfolane or nitromethane (5 ml) was added to a tract was washed successively with dilute ammonium hydroxide solution of nitronium tetrafluoroborate (0.005 mol) in the same and brine. The ethereal extract was dried over anhydrous so- solvent (10 ml) at 25°C. The reaction mixture was stirred at this dium sulfate and analyzed by GLC. temperature for 1 hr and then quenched with 10% aqueous so- Nitration with N-Nitrato-4-nitropyridinium Tetrafluoro- dium bicarbonate solution. The product was extracted with borate. To a stirred suspension of nitronium tetrafluoroborate ether and the ethereal extract was washed successively with (0.01 mol) in acetonitrile (25 ml), 4-nitropyridine N-oxide (0.01 10% aqueous sodium bicarbonate solution and brine. The ethe- mol) was added in small portions for 30 min at 0°C under a dry real extract was dried over anhydrous sodium sulfate and ana- nitrogen atmosphere. The reaction mixture was warmed to 250C lyzed by gas/liquid chromatography (GLC). over a period of30 min and then a solution ofnaphthalene (0.02 Nitration with Methyl Nitrate/Methyl Fluorosulfonate. A mol) in acetonitrile (10 ml) was added and the reaction mixture mixture of methyl nitrate (0.005 mol) and naphthalene (0.01 was stirred for 1 hr. It was quenched with dilute hydrochloric mol) in acetonitrile (24 ml) was stirred under nitrogen at 25°C acid and extracted with ether. The ethereal extract was washed and methyl fluorosulfonate (0.005 mol) was then added drop- with water and 10% aqueous sodium bicarbonate solution and wise to the reaction mixture. The reaction mixture was stirred dried over anhydrous sodium sulfate. The products were ana- at 25°C for 4 hr and subsequently quenched with 10% aqueous lyzed by GLC. sodium bicarbonate solution and extracted with ether. The is ethereal extract was dried over anhydrous sodium sulfate and Support ofour work by the U. S. Army Office ofResearch gratefully analyzed by GLC. acknowledged. Nitration with Acetyl Nitrate and Benzoyl Nitrate. A solu- 1. Alcorn, P. G. E. & Wells, P. R. (1965) Aust. J. Chem. 18, 1377- tion ofacetyl chloride or benzoyl chloride (0.01 mol) in aceton- 1389. itrile (5 ml) was added dropwise to a solution of silver nitrate 2. Perrin, C. L. (1977) J. Am. Chem. Soc. 99, 5516-5518. (0.01 mol) and naphthalene (0.005 mol) in acetonitrile (25 ml) 3. Eberson, L., Jonsson, L. & Radner, F. (1978) Acta Chem. Scand. at 25°C. The reaction mixture was stirred for 1 hr and quenched B. 32, 749-753. with 10% aqueous sodium bicarbonate solution. The reaction 4. Eberson, L. & Radner, F. (1980) Acta Chem. Scand. B. 34, 739- 746. mixture was extracted with ether and the ethereal extract was 5. Draper, M. R. & Ridd, J. H. (1978) J. Chem. Soc. Chem. Com- washed successively with 10% sodium bicarbonate solution, mun., 445-446. dilute ammonium hydroxide, and brine. The ethereal extract 6. Hughes, E. D., Ingold, C. K. & Reed, R. I. (1950)1. Chem. Soc., was dried over anhydrous sodium sulfate and the products were 2400-2440. analyzed by GLC. 7. Kuhn, S. J. & Olah, G. A. (1961) J. Am. Chem. Soc. 83, 4564- Nitration with Dinitrogen Tetroxide. A solution of dinitro- 4571. 8. Hoggett, J. G., Moodie, R. B., Penton, J. R. & Schoefield, K. gen tetroxide (0.5 ml) in acetonitrile (5 ml) was added with stir- (1971) Nitration andAromatic Reactivity (Cambridge Univ. Press, ring to a solution of naphthalene (0.01 mol) in acetonitrile (10 Cambridge, England). ml) at 25°C under a dry nitrogen atmosphere and the reaction 9. Olah, G. A., Fung, A. P., Narang, S. C. & Olah, J. A. (1981)J. mixture was stirred at this temperature for 1 hr. The reaction Org. Chem., in press. Downloaded by guest on September 26, 2021