3683
GEORGE
A.OLAH A N D
Reaction of K'TCNEwith Acid.-K+TCKE(1.00 g., 5.98 mmoles) was added to 10 ml. of ice-cold 6 h'hydrochloric acid. The suspension was stirred 1 minute, and the x-hite precipitate which formed was collected on a filter and dried. The product (0.63 g., 64% yield) was shown to be a 1: 1 mixture of T C N E and tetracyanoethane by comparison of its infrared spectrum with t h a t of an equimolar mixture of authentic T C S E and tetracyanoethane. Stability of K+TCNEy to Heat.--,l sample of K + T C S E was heated in an atmosphere of nitrogen a t 150" for 3 hours. The infrared spectrum of the product showed it t o be substantially unchanged. Another sample was heated a t 200" for 3 hours. The infrared spectrum of this product
[CONTRIBCTIOS
STEPHEN
J. KUHN
indicated gross decomposition. T o test the stability of solid K'TCSEto atmospheric oxygen and water vapor, a crystalline sample was allowed to stand in a n open beaker. Periodic examination of the crystals by infrared spectroscopy showed no apprecmble decomposition in 1 month
Acknowledgments.-The authors wish to thank Dr. T. L. Cairns for many helpful discussions. In addition, we wish to thank Dr. P. E. Bierstedt for determination of the electrical resistivities and 1Tiss Lucille E. U7illiams for determination of the halfwave potentials.
s o , 62, EXPLORATORY RESEARCH LABORATORY DO\\- CHESIICAI. OF
Aromatic Substitution
Vol. 84
CANADA,
LTD.,S A R N I A ,
OSTARIO, C A N . ]
X K 1 Steric Effects in Nitronium Salt Nitrations of Alkylbenzenes and Halobenzenes
BY GEORGEA . OLAHAND STEPHEN J. Kurm RECEIVED J A N V A R Y 2, 1962 The observed differences in isomer distributions and relative reactivities in the nitrations of alkylbenzeties and halohen zenes with a number of stable nitronium salts such as XOn+BFI-, NO.+ClO1-, NO-!+PFe-, NOy+XsFfi- atid SO:!+HSzO; in tetramethylene sulfone and nitromethane solution are discussed. I t is suggested t h a t these differences are due tc~ steric effects caused by solvent participation and ion pair interactions.
Introduction The nitronium tetrafluoroborate (IY02'BF4- ) nitration of alkylbenzenes? and halobenzenesY has been discussed previously. It was pointed out t h a t solvent participation and ion pair interaction play a certain role in the isomer distributions and relative reactivities of the reactions. I n the present paper this question is further discussed on the basis of extended experimental work. Results Competitive nitration of benzene with alkylbenzenes and with halobenzenes was carried out with different stable nitronium salts. The nitronium salts used were T\'02+BF4-, N021C104-, XOz+PFs-, N02+AsF6- and r\702+HS20i-. The use of these stable nitronium salts in nitrations was discussed previously4 together with details of their preparation and handling. Two solvent systems were used for the kinetic nitrations : tetramethylene sulfone and nitromethane. The solubility of NO?+BF4- in tetramethylene sulfone a t 25' is about i%.4 The solubility in nitromethane is, however, very low (less than 0.2%,), and nitrations with XOz+BF4- in nitromethane solution could be carried out only in very high dilutions with considerable preparative difficulties. N02'PF6- and X02+AsF6- are quite soluble both in tetramethylene sulfone and nitromethane. The nitromethane solutions can reach concentrations of more than 50% (by weight). The solubilities of nitronium perchlorate and hydrogen disulfate are again negligible in nitromethane, and do not allow investigation of homogeneous nitrations. X02+SbF6- although it is quite soluble was not investigated in kinetic nitraXI,J . Ans. Chein SOL.,84, 1696 (1962) (2) G . A . Olah, S. J . K u h n a n d S. H. Flood, ibzd., 8 3 , 4571 (1961). (3) G. A. Olah, S. J. K u h n and S . H. Flood, ibid., 83, 4581 (1961). (4) S J . K u h n and G. A . Olah, ibid., 83, 4564 (1961). (1) Part
tions, due to secondary rcactioiis caused by SbF6 with the solvents. The method of competitive nitrations and the gas-liquid chromatographic analysis of the products was identical with t h a t previously The results of these competitive nitrations are summarized in Tables I and TI. Discussion of Results T h e differences in nitrations of alkylbenzenes and halobenzenes with different stable nitroniuni salts in tetramethylene sulfone and nitromethane solutions can best be discussed treating separately the observed differences on substrate-selectivity (c.g.> relative rates) and positional-selectivity (c.g., isomer distribution). Effects on Substrate-selectivity. (a) Tetramethylene Sulfone as Solvent.-- Only slight differences in relati\-e reactivity over benzene were observed. These are in accordance with possible steric interference depending on the size of the aiiion inlrolved. BF4-- and C10,- are isosteric and close similarities are expected. PFa- and -1sFe- are larger than the fluoroborate or perchlorate ions and this may explain the observed decrease in relative rates. This is most predominant in the case of the mesitylene :benzene ratio, where the steric interference indeed should be the largest. HS20i-, the largest of all anions investigated, shows also the lowest relative rates. I t is significant to compare the mesitylene :benzene ratio, which is given in Table 111 for the nitronium salts investigated. (b) Nitromethane a s Solvent.-Relative rates of competitive nitration of alkylbenzenes and benzene using nitromethane as solvent are generally lower then those observed in tetraniethylene sulfone solution. Again the most predominant differences can be observed by comparing the mesitylene : benzene ratios.
Oct. 3, 1962
36%
NITRONIUhl S a L T N I T K a T I O N S OF B E N Z E N E S
TABLE I COhlPElITIVE KITRATIOS OF
BESZESE,hLKYLBENZESES AND HALOBENZESES TYITH SULFOXE SOLUTION AT 25’
1 .oo 1 .BO L64
1.00 1 40 1 46
1, G
1. 5 2
1.18
1.96 2.71
1.98 3.25 0.46 0.20
1 60 1.60 0.54 0.21
0,45
.. 1.0
Y.3 23.8
0.11
,--
liel rate
Aromatic
Benzene Toluene o-Xylene
1.00 1.52 1 .60
iii-Syletie
1.71
p-XJ-leiie hIesit~-lerie Fluorobenzerie Clilorobetizeiie
1 8.i 1.65 0.39 0.12
9 1 . L5 id.2
11.7
..
-“3.7
..
oi,tho
I_-_-
Benzene Toluene E t h ) lbenzene t-Butylbenzene o-Xylene
Iiel. rate
para
1 .00
64.7
3.2
32.1
p-S,lctie Alesit! lene Fluorobeiizcne Chlorcibeiizene D ro11ioberi7cne
. ( I
1.72 0.90 .41 . 14
h’ITROSIUM SALTS I S SITROMETHANE
-----EOn Rel. --Isomer, rate ovtho
pura
1 9 333 6 1 385 13 5 7-13 3-SO?-o-xylene 4-KOn-o-xylene 2-hTO~-iiz-sylene 4-SO?-1iz-\ylctle
1.00 0.97
66.6
+&Fs
‘lo-----mela
para
2.1
31.3
.x1 .41
1.22
.i3 .j 0
.30
AT
3.25 2.71 1.60
TABLE
COAIPARISON AND
014’
C]OMPETITI\’E
k,r:kB
k,r :k~
S O ?“Al~FG1 - 0 2 -HSyOi-
I\: S I T R A T I O S OF hIESITYLENE
BESZESE IN XITROVETHANE SOLUTIOS A T 25’
25’
kar : kH
so.,
para
11
64 8 55.4 12 2 68 6Yc 31 14 6‘ 85 44(
‘rAB1-U 111 COMPARISOS OF CO3IPETITI\’E SITRATION 01: 1\IESITYLESL: ASD BESZENE IS TETRAXIETHYLENE SULFONE SOLUTIOS
KO2 T l O , 1 - 0 2 -B F;-PFs
.8
70.8
Yo-
Isomer, mela
ortho
7p-~02+pFb---------r Rei. -Isomer, %-----. rate ortho mela
--
iii-Syle1lc
s;,
. . ..
1.23
73.0
1.00 0.91 .76 .62 .74
14.2 29.2
1.00 1 48 1.10
65.5 2.6 31.9 71.8‘; 3-SO2-o-sylene 25.27, 4-SO?-o-sylene 12.1% 2-SO2-in-s>-lene 87. 9yi -I-SO?-wz-xyiene
27.0
--
Iiel rate
para
iiie!a
BESZEIUE, ALKYLBENZESES A S D HALOBENZESES \VITII SOLUTION AT 25’
Sop +BFA-----Isomer, Y--oriho iize:a
1.19
88.3 76.3
,---- _-N O 2-HSpOi------.
s02+AsFe--------Isomer. $%--
T.4BLE
COMPETITIVE r\-ITRATIOR. O F
Aromatic
liel. rate
para
1. 00 1. 6 7 1 .73
Benzene Toluene o-Xylene
p-Xylene ?*Iesitylene Fluorobenzene Clilorobenzeiic
Rel. rate
Iiel. rate
TETRAMETHYLESE
7 -
I
7-
Aromatic
-NO: +ClO~-----. -Isomer, Yo---ortho iiie!a
S I T R O S I L W SALTS IS
1.65 0.90
The mechanism of nitronium salt nitrations in tetramethylcne sulfone previously was in terms of a rate-determining activated state of the oriented r-coniplex type. Low substrate selectivities observed in nitromethane solution shorn- a similar pattern. Differences due to basicities of alkylberizenes must be compensated to a large degree, or even overshadowed by steric re-
XO?+BFAS O ?‘PFs-
1.22 0.41
SOn+.lsFs-
0.42
quirements because of higher association of the reagents (ion pair nature) in nitromethane, or more stable complexing with the solvent. S o differences in the spectra of nitronium salts in either tetramethylene sulfone or nitromethane could be demonstrated b y infrared and Raman spectroscopic investigation of the solutions, as compared with spectra of the crystalline complexes as mulls. KO shift of the 23SO cni.-l infrared or the
3666
GEORGEA. OLAHA N D STEPHEN J. KLEIN
1400 cm.-l liaman-band of X02+could be observed. However, no differences are expected, if the complexes are present in the solutions in the form of closely associated ion-pairs. T h a t the solvent must complex the nitroniurn salts to a certain degree was obvious from the solubility of these ionic salts in non-aqueous organic solvents to the observed degree. Also the considerable differences observed with different nitronium salts would be difficult to explain otherwise. T h e high solubility of T\'Or+PFe- and NOz+AsF6(as well as K02+SbFe-) must also be due partly to the higher association of these salts as compared F4-. with NO2+-B A further proof of complexing of the nitronium salts by solvent was obtained from experiments in which solutions of nitronium hexafluorophosphate in one of the solvents (nitromethane or tetramethylene sulfone) were used for nitrating aromatics dissolved in an approximately equal amount of the other solvent. The data obtained (Table V) point to the fact t h a t a nitromethane solution of NOz+PFs- gave. in the nitration of a toluene: benzene mixture dissolved in approximately the same amount of tetraniethylene sulfone, the same relative reactivities as nitration using nitromethane alone as solvent. This was also observed in reversed experiments for tetramethylene sulfone. TABLE V COMPETITIVESITRATIOS OF MESITTLESE AND BEXZENEIS MIXEDSOLVENTS SOpPFb in solvent
Bromatics in solvent
Tetramethylene sulfone Tctrnmetli\~lenesulfoiic Sitromethane Sitromethane Sitromethane Tctrnriietliylene sulfoIic Tetr:imethylene sulfone Kitrotiictliuiie
k x :kB 1.GO 0.43 0.37 1 .K2
Effects on Orientation-selectivity.-?‘abies I and I1 show t h a t nitration of aromatics with prepared nitronium salts shows low substrate selectivity and high positional selectivity in both tetraniethylene sulfone and nitromethane solutions. Only minor change in the isomer distribution of the nitration of toluene can be observed when the same nitronium salt is used in tetramethylene sulfone or nitromethane. T h e relative amounts of ortlzo isomers are slightly decreased when nitromethane was used instead of tetramethylene sulfone. With increasing size of the anion of the nitronium salts, the amount of the ortho isomer also decreases slightly. This is best observed in the case of the iX02+HSr07- nitration of toluene where the ovtho isomer formed was found to be (i2.170 coinpared to 66-6770 of ortho isomer in nitrations with other nitronium salts. There seems to be a parallel between substrate selectivity and positional selectivity changes (affecting the o-positions), both due mainly to steric factors. I n excess toluene alone, steric effects due to solvent participation or ion pair interactions are easily demonstrated. T h e nitronium salt reacts practically instantaneously with toluene and no solubility data could be determined, b u t the solubility of the ionic salt in the hydrocarbon must be very low. -1comparison of the isomer distribution of nitrotolucnes obtained using excess toluene as
l-ol, s4
solvent with data of nitration in tetraniethylene sulfone and nitromethane solutions is given in Table VI. TABLE 1'1 COMPARISOSOF ISOMER DISTRIBCTIOS OF SITRATIOS OF TOLUEXE I N DIFFEREPTSOLVESTS AT 25' Sitrating Isomer distribution, 5b agent
ortho
Solvent
K0zfBF4-
Tetramethylene sulfone Sitroniethane Toluene S02'C10,Tetramethylene sulfone Toluene S02+PF6Tetranietliylene sulfone Sitroniethane Toluene K02+HS207- Tetranietli~-lenesulfone Toluene
inera
para
65.4 2 . 8 31 . 8 G4.7 33.8 66.2' 55.0
67.6 04.8 55,IJ 62. 1 4!3.2
3.2 3.2 3 .4 3.1 1.4 1.9 2.2 : 2 3 . ri
32.1 13.0 30.4 41.6 31 . O 33.3 42.8 34.7
47.:3
I t is worthwhile to iiientioii t h a t according ti) observations of Fischback and Harris5 mixed acid (HNOy H2S04)nitration of toluene a t -13' gives an isomer distribution of 47.5% ortho, l . l c & meta and 51.4% para isomer, as compared with the isomer distribution a t 25': 57.4% ortho, 3.9% meta and 38.7 para. It was suggested t h a t the low ortho ratio a t lower temperatures is due to the higher degree of solvation of the nitronium ion. Friedel-Crafts nitration4 of toluene with S O r C l TiC14 and T\'OzC1 XgBF4 in tetramethylene sulfone, nitromethane or toluene solutions similarly gave significantly different isomer distributions (Table VII). TABLE VI1
+
+
+
Xi tr:itin&
agent
Catalyst
Solvent
'l'enip., Isonier di\tril,ution, ?'o ~ ~ r / h ~ : "C. uitlio me!u payil piii ii
SOrCl TiCIi
TXS
25
SO&l TiC14 S02Cl XgBI:,
Toluciic
2.5
CI-IaL-0::
15
6 1 . 3 4 . 2 X s - )1 . 7 S 11 :4 2 .1 .X.3 0 7.; 0 8 . S 2 . 1 29 . 4 2 . X 3
+
The NOzCl ;IgBF4 xitration is a typical nitronium tetrafluoroborate nitration with in sitil preparation of the nitronium salt. n'02CI f TiC14in tetramethylene sulfone solution also shows a typical nitroniuni nitration ratio, although ionization may be less complete. U h e n toluene is used as solvent, i t is suggested that primarily only a 1 : l donor-acceptor addition compound is formed, which then reacts with the aromatic as a considerably bulkier nitrating agent than NO?+ gixring a much lower ortho :para ratio. Experimental 'l'lic alkylbciizeiic~ a n d haloherizcnes u i e d \\-ere U I I I I inercial materials of liighest available purity. S i t n i methane was E a s t n u n Kodak Co. spectro grade. TetraInethylene sullotie was obtained from Shell Chemical Co. and purified by lractional distillation under reduced pressure. Sitroniutn salts were prepared in this Laborator!-.' 1. Competitive Nitrations in Tetramethylene Sulfone.Benzene (02.5 mole) and 0.25 mole of alkyl- or halobenzenes were dissolved in 70 g. of tetrainethylene sulfone. A solution of 0.05 mole of nitroniuni salt in GO g. of tetramethylene sulfone was added cirop\vise to the vigorously stirred reaction mixture and the temperature \\-as kept a t 2,: i O.,io. :lfter stirring for 15 minutes the mixtures w r e neutrdized \\-it11 KFI., gas, filtered aud analy.cri by gasliquid clirotnatogrnlili!-. (.i R. )C . Fischhack and G. H. Harri,, I x r w n a l ctnnrnitnicatiot1.
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 nitroniethanc, 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) w a s 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 and 0.22 mole of mesitylene dissolved in 70 g . of the other solvent under crmditicins identical with previous nitrations.
[COSTRIBCTION S O .65, EXPLORATORY
3087
A4LKYLBENZENES
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 mil 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 Raman spectroscopic investigations.
RESEARCH LABORATORY D O W CHEMICAL O F
CATADA,
LTD.,S A R S I A ,
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 . %IC., 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 b y employing Ingold's competitive nitration technique, i t was possible to compare relative reactivity of aromatics. T h e 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. T h e 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).
