Nov. 30, 1961
hTITRONIUM
[CONTRIBUTION No. 44
FROM THE
TETRAFLUOROBORATE NITRATION
GS1
EXPLORATORY RESEARCH LABORATORY, Dow CHEMICAL OF CANADA,LTD., SARNIA. ONTARIO, CAKADA]
Aromatic Substitution, E. Nitronium Tetrafluoroborate Nitration of Halobenzenes in Tetramethylene Sulfone Solution BY GEORGEA. OLAH,STEPHEN J. KUHNAND SYLVIA H. FLOOD RECEIVED JULY 1, 1961 Identical reaction conditions as described previously for alkylbenzene nitrations were used in the investigation of the nitronium tetrafluoroborate nitration of halobenzenes in tetramethylene sulfone solution. Relative reactivities compared with benzene and isomer distributions were determined using a gas chromatographic method. The relative rates of nitrations of halobenzenes show fair agreement with complex stabilities with *-complex forming agents. A secondary isotope effect, similar t o t h a t observed in nitrations of benzene-& and toluene-& was observed in nitration of 4-D-fluorobenzene. The mechanism of the reaction is discussed in view of the experimental results.
The analysis of mixtures of known composition which were dissolved in identical amounts of solvents and treated similarly as in the case of competitive nitrations, showed that the ratio of the peak areas obtained in gas-liquid chromatography was proportional, within 37,, to the ratio of the quantities of the materials. Peak areas, as obtained by Results the use of the electronic integrator, were found in Competitive nitrations of benzene and haloben- good agreement with those measurred as a control zenes were carried out (as described in detail pre- for a number of experiments by constructing triviously) by treating equimolar quantities of the angles made up of tangents to the Gaussian curves aromatics in tetramethylene sulfone solutions, kept and the intercepts on the base line or by weighing a t a constant temperature bath of 25 f 0.5'. A the cut-out peaks and determining the areas from solution of nitronium tetrafluoroborate in tetra- 'the weights. methylene sulfone was dropped into the wellThus, the gas-liquid chromatographic method stirred solution of the substrates. The mole ratio of used was found reliable to the extent required and the combined aromatic substrate : nitronium tetra- it was also shown that no significant losses of prodfluoroborate was 10:1, this a practically constant ucts take place during the preparation of the excess of the aromatics was maintained. Under the reaction mixtures for analysis (water washings). experimental conditions used, a quantitative monoI n a set of alternate experiments, water washing nitration takes place. N o traces of dinitro products of the nitration mixtures for removal of the acid or any other by-product was detectable in the reac- formed and simultaneously of the major part of the tion products either by gas chromatographic or solvent, tetramethylene sulfone, was omitted. Inspectroscopic methods. The mixtures were ana- stead the nitration mixtures were neutralized with lyzed by gas chromatography. From the areas of anhydrous ammonia, precipitated ammonium fluothe individual peaks (obtained by the use of an borate was filtered off and the reaction solutions electronic printing irtegrator), mol yo figures were were directly analyzed by gas-liquid chromatogcalculated for each product after first determining raphy. Comparison of data obtained by this relative response data following the method of method with those obtained by washing out the Messner, Rosie and Argabright. reaction mixture with water to remove acid showed The observed reactivities of the halobenzenes rel- no significant differences. ative to that of benzene, together with the isomer The method of competitive reaction rate deterdistributions of the mononitro products, are sum- mination can be applied only if the observed relamarized in Table I. (All data reported represent tive rates are dependent on the aromatic substrate. the average of a t least three parallel experiments.) Changing the concentration of either of the aromatic components in competitive experiments in TABLE I tetramethylene sulfone solution from the 1: 1 ratio COMPETITIVE NITRATIOX OF HALOBENZEXES AND BENZENE to 3 : 1 and 1: 3 showed that the relative rate ratio WITH NO?BF4 IS TETRAMETHYLENE SULFONE SOLUTION AT remains almost unchanged if a first-order depend25O ence on the aromatic concentration is accepted Relative reactivity -Isomer distribution, %--(Table 11). Aromatic k.kr/kbenrene Ortho .Veta Para Increasing the speed and efficiency of stirring Benzene 1.0 had almost no effect on the relative reactivities obFluorobenzene 0.45 8.5 .. 91.5 served. Changing the temperature of the reactions Chlorobenzene .14 22.1 0.7 76.6 had only a slight effect in accorance with a regular Bromobenzene .12 25.7 1.1 73.2 dependence of the relative rates on ternperaIodobenzene .28 36.3 .. 63.7 ture. Not detectable due to peak, interference on chromaIn tetramethylene sulfone solution, the relative tograph. reactivity change of fluorobenzene/benzene was (1) Part VIIJ, G .A . Olah, S. J. K u h n a n d S. H. Flood, J .A m . Chew.. observed between +10 and + 5 5 O (the relatively SOL, 83, 4571 (1961). high freezing point of the solvent prohibited deter(2) A. E. Messner, D. M. Rosie and P. A. Argabright, Anal. Chcm., Sl [Z],230 (1959). minations a t lower temperatures).
Introduction
I n the previous paper of this series1 the nitronium tetrafluoroborate nitration of alkylbenzenes in tetramethylene sulfone solution was discussed. This work was now extended to similar investigation of the nitration of halobenzenes.
GEORGEA
4382
OLAH,
STEPHEN J. K U A N
.4ND
SYLVIAH. FLOOD
VOl.
s3
alkylbenzenes. Therefore, no empirical sequence of relative sigma basicities is available. Recently, however, i t was observed3that H F SbF6 forms a BEKZENE IS COMPETITIVE S~T.P.~TIOSS fairly stable benzenonium complex with fluoroR a t i o of Ohsd. relative Huorobenzene: benzene rate ~ C E H ~ F / ~ C ~ H , benzene. 3 1 1 .x? 0,44 The question of relative basicities of halobenzenes 2 1 0.88 ,-I1 is more complex than that of the alkylbenzenes. 1 1 .45 ,45 Halobenzenes have two different donor centers. 0 1 .21 . -1-8 They can act as T-donors involving the ring a1 3 .14 ,42 sextet and as n-donors involving the unshared Average 0 . 1,i halogen pairs. Consequently, complex stabilities could be dependent on two different types of comTABLE 111 plex formation. As an example, we can take the TEMPERATCRE DEPENDENCE OP COMPETITIVE \rITR.4TIOV O F iodobenzene Ag+ system in which n- and not aFLUOROBENZENE A S D BEXZESE complexing could account for the high relative staTemp., kfluorobenSeno/ --Isomer distribution. %-bility observed. OC. kboneene Ortho Mela Para Brown and co-workers4 measured the solubilities 10 0.41 6.4 .. 93.6 of HC1 in halobenzenes by vapor pressure measure15 .43 7.8 .. 92.2 ments and found the solubilities. thus the correlated 20 .44 8.4 .. 91.6 basicity, to follow the order of C6H6 > CGHSF> 25 .45 8.5 .. 91.5 CF~H~CI > C&Br > CsH51. .48 8.6 .. 91.4 30 \Vatanabe6 found the ionization potentials of 35 .50 8.5 .. 91.5 halobenzenes to be in the order of fluorobenzene 9.19 40 .51 8.7 .. 91.3 chlorobenzene 9.07, bromobenzene 8.98 and iodo45 .52 9.1 .. 90.9 benzene 8.73 e.V. 50 .53 9.6 .. 90.4 As it is generally accepted that the lowering of the The temperature dependence of the reaction in- ionization potential means increased basicity of dicates that with increasing temperature, the selec- an arornatic, the relative order of basicity is C6H6T > tivity decreases. C6H5Rr > C&C1 > C6H6F. Addition of water up to an equimolar amount of It should, however, be kept in mind that the the combined aromatic substrate had no effect on ionization potential of benzene is 9.245 e.V., thus higher than that of all halobenzenes. Assuming the the observed relative reactivities. generally accepted --I > T effect of halogens in Discussion of Results halobenzenes, no simple correlation of relative basiciThe observed relative reactivities of haloben - ties of halobenzenes and benzene is possible based zenes and benzene show good agreement with rela- on ionization potentials. tive stabilities of complexes of halobenzenes with A comparison of the shift of the infrared fundaAg+, Br2, 1 2 , IC1, SOz, picric acid, HC1 and H F , mental H-X stretching frequency of a number of which are considered as T-complex-forming agents. proton donors such as HCl, HBr, HI, C6HsOH, Table I V shows a comparison of the relative CH,OH, n-C4H90Hand pyrrole, was reported by rates of NOz+BF4- nitration in tetramethylene Josien and Sourisseaue for chlorobenzene and brosulfone solutions with relative stabilities of com- mobenzene. In order to get information on all the plexes of halobenzenes. Differences may partly be halobenzenes and thus establish a relative order of due to varying steric hindrance. basicity similar to Cook’s’ method of determining the relative basicities of the alkylbenzenes, these TABLE IV investigations were extended by us to include COMPARISON OF RELATIVE STABILITIES OF CONPLEXES OF fluorobenzene and iodobenzene. We also included HALOBENZENES A N D NITRATION RATESWITH NO2 +BF4phenylacetylene and methanol4 as acids. Table V Aromatic Ag+ Brr 11 IC1 SOa HC1 N02” summarizes the data obtained using a Perkin Elmer, Benzene 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Model 221-GI grating spectrophotometer with soFluorobenzene 0.18 .. .. .. . . 0 . 7 4 0 45 0.86 0.50 .. 0.58 .50 .14 Chlorobenzene ,29 dium chloride optics. I n all the cases investigated, Bromobenzenb .40 1.13 0.86 0.59 .. .41 .12 the spectroscopically observed shifts followed the de2.08 1.53 1.5 .. .. .40 .2S Iodobenzene Reference b c c d * f creasing order C&f6 > C G H ~>I C6H6Br > C6H6C1 > a L. J. Andrews and R. M. Keefer, J . Am. C h m . SOC., CsHsF. 72, 3113 (19.50). b R. M. Keefer and L. J. Andrews, Kinetic Isotope Effect.-As in the case of the bid., 72, 4677 (1950). L. J . A n d r e w and R. hl. Keefer, nitronium tetrafluoroborate nitration of benzene-& bid., 74, 4500 (1952). d L. J . Andrews and R. M. Keefer, bid., 73, 4169 (1951). e H. C. Brown and J. D. Brady, and toluene-&, also in the case of the nitration of 4-D-fluorobenzene, a small secondary reverse isotope bid., 71,3573(1949); f Present work. effect was observed. The Question of Relative Basicity of HalobenCompetitive nitration of benzene and 4-D-fluorozenes.-Contrary to the well investigated relative benzene, compared with that of benzene and fluorobasicities of alkylbenzenes, there are only few data (3) Unpublished results of G. A. Olah a n d S. W.TOlgYeSi. available on relative a-complex basicities of halo(4) H . C. Brown and J . D. Brady, J. Am. Chem. Soc., 74, 3570 benzenes and no data a t all on &complex basicities. (1952). ( 5 ) K. Watanabe, J. Chem. Pkys., 2 6 , 542 (1957). Halobenzenes do not form a-complexes with H F (6) hl. L. Josien a n d G . Sourisseau, “Hydrogen Bonding,” Ed. L). BF3 (or AlCla HC1 or AlBra HBr) under condi- Hadzi, Pergamon Press, New York, N. Y.,1959. p. 135 tions similar to that of a-complex formation of (7) D. Cook, J. Chem. P k y r . , 26, 7 8 s (1366). TABLE I1
+
CONCENTRATION ~TARIATION OF FLUOROnESZEXE A S D
i
+
+
(1
+
+
+
NITRONIUM TETRAFLUOROBORATE NITRATION
Nov. 20, 1961
4593
TABLE V SHIFTOF INFRARED FUNDAMENTAL H-X STRETCHING FREQUENCIES OF PROTON DONORS I?; HALOBENZEYES Donors
Gas, cm
CCl,,cm
HBr
HCI -1
-1
2886 f 1 2831 f 1
HI
CsHsOH
CHaOH
CHsOD
n-CaHoOH
Pyrrole
P h e n \ Iacetylene
2558 f 1 2230 f 1 3654 f 1 3687 f 2 2720 i 2" 3680 zk 2 3.530 f 2 33t0 i 2 2518 f 1 2204 f 1 3610 f 2 3645 f 2 2689 i 2b 3640 3t 2 3498 =t2 3313 i 2
Acceptors
CsH6F 2810 f 2 2518 f 2 2190 f 2 3606 f 2 3643 f 2 2 6 8 7 f 2 3630 f 2 3488 zk 2 3 3 0 i f 2 2779 f 2 2481 f 3 2182 f 3 3578 zk 2 3624 f 3 2677 f zb 3616 f 3 3475 k 2 3304 f 2 C6H6C1 2769 f 2 2476 f 3 2176 f 3 3568 f 3 3618 f 3 2669 f 2 3610 f 3 3470 =k 2 3302 f 2 CgHLBr 2761 f 2d 2468 f 2 2167 f 2 3559 f 2 3610 f 2 2667 f 2h 3609 f 2 3467 i 2 3300 f 2 C~HJ 2756 f 2d 2452 f 2 2164 f 3 3657 f 2 3607 f 2 2660 f 2c 3606 f 3 3460 3z 2 3290 f 2 CeHe Data for chlorobenzene and bromobenzene (with the exception of those for phenylacetylene and CHpOD) are from 51.L. Josien and G. S o u r i e s ~ e a u , ~Those for fluoro- and iodobenzene are from the present investigation. a E. F. Barker and W. Gordy J . Chem. G Bosschieter J . Chem. Phys., 6 , 563 (1938). b M. Tamres J . Am. Chem. Sac., 74, 3375 (1952) Corrected value Phys., 7 , 9 3 (1939); L. H. Jones and R. M. Badger J . Am. Chem. SOL,73,3132 (1951).
benzene, gave the following results (analyzed by gas-liquid chromatography). k ~ ~ ~ ~ r / k5 c s0.45 n ~ kr-D-CsHdF/kC,He
kD/kn
= 1.14 f 0.03
0.51
As p-substitution in nitration of fluorobenzene takes place to an extent of 91.5%, the observed kinetic isotope effect should be corrected accordingly and is slightly larger ( k ~k~, = 1.22). The previously established kinetic isotope effect for the nitration of C6De1 was again observed in competitive nitration of C6D6 with fluorobenzene kcaH,F/kcpEs
= 0.45
kcsA.F/kcsD.
= 0.39
kDin
= 1.16 f 0.03
The reason for this must be not only the lower T electson density of the ring in halobeiizenes, but the absence of alkyl groups capable fo stabilizing through conjugation (in tlie 0- and p-positions) benzenonium ion formation. The recent observation on the preparation of a stable benzenonium complex from fluorobenzene HF SbF63
+
+
Ii H
is in agreement with the fact that the fluorine substituent can effect, through conjugation, a considerable stabilization of the benzenoniurn ion in the $position. N .m.r. investigation of the complex in liquid SOz solution showed, that protonation had taken place nearly exclusively in the 4-position. Known n-complex stability data of halobenzenes, as shown in Table IV, give a fair correlation with relative rates of nitration of halobenzenes. These data, however, relate always to the over-all donoracceptor complex stabilities with different acceptors and could also involve a considerable amount of ncomplexing, instead of ring n-complexing. Therefore, it is easily understandable why the observed relative nitration rates (involving always the ring x-electron system) are lower (first of all in the case of bromo- and iodobenzene) than relative complex stabilities, say with Agf. The anomalous behavior of iodobenzene in complex formations with silver salts shon-ing considerably higher stabilities than the other halobenzenes was discussed previously by Andrews and Keefer.l5 They gave a reasonable explanation for the anomalous behavior by suggesting that in the complex Conclusions formation the silver ion bonds to the iodine atom Correlation of the observed relative reactivities (and not the aromatic n-sextet) Ar-I -+ Ag+. The of halobenzenes, obtained in homogeneous nitra- existence of such complexes is not surprising since tions with nitronium tetrafluoroborate in tetra- structurally they are similar to the known diphenylmethylene sulfone solution, with relative stabilities iodonium salts and iodobenzene dichloride. of certain n-complexes of the halobenzenes gave a The stabilities of the n-complexes do not change fair agreement. No data are available on a-complex greatly with the nature of the halogen substituents BFa). (see data of Table IV). It is obvious that the absoforniing ability of halobenzenes ( H F It seems that under conditions where the alkyl- lute value of the density of tlie n-electron donor benzenes give stable benzenonium ions, no similar system could not change drastically by inductive complex formation takes place with halobenzenes. and conjugative effects of one halogen atom. Thus i t can be explained that the relative reactivities (8) H. S. Klein and A. Streitwieser, Jr , Chcmirfry 6' Ziidrcstry, 180 (1961). observed iri N02'BF4- nitrations in the present The explanation for the observed small isgtope effect for the nitration of 4-D-C&F most probably is that in the transition state, the initial sp2 carbon hydrogen bond must have been changed a t least somewhat in the direction toward sp3. Owing to the low frequency of the out-of-plane bending mode of aromatic carbon-hydrogen bonds, it could be expected that this would increase the zero-point energy and, consequently, cause a secondary aisotope effect with the heavy molecule reacting faster. The opposed hyperconjugative effect of the carbon-hydrogen bond removed somewhat from the ring plane with the p-orbitals of the other five carbon atoms, will tend to decrease the zeropoint energy but will not cancel it completely. Streitwieser recently demonstrateds that aromatic deuterium is electropositive relative to protium in the normal state. I t is effectively electron donating relative to hydrogen, presumably because of anharmonicity effects. This finding is in agreement with the observed secondary kinetic isotope effect in our investigated nitrations.
+
GEORGEX.OLXII, STEPHEN J. KUHNAND SYLVIA H. FLOOD
4384
Vol. s3
TABLE VI ISOMER DISTRIBUTION IN Halobenzene
Sitrating agent
Solvent
THE
MONONITRATIOS OF HALOBEXZESES
Temp., ‘C.
OYlho
.Ue/a
Para
Ovlho: p a r a ratio
Anal>tical rnethode
Keference
12.4 .. 87.6 0.14 F.p. b 8.7 .. 91.3 .095 G.L.C. c TMS 8.5 .. 91.5 ,087 G.L.C. Chlorobenzene “ 0 3 HuSO~ 30.1 .. 69.9 .13 F.p. i Acetyl nitrate Nitromethane 29.6 0.9 69.5 Isotopic dilution KOzBF4 TMS 22.7 0.7 76.6 G.L.C. iL .. 62.1 H8Oa 18 37.6 Bromobetizerie HXOa F.p. 1 Acetyl nitrate Nitromethanc 25 36.5 1.2 62.4 Isotopic dilution c NOfBFd TMS 2.5 25.7 1.1 73.2 G.L.C. HzSOa 18 41.1 .. 58.7 Iodobenzene HSOa F.p. d Acetyl nitrate Sitrate methane 25 38.3 1.8 59.7 Isotopic dilution NOzBF, TMS 25 36.3 .. 63.7 G.L.C. a . 4.F. Holleman, “Die direkte Einfiihrung von Substituenten in den Benzolkern,” Leipzig, 1910, p. 175; A. l?. Holleinaii, Present Chem. Revs., 1, 187 (1925). * J. R.Knowles, R. 0. C. Norman and G. K. Radda, J . C h e w Suc., 4885 (1960). work. J. D. Roberts, J. K. Sanford, F. L. J. Sisma, H. Cerfontain and R.Zagt, J . Am. Chem Soc., 76, 4525 (1954). e TMS = tetrainethylene sulfone, G.L.C. = gas-liquid chromatography, F.P. = freezing point method. Fluorobenzerie
HXOa
H NO3 SOnBFa
HSO, Acetic anhydride
18 25 25 18 25 25
work show only small differences of the same order the present investigation) is possible without necesof magnitude for benzene and halobenzenes. (The sarily involving a change of the isomer distribution halobenzenes take part with their T-sextet as enti- in the direction of statistical distribution (demonties in the rate-determining a-complex-forming strated by an increase of the m-isomer). Our competitive nitrations showing low selecstep.) This order of selectivity of the aromatic substrate is different from reactions where relative tivity of the halobenzenes gave isomer distributions stabilities of intermediate g-complexes in indi- of the mononitro products showing no significant vidual positions are involved. These, of course, amount of the m-isomers Isomer distributions reported for the mononitrafrequently show relative reactivities of orders of magnitude smaller than that of an individual ben- tion of halobenzenes are compared with the results of the present work in Table VI. zene position. Concerning the directing effect in the nitration of It is difficult a t the present time to give a “general” order of basicity of halobenzenes based on the halobenzenes, the nature of the a-complexes inavailable data. In present investigations inter- volved in the rate-determining interaction must be action with NOl’ as acceptor acid was established. considered. It is necessary to be aware of the fact Qnly an informative correlation with relative that whatever the nature of the primary interaction basicities observed in one system should not neces- between a strong electrophile (NOi+) and the arosarily mean a similar relative order against another matic substrate should be, in the case of substituted acceptor] with considerably changed steric and benzene (e.g., aromatics showing a non-uniform T other properties. Any oversimplification of the electron distribution) the outer complex must be problem, particularly in view of the two distinctive an oriented one. The electron distribution of the donor systems of halobenzenes ( T - and n-donor), halobenzene may give an explanation for the nature of the oriented T-complex formation and subsequent seems to us without proper foundation. I effect The next question to be answered is how the lorn directing effect in nitration. The -1 > substrate selectivity, corresponding to relative a- of the halogen atoms results in the region of the ocomplex stabilities, compares with the observed positions generally to some degree of neutralization of the two opposed effects. As the inductive isomer distributions. The greater the reactivity of an electrophilic effect diminishes with the distance in the p-posisubstituting agent, the smaller its s e l e c t i ~ i t y . ~tion, the conjugate effect becomes predominant. lo This means low selectivity with different aromatics This is best shown by comparing the ortholpara and also a simultaneous change of the isomer dis- ratios of nitrohalobenzenes obtained, which gives a tribution toward the statistical value (407, ortho, sequence quite obviously opposed i o the steric k3o/c meta and 20YGpara, representing two 0-, two ortho effect. m- and one 9-position in a monosubstituted benOilho ’ p a u n zene). This generally is demonstrated by an inFluorobenzeric 0.09 crease in the concentration of the m-isomer. The .30 Chlorobenzene obvious explanation is the decreasing role of small .35 Bromobenzene activation energy differences of different individual .57 Iodobenzene positions compared to the over-all activation energy of the reaction. This, again, is valid only if the The over-all effect can be further increased by reaction is dependent on relative stability of m- polarization by the strong electrophile, NO?+. complexes but izot if competition of individual The lower o-isomer ratios obtained in nitrations molecules is involved (according to relative T - of chloro-, bromo- and iodobenzenes with nitrocomplex stability). In this case, a low selectivity nium tetrafluoro-borate in tetramethylene sulfone with different aromatic substrates (halobenzenes in
+
(9) M. J. S. Dewar, A w n . R p p t
, 63, 132 (195G).
(10) For a more detailed discussion in the case of nuorobenzene, see G. Olah, A. Pavlath a n d G. Varsanyi, .i. C h v i n . Soc., 1823 (1957).
Nov 20, 1961
4383
hTITRONIUM TETRAFLUOROBOKATE NITR.kTION
methylene sulfone, was added dropwise. The reaction mixture was vigorously stirred and the temperature was kept constant a t 25 f 0.5O during the reaction. The reaction time was generally 15 minutes. After t h a t , 150 ml. of ether was added, the mixture was washed with 300 ml. of water and the separated organic layer was washed again with 200 ml. of water and dried with CaC12. The resulting dry mixture was analyzed by gas-liquid chromatography. (.b) In Tetramethyl Sulfone Solution (Neutralization of Acid with NHa).-Benzene (0.25 mole) and 0.25 mole of the corresponding alkylbenzene were dissolved in 70 g. of tetramethylene sulfone. Into this solution, 0.05 mole of 1\102BFq dissolved in A0 nil. of tetramethylene sulfone, was added dropwise. The reaction mixture was vigorously stirred and the temperature kept constant a t 25 & 0.5" during the reaction. Xfter the addition of the nitronium salt was completed, the mixture was stirred for another 15 minutes. The solution was then neutralized with anhydrous ammonia gas, precipitated ammonium fluoroborate was filtered off and the solution analyzed directly by gas-liquid chromatography, Analytical Procedure.-Gas-liquid chromatography was carried out on a Perkin-Elmer model 154-C vapor fractometer, using a thermistor detector equipped with a PerkinElmer model 194 electronic printing integrator. A 4meter by 0.25" stainless steel column packed with polypropylene glycol (UCON LB 500-X) supported in diatomaceous earth was used. The column temperature was 180" for all mononitroalkylbenzene determinations; 60 ml. of hydrogen per minute was used for carrier gas. Samples of 100 p ml. were generally injected. The accuracy of the analytical method was checked by TABLE TI1 preparing and analyzing mixtures of the mononitrohaloRelative r a t e s of mono- Relative r a t e s of monobenzenes, halobenzenes and solvent of known composition nitrations with acetyl nitrations with h71z+BF~approximating those obtained upon nitration of the same nitrate" in acetic in tetramethylene alkylbenzene mixtures under our experimental conditions. Aromatic anhydride b y Ingold sulfone W e found the error never exceeded 5 3 7 , and was this large Benzene 1.00 1.00 only in the case of the minor constituents. An error of less. Fluorobenzene 0.15 0.45 was found for most constituents. than =t1y0 Chlorobenzene .033 .14 Observed retention times of mononitrohalobenzenes are' given in Table VIII. Bromobenzene ,030 .12 TABLE 1.111 Iodobenzene .18 .28 RETENTIONTIMESOF MONONITROHALOBESZENES AT In a forthcoming paper of this series a more deCOLGMN TEMPERATURE 180'
solution, as compared with values obtained in pre vious conventional nitrations, may be partly due to steric effects. NO*+, by dissolving the nitronium salt in tetramethylene sulfone, is complexed to some extent by the solvent and the main rate determining step, as pointed out previously,' is considered as (he transition of the XO2+-solvent complex to the NOz+-aromatic n-complex. The cornplexed nitronium salt, being bulkier, could effect a larger steric ortho effect in the case of halobenzenes having a sufficiently large halogen substituent. The question of steric effects and solvent participation will be discussed in more detail in a forthcoming publication. It is necessary to call attention t o the fact that in the case of halobenzene nitrations, the NOz+BFdnitrations (in tetramethylene sulfone) differ less from the classical nitration results of Ingold'l obtained with acetyl nitrate in acetic anhydride solution than in the case of the previously investigated alkylbenzenes. The differences are, however, significant as shown in Table VI1 and show again a considerably lower substrate selectivity without, however, any significant change in isomer distribution giving rise to a substantially increased m- isomer ratio.
tailed comparison of our data obtained in homogeneous stable nitronium salt nitrations with classical conventional nitrations will be given and some explanations will be forwarded to account for observed differences. Acknowledgments.-Dr. \V.S.Tolgyesi prepared the 4-D-fluorobenzene used in the present investigation. \%reare grateful to Dr. D. Cook and Miss D. Anderson for the infrared spectra and Dr. E. B. Baker (Physical Research Laboratory, The Dow Chemical Co., Midland, Mich.) for the n.m.r. investigations. Experimental As nitronium tetrafluoroborate is very hygroscopic, all operations involving the nitronium salt preparation must be carried out with usual precautions to avoid moisture. Materials.-Benzene, halobenzenes and nitrohalobenzenes were commercial materials of highest available purity. They were purified by fractionation in a laboratory column rated a t 50 theoretical plates or by repeated crystallization to constant 1n.p. Their purity was checked by gas-liquid chromatography. NOziBF4- was prepared as described previously.12 Tetramethylene sulfone was a commercial product (Shell Development Co., Emeryville, Calif .). It was purified by being twice vacuum fractionated. It give a specific conductivity of 1.35 X 10-7 mho/cni. General Procedure for Competitive Nitration. (a) In Tetramethylene Sulfone Solution (Water Washing) .Benzene (0.25 mole) and 0.25 mole of halobenzene were dissolved in 70 g. of tetramethylene sulfone. Into this solution, 0.05 mole of XO;BFI, dissolved in 60 g. of tetra(11) M . L. Bird a n d C K . Ingold, J . Chem. Soc., 918 (1938). (12) S. J . K u h n a n d G . A . Olah, J . A m . Che-1. Soc.. 83, 4504 (19Gl).
Compound
Sitrobenzene o-Kitrofluorobenzene p-Nitrofluorobenzene o-Xtrochlorobenzene m-Xitrochlorobenzene p-Sitrochlorobenzene
Retention time, min.
16 1; 15
36 30 32
Compound
o-Nitrobromobenzeiie nz-Sitrobromobenzene p-Sitrobromobenzene o-Ktroiodobenzene p-Sitroiodobenzene
Retention time, min.
59 52 54 i0
65
Determination of Kinetic Isotope Effects. ( a ) Competitive Nitration of CsD6:C6HjF.-Fluorobenzeiie (0.05 mole) and 0.05 mole of benzene-& were dissolved in 35 g. of tetramethylene sulfone and into this solution, 0.01 mole of SO?BFI in 15 g. of tetramethylene sulfone was added dropwise with constant stirring. The temperature was kept a t 25 f 0.5' during the whole reaction (15 minutes); 50 ml of ether was then added and the reaction mixture was washed with 50 ml. of water. The separated organic layer was again washed with 30 ml of water, then dried with CaC12 and analyzed by gas-liquid chromatography. ( b ) Competitive Nitration of C6H6:p-D-C~H1F.--Benzene (0.05 mole) and 0.05 mole of p-D-C6HpF prepared according to Hall, Piccolini and Roberts,13 were dissolved in 35 g. of tetramethylene sulfone and into this solution 0.01 mole of h'OnBFa in 15 g of tetramethylene sulfone was added dropwise. The mixture was stirred and the temperature was kept a; 25 =t0 . 5 O . On completion of the reaction (15 minutes), 00 i d . ether was added and the reaction niisture was washed with 50 i d . of watcr. The separated organic layer was washed again with 30 nil. of water. -4fter drying with CaC12, the mixture was analyzed by gas-liquid chromatography. (13) C . E. Hall, R. Piccolini and J. D Roberts, h d . , 77, 4540 (1955).
