Oct. 5, 1963
NITRATION OF
2 . Competitive Nitrations in Nitromethane.-Benzene (0.25 mole) and 0.25 mole of alkyl or halobenzenes were dissolved in 50 g . oi n i t r ~ ~ ~ n c i :\ l ~solutioti ~ ~ i ~ ~of. 0 0.5 inole I J ~iiitroniuni salt tliswlvcd i i i (i0 g . of iiitrorricth:irie \vas added to the vigorously stirred substrate sulutiou. Since S 0 2 + B F 4 -is only slightly soluble i n nitrornethanc, a very dilute solution must bc used i n thcsc experinients. T h e 1111. of water, resulting mistures i\-ere waslied twice ivitli dried over CaCL and analyzed by gas-liquid chromatography. 3. Nitrations in Mixed Solvents.-The nitroniuni salt (0.05 mole) n-as dissolved in 60 g . of one of the solvents (nitromethane, tetramethylene sulfone) and this solution !vas used in the competitive nitration of 0.25 mile of benzene arid 0.22 mole of mesitylene dissolved in 70 g . of the other solvent under crmditicins identical with previous nitrations.
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4. Nitrations in Toluene.-Toluene (0.05 mole) was added in srnall portions to 0.05 mole of solid nitronium salt, while tlie ternperatur~\vas kept iLround 23'. The inistures ol,tained wcre washrd tiiice \zit11 2.5 nil. of \\-ater, dried over CaCln mi1 analyzed by gas-liquid chromatography. T h e determination of relative rates and isoiner distribution were carried out b>- gas-liquid chromatograph>- using a Perkin-Elmer inodt.1 I54C gas chromatograph a s described in a n earlier paper in this series.2
Acknowledgment.-The authors are grateful to Mrs. S. H. Flood for part of the experimental work and the gas-liquid chromatographic analyses, to Dr. D. Cook for the infrared and 1)r. J. C. Evans for the Ranian spectroscopic investigations.
[COSTRIBCTION S O .65, EXPLORATORY RESEARCH LABORATORY DOW
CHEMICAL O F
CATADA,
LTD.,SARSIA, ONTARIO, CAN.]
Aromatic Substitution. XIII.la Comparison of Nitric Acid and Mixed Acid Nitration of Alkylbenzenes and Benzene with Nitronium Salt Nitrations BY GEORGE-4. OLAH,STEPHENJ. KUHN,SYLVIA H. FLOOD AND J o m C . E v i l i ~ s ~ ~ RECEIVED JANUARY 2 , 1962
X coniparisoii of nitric acid arid mixed acid nitrations of alkylbenzenes and benzene in nitromethane, acetic acid, acetic anhydri-le, tetramcthyletie sulfone and sulfuric acid solutions with nitroniurri salt nitrations \vas carried out. In sulfuric acid solutions or in concentrated organic solutions using mixed acid, where detectable amounts ~f S O ? are present, nitrations s11o;v a low substrate but high positional selectivity, in complete agreement with previous observations of nitrations with preprepared S O 2 + salts. In dilute organic solutions of the acids there is no spectroscopicall>-( Kninan and infrared) detectable amount of nitroniuin ion and the slow kinetic step of these reactions must be considered t o be the formation of S O ? + . The ititeraction lvith the aromatic substrate must be taking place even before the nitroniurn iwi is complctel>-formed and its weaker electrophile precursor shows higher substrate selectivity. Cryosco;iic investigation of tetramethylene sulfone solutions of S O r + B F 4 -gave evidence of very limited ion separation. Therefore, nitrations with nitroniuin salts in organic solvents cannot he considered as interaction of the free NOp+ ion with aromatics, b u t as nucleo2hilic displacement of t h e solvated SOu"BF4- ion pair by the aromatics. A l n interaction of this ti-pe is in accordance with substantial activation energy needed for the formation of an oriented n-complex type activated state, suggested for nitronium silt nitrations previously +
Introduction Our present knowledge of the nature of electrophilic aromatic nitration goes back to Euler2 who first suggested NO*+as the active nitrating agent. H a n t ~ s c h , n~ ' a l d e r ~ , ~Ri and Eyring,j Ingold, Lapworth and co-workers,E Price' and Chedin,8 added to the theory of Iv02+nitration, b u t i t was not until 194G t h a t N 0 2 T was finally established as the active nitrating agent. Kinetic and spectroscopic evidence obtained from the work of Bennett, Brand and M7illiams, \Vestheher and KharaschlO and in particular by Ingold, Hughes and their co-workers" were the major contributions to the mechanism of aromatic nitrations. Since much of this work is widely known and has been repeatedly reviewed, no further review seems to be necessary. (1) ( a ) P a r t X I I , J . A m . C h e m . Soc., 84, 3684 (1902); Chemical Physics Research Laboratory, T h e Dow Chemical Co., A4idland, Xlich. (2) H Euler, A n i z . , 330, 280 (1903). ( 3 ) A . Hantzsch, Be?., 58, 911 (192.5). (4) P. XValden, Angew. Chern., 37, 390 (1924). (6) T. R i a n d E. Eyring, J . C h e m . P h y s . , 8 , 433 (19tO). (6) C . K . Ingold, A. Lagworth, E . Rothstein and I ) . W a r d , J . C h e m . soc., 1 n i 9 (1!)31j, ( 7 ) e. C . Price, Cizem. R e v s . , 29, 2 1 ( 1 9 4 1 ) . ( 8 ) J Chedin, C u i ~ i p l r e m l . , 2 0 0 , 1:;97 (193:). ('2) ( > RI. 13ennet1, J C . U . Brand a n d J . \i'illiams, J A m . Chein. j i i c , 68, 809 (194(i). (10) F. H Xyestheimer and R I . S. Kharasch, i b i d . , 68, 1871 (1916). (11) C . K . I n g o l d , 1,:. U. H u g h e s , e l ai., S u t u r e , 168, 448 (1946); J. Chem. S O L . 2400 , (IXO).
Results and Discussion Nitronium salts have become easily available and have been developed as preparative nitrating agents.'? The use of nitronium salts in tetramethylene sulfoneI3 and nitromethaneb solution has allowed investigation of the kinetics and mechanism of the nitration of aromatics (benzene, alkylbenzene, halobenzenes) . All of these nitrations are very fast, b u t by employing Ingold's competitive nitration technique, i t was possible to compare relative reactivity of aromatics. The relative rates show first-order dependence on the aromatic substrates and thus permit a comparison of relative reactivities under competitive conditions. Nitronium salts in organic solutions with reactive aromatic substrates show small substrate b u t high positional selectivity. The relative reactivities correspond t o known n-complex b u t not to cr-complex stabilities of the substrates. I t was of some interest t o try t o compare these results with classical nitric acid nitrations, in which nitronium ion has been suggested to be the active nitrating species. Therefore an attempt was made to compare nitronium salt nitraticns with nitric acid nitrations, carried out in organic media (acetic anhydride, acetic acid, nitroniethane, tetramethyl( 1 2 ) G . Olah, S K u h n a n d A. l f l i n k o , i b i i . , 4237 (1!13iI, S J . K u h n a n d G. A. olah, J . A m . Chern. s o c . , 83, 4 5 l i l ( I U l j l ) (18) G . A Olah, S. J . K u h n and S. H. I?lood, i b i d . , 8 3 , 4 5 7 1 (19iil). (14) G. A . Olah, S. J. K u h n a n d S.H. Flood, ibrd., 83, %381(1961).
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ene sulfone) and with mixed acid nitrations in the same solvent systems. Nitration with Nitric Acid in Organic Solvents.--IYith nitric acid (used both as solvent and as nitrating agent) Ingold has found t h a t nitrations are first order with respect to aromatic compounds. Allthough,according to Kaman spectroscopic observations, :: anhydrous nitric acid contains approximately lY0 of nitronium ion, solutions of nitric acid in organic solvents such as nitromethane contain no detectable amount of nitronium ion. Ingold, Hughes and co-workers have found t h a t sufficiently reactive aromatic compounds (benzene, toluene, ethylbenzene, etc.) exhibit, in nitrations in nitromethane solutions, zeroth order kinetics, ;.e., the reaction rates are independent of the nature or concentration of the substrate indicating t h a t the aromatic compound takes no part in the rate-determining step. This step must therefore be limited to the nitric acid (with the possible assistance of the solvent) and was found to be the formation of X02+. The necessary- condition in the formation step of the nitronium ion is that the rate of the recombination of nitronium ion with water should be much slower than the rate of reaction of the nitronium ion with the aromatic compound. In nitromethane solution there will be a t the start of the reaction a minute concentration of water arising from the ionic self-dehydration of the nitric acid. During the reaction this will be increased, b u t the water concentration will always be much less than t h a t in the partly aqueous solution. In nitromethane solution the zeroth order rate was observed by Ingold and Hughes in almost all cases involving compounds with reactivities equal to or greater than t h a t of benzene. The addition of small quantities of water to nitromethane has little effect on the zeroth order rate. This is expected, since water does not enter into the pre-equilibrium step and therefore does not reduce the concentration of the nitracidium ion. IThen more water is added, a point is reached where the water competes with the aromatic compound for the limited supply of the nitronium ion. The kinetics then change to a first-order form. 16 The relative reactivity of toluene and benzene in competitive nitration with nitric acid in nitromethane was found by Ingold" to be 21, using a dilatometric technique. Employing gasliquid chromatography as an analytical method, the nitric acid nitration of benzene. toluene, ethylbenzene, $-xylene and mesitylene were cornpared in competitive experiments (Table I ) . The results are in excellent agreement with Ingold's data for the toluene : benzene reactivity ratio and also establish high substrate selectivity and the usual isomer distributions in the nitration of the other alkylbenzenes. Nitric acid nitration of toluene and benzene in acetic anhydride and acetic acid solutions was investigated by Ingold and Hughes during their sTTstematic study of nitrations." Knowles. Korman and RaddaI6 redetermined relative rates (I.?) E . 1 3 . Hughes, C K . Ingold and R R. Pearsun, .I C I i ~ ? i i.So(., . 4957 (1958). ( I t ! ) J . R Knowlep. R 0 C . S o r m a n and C,. K . RadBa, J . Chriii Sac., 48% . Cook, S J . Kuhn and C . A . Olah, ibid.. 3 3 , 1669 (10G0). t25) R. I. Gillespie, E . I). Hughes and C. K . Ingold, J . Chem. Soc., considerably increased substrate selectivity over 2473 (1950). that obtained with NO2+ salts. On the basis of (20) R. I,. Burwell, Jr., and C . H. Langford, J . A m . Chem. Soc., 81, 3799 (1959).
(27) P. W. Foster, unpublished results, personal communication.
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the experimental data it is suggested that in these nitrations a weaker nitrating species than NO2+ must be involved in the primary interaction with the aromatic substrates. This incipient nitronium ion then attaches itself to the aromatics in a step giving high substrate selectivity. U'hether the incipient nitronium ion is the nitracidium ion (HaNO:jT), protonated acetyl nitrate (CH:,COOHNO%+)or probably a transition state of any of those unstable species to KO%+, in which water is loosened, but not yet completely eliminated, is difficult t o say and no direct physical evidence is available. Nitric acid nitrations in nitromethane, acetic acid, acetic anhydride (in the last of course acetyl nitrate is formed and must be considered the starting material for the formation of XO2+ or its precursor), or in tetramethylene sulfone solution all must be considered specific nitration systems not containing detectable amounts of NO%+.Nitric acid in sulfuric acid (in which it is completely ionized), or mixed acid in concentrated organic solutions (containing Ihu'O2+) indeed nitrates as NO?+, which is solvated by the acid to a certain degree. Dilute organic solutions of mixed acid (not containing detectable amounts of NO2+) behave similarly t o nitric acid nitrations. If we assume that an activated state of the oriented n-complex type is formed irreversibly in nitrations with nitronium salts, one must explain from where forces resisting its formation to that extent come. The NOr+ ion, even in nitrations with nitronium salts, is not present (at least in the organic solvents investigated) as separated ion. Cryoscopic investigations (involving solutions of about 3 weight per cent. concentration in tetramethylene sulfone, in contrast t o more concentrated, about 7 weight per cent. solutions used in relative rate determinations) suggest t h a t ion separation, if any. is very limited in tetramethylene sulfone solution of N02+BF4- (
