Oct., 1960
SUCLEAR AkGNETIC
RESONANCE SPECTRA O F
1555
TRIPHENYLCllRBONIUM I O N S
NUCLEAR MAGNETIC RESOSASCE SPECTRA OF THE TRIPHENYLCARBOSIUM AND R/IETHYLDIPHENYLCARBONIUI\1 10s BY D. E. O’REILLYBND H. P. LEFTIK Mellon Institute and the Gulf Research & Development Co., Pittsburgh, Penna. Received M a y 0,1960
Proton magnetic resonance spectra of triphenylcarboriium and rriethyl3iphenylcarbonium ions were measured in solutions of triphenylcarbinol and diphenyleth lene in sulfuric acid. Chemical shifts for ring protons urre obtained by the moment method and interpreted in terms o r stereochemical and electronic structures of the ions. The calculated “mesomeric” contributions to the chemical shift are correlated with charge densities calculated by the LCAO and SCRIO methods. Chemical shifts in the spectrum of triphenylcarbinol in sulfuric-acetic acid solvent are given and the Jo acidity function for this solvent system is evaluated from the resonance data.
Introduction The postulation of carbonium ion inter mediates has been a useful device for the successful correlation of kinetic and product distribution data for diverse organic reactions. While in many instances the existence of such ions is mere conjecture, it has been known for some time that phenyl substituted carbonium ions (e.g., triphenylcarbonium ion) are relatively stable and can be formed reversibly.’ Najor evidence establishing this fact has been provided by conductivity, cryoscopic and spectrophotometric measurements. Little is known, however, about the internal structure of these ions. Xuclear magnetic resonance methods are ideally suited to such investigations. Thus, for example, chemical shift and spin coupling data can provide information concerning both electronic and stereochemical features. The present publication presents the results of an n.m.r. study of the triphenylcarbonium and methyldiphenylcarhonium ion$. Results and Discussion [t has been well established that triphenylwrbinol ionizes in sulfuric acid to give stable carbonium ions according to the reaction
’I’riphenylcarbinol dissolved in CS, exhibits two resonance absorptions at -99 and 79 c./s. (relative to pure water at 2 5 ” ) , due to the aromatic ring protons and the alcoholic proton. The aromatic protons exhibit a chemical shift very close to that for benzene dissoh-ed in CS,, and are all magnetically equivalent. This effect is the result oi cancellation of the inductire effects by the ring-current effects of the various groups as illustrated by the data of Table I. TABLE I CHEMICAL SHIFTS FOR TOLUENE AND DERIVATIVES I N (RELATIVE TO BENZENE AT 40.00 MC./S.) Compound
(6,
cs2
C./S.)~
6 8 5 2
C6H 5 CH 3
(CeHd&He (CsHs)aCH 4 1 (CsHs)CHgOH 4.0 (C6HdsCOH 0.7 C6Hs 0.0 (I 8 = [ ( H - H0)/HO]. Y O where Ho i s the resonance field for the reference and Y O is the fixed resonance frequency.
Figure l b shows the proton n.m.r. spectrum of 1,l-diphenylethylene (DPE) at 5 mole yoin concen(CsH:)&OH + 2H2S01+ (CsH6)3C+ 2HS04HaO+ trated sulfuric acid. The aryl protoii resonance The carbonium ion is stabilized by resonance dis- consists of seveii partially resolved peaks. The tribution of charge. Many structures have been proton 1i.in.r. spectrum of DPE in CS? consists of proposed for this ion; however, only two merit se- two resonance absorptioiis due t o the ring protons rious consideration. In the first of these, the ion is (-98 c./s.) and the ethylenic protons ( - 2 5 c./s.). propellor shaped with all three rings equivalent in The positions and approximate intensities of n.m.r. the resonance hybrid (Model A).2 I n the other bands for triphenylcarbonium (TPC) and methyl(Model B) the positive charge is distributed over diphenylcarbonium (hIDPC) ions in concentrated only one or two of the rings.3 The proton magnetic sulfuric acid are given in Table 11. resonance spectrum of triphenylcarbinol in concenTABLE I1 trated sulfuric acid is consistent with Model A, wherein all rings are equivalent for times longer POSITIONS AND RELATIVEINTEXSITIES OF THE PROTOK than of the order of low2second, ie., of the order of N.M.R. ABSORPTION OF TPC AND MDPC IN COXCEXTRATED the frequency separation between proton absorp- SULFURICACIDAT 40.00 MC./S. (RELATIVETO BENZENEIN tions described below. The rings are probably CSZP __-_ TPC-----_-_Af DPC--equivalent for a time period shorter than -8, C./S. I - 8 , c .. I second, but evidence for this is not furnished by 164 21 the n.ni.r. spectra. -6 1 Figure l a shows the proton n.m.r. spectrum at 7 2 6 2 40.00 mc./s. of triphenylcarbinol in concentrated 13 8 14 8 sulfuric acid a t a concentration of 5 mole yo. The 20 4 20 45 resonance signal consists of five resolved peaks.
+
+
(1) J. E. Leffler, “The Reactive Intermediates of Organic Chemist r g ,” Interscience Publishing, Inc., New York, N. Y., 1956, Chapter V. (2) G. N. Lewis, T. T. Magel and D. Lipkin, J . Am. Chem. Soc.. 64, 1774 (1942). (3) M. S. Nevman and N. C. Deno. iZ$d,, 73,3644 (1951).
10 33 L5 40 3 With correction for the difference in hiilk-suscrptibilitv brtmwn CR2and H2S04 27 38
2 5 1
ZG
Vol. 64
D. E. O'REILLYAND H. P. LEFTIN
-121
-129
65
Fig. 1.--(a) Proton n.m.r. spectrum of triphenylcarbinol dissolved in ccincentrated sulfuric acid; (b) Proton n.m.r. spectrum of I, 1-diphenylethylene dissolved in concentrated sulfuric acid. Abscissas are labeled in c.p.6. a t 40 Mc./sec., relative to pure water.
For theoretical purposes, the chemical shifts for the aryl protons of TPC and MDPC are referred to benzene in CSn. A bulk-susceptibility correction4 to account for the shift due to the difference between the susceptibilities of H2SO4and CS, has been made. Spectrit of triphenylcarbinol in liquid SO2and acetic acid were also obtained. The shift of the most intense peak of TPC from that of triphenylcarbinol in the above-mentioned solvents agrees within 1 C.P.S. with that obtained for CS2 solvent after a bulk-susceptibility correction is made, indicative of the lack of influence of solvent on the chemical shift of the aromatic protons in triphenylcarbinol. As a consequence of the formation of carbonium ions, the aromatic protons of TPC and MDPC undergo a chemical shift to lower field and become resolved into several lines. Chemical shifts to low field can result4 from (a) a decrease in electron charge density at the carbon atom to which a proton is bonded, or (b) the magnetic field produced by the current induced in the pi electron loop of the rings upon application of a magnetic field. In order to obtain the chemical shifts for the various ring protons of T P C and RIDPC from the observed spectrum, the moment method of Anderson and McConnel16 was employed; for this purpose, the first, second and third frequency moments of the observed spectra were determined. For reasons discussed below, the ring protons of TPC and MDPC were considered to consist of three equivalent groups; the ortho, meta and para position pro( 4 ) J. A . Pople, w. G. Schneider and H. J. Bernatein, "High Resolution Nuclear Ms,jnetic Resonance." McGrrtw-Hi11 Book Co., Inc., New York, N. Y..1959. ( 5 ) W. Anderson and H. I f . McConnell, J. Chem. Phys., 26, 1496 (1967).
tons. In this case, the moment method allows the evaluation of the individual chemical shifts for these protons, since frequency moments up to and including the third do not involve spin-spin coupling parameters. Thus, evaluation of frequency moments up to the third does permit evaluation of chemical shifts but yields no information on spinspin coupling constants. With the use of Fig. 4 of ref. 5 , chemical shifts for the various protons of TPC and MDPC were evaluated and are given in Table 111. The solution for 6,, ,6 and 6, for TCP from the calculated moments is unique. For MDPC, three solutions are possible. Of these, the one given in Table 111 corresponds continuously to that of TPC, and furthermore, is the only one n-hich yields a value of 6, larger than ,6 or 6, analogous to the result obtained for TPC. For both spectra the assignment of two of the shifts to ortho or meta protons was made on the basis that the ortho shift has a larger absolute value than the meta shift. This assumption is justified below. It must be iioted that the error in the magnitude of the calculated shifts may be as large as j=20% due to errors in the intensities of resonance lines displaced most from the mean shift. However, the order of the calculated shifts is believed to be correct, i.e., 16, I > 60
I > 16, I.
TABLE I11 ortho, meta AND para PROTON AND MEANCHEMICAL SHIFTS FOR TPC AND MDPC AS EVALUATED BY THE MOMENT METHOD@ TPC a0 (6,)
G?n(&) 6,
- 17 -11 -32
MDPC
-24
- 14 -32
-
Gmean - 17 22 Shifts relative to benzene in CSJ at iufinite dilution a t 40.00 mc./s. Alternative aesignment. a
The central carbon atom of TI'C and hlDPC was assumed to form three coplanar bonds so as to satisfy the requirements of the usual resonance structures that can be written for the molecule. In addition, free rotation of groups bonded to the central carbon does not occur due to the partial double bond character of these bonds, However, the phenyl rings of TPC or RlDPC cannot be perfectly coplanar due to steric repulsions of ortho-hydrogens. As already mentioned, inter-ring-current shifts will produce a significant contribution to the observed shifts of the ring protons of TPC and MDPC. For evaluation of this effect, the orientations of the phenyl rings relative to one another and the average time spent in each orientation must be knowri. This information is, however, not presently available. A model for TPC is proposed for evaluation of the ring-current shift. The central carbon bonds are assumed to be coplanar and the phenyl rings to be twisted, all in the same sense, out of the plane of the bonds to the central carbon atom. The magnetic field produced by an induced ring current may be represented by the field of a magnetic dipole; hence the chemical shift, AUJ, of proton i produced by the ring current in ring j may be evaluated by the equation
NUCLEAR MAGNETIC RESONANCE SPECTRA OF TRIPHEXYLCARBONIUM Ioss
Oct., 1960 Auij =
eeae (3 COS' 2mc'R3ij
-
dij
- 1)
(1)
where a is the effective radius of the phenyl ring current loop, R i j is the distance from the center of phenyl ring j to proton i, &j is the angle between the perpendicular t o ring j and radius vector Rij and the remaining symbols have their conventional significance. The ring-current shifts for ortho, meta and para protons of the phenyl rings have been evaluated for several different angles of twist, 0, as well as the distance between adjacent ortho protons as a function of 8. Shifts were calculated relative to that for the protons of benzene. The results are shown in Fig. 2. At 0 = 23", the distance between ortho protons is equal to twice the expected van der Waals radius of hydrogen (1.80 A.). The corresponding ring-current shifts are: employed in the following discussion. A second contribution to the chemical shift for the protons of T P C and MDPC relative to benzene will result from the positive charge of the molecule. Moreover, a linear relationship is expected between proton chemical shift due to positive charge density on the carbon to which it is bonded and positive charge on the carbon, Le. a+ = 4 P (2) where 6+ is the proton chemical shift due to positive charge, p is the positive charge density on the carbon atom to which the proton is attached and q is a constant. Some justification of equation 2 may be given by noting that the chemical shifts for hydrogen bonded to a carbon sp2orbital is expected to be proportional to the ionic character of the C-H bond. A positive charge density placed on the carbon due to a decrease in pi electron density may be expected to cause a proportionate increase in the ionic character and, hence, to increase the chemical shift of the hydrogen from that for neutral C-H bond. The T P C ion has been treated by the conventional Huckel LCAO method by Streitweiserd and by the self consistent molecular orbital (SCMO) method by Brickstock and Pople.' The results of these two methods differ considerably as shown in Table IV. TABLE IV RINQ CURRENT AND POSITIVE CHARQl (MESOMERIC) CONTRIBUTIONS TO THE CHEMICAL SHIFT FOR TPC AND VALUES O F THE POSITIVE CHARQE DENSITYCALCULATED BY THE LCAO AND SCMO METHODS "6-7
Itingcurrent,
Position
Total obsd.
0 = 23'
ortho
-17
-11
.
Mespnieric
-
--
-aP
LCAO
SCMO
6 0.074 0.05 mela -11 -4 -7 .OOO .06 para -32 -4 -28 .074 .19 Relative to benzene in CSZa t infinite dilution, corrected for bulk-susceptibility of solvents. * In units of electronic charge.
The "mesomeric" contribution to the chemical shift, 6+, was calculated from the observed shifts by subtraction of the ring-current contribution calcu( 6 ) A. Streitweiser, Jr., J . Am. Chem. Soc., 74, 5288 (1952). (7) A. Bricketock and J. A. Pople, T r a m . Faraday SOC.SO, 901 (1954).
G'c'o
r
I
I
I
1557
D. E. O'REILLYAND H. P. LEFTIN
1558
-70
r
o
I 0 0 I! 00
I O
20 3.0 40 50 60 H 2 S 0 4 C O N C E N T R A T I O N , MOLE L I T E R - ' .
10
Fig. 3 --Ha and J o functions versus sulfuric acid concentration in sulfuric acid-acetic acid mixtures. Ho data are those of ref. b, Table VI; J o data are those of Table VI.
lated fi-om equation 2 with 0 = 15" are given in parentheses in Table V. As was noted above and in Table 111, there is an ambiguity i r the assignment of the values of 6, since the assignment of the shift for the ortho and meta protons, as determined by the moment method, is not unequivocal. If the alternative assignment denoted in parenthesis in Table I11 is adopted, the chemical shifts continue t o be in best agreement with the charge densities as computed by the SCMO method rather than the Huckel method, although the relationship given by equation 3 it: riot quantitatively obeyed. The relative stability of triarylcarbonium ions is strongly determined by the electronic effects of ring substituents. Major evidence in support of this fact is based on spectrals and conductivity9 data. This fact has been used in setting up an acidity scale for non-aqueous media analogous to that of Hammett.lo Gold and Hamesl' defined the J Oacidity function by Jo
= (pKR+
- log CROH cR+)
(3)
for the reaction 1'
+ HzO
ROH
+ H+
(4)
where clearly
When J o values for a solvent system are known, thermodynamic ~ K + R data can be obtained from a spectrophotometric examination according to equation 3, since the electronic spectra of carbonium ( 8 ) G . Etranch and H. Walba, J . A m . Chem. Soc., 7 6 , 1564 (1954); S . C. Deno, .J. J. Jariizelski and 9. Schriesheim. J . O w . Chem., 19, 155 ( 1 954). 19) N. N. Lichtin and P. I).Bartlett, J .
A m . Chem. SOC.,73, 5530 (1951); N. N. Lichtin and H. P. Leftin, THIS JOURKAL, 60, 164 (1956). (lo) L. 1'. Hammett, "Physical Organic Chemistry," McGraw-Hill Book Co., Uew York. N. Y., 1940,Chapter IX. (11) V, Gold end B. R', vs Hawes, J . Chrm, Sou-, 2102 (1951).
Vol. 64
ions differ markedly from those of their carbinol precursors. Conversely, once having established reliable ~ K Rdata, + the acidity function J o of many solvent systems can be established. Nuclear magnetic resonance techniques have been successfully applied to studies of chemical equilibria.4 In order to test this technique for equilibria involving carbonium ions, the n.m.r. spectrum of triphenylcarbinol was examined in a series of solvents composed of sulfuric-acetic acid mixtures covering the range 0 to 100 wt. yo H2S04. The proton resonance of the aromatic protons consisted of a single broadened line in the mixed solvents studied. The signal shifted to lower field with increasing acid strength as illustrated in Table VI. As before, a bulk-susceptibility correction was made for each sample. The diamagnetic susceptibility of a mixture was computed by the Wiedemann additivity law4 from the known concentrations and susceptibilities of the components. TABLE VI CHEMICALSHIIW OF AROMATICPROTOSS(TRIPHENYLCARBINOL I N ACETIC-SULFURIC -4CID MIXTURES,%") Solvent (wt.7. Iizsol In CHCOOH)
Chemical shift (c./P. a t
J\Ioles/l.
Hob
40 me. 's.)
Calcd. Jo
0 0.00 0 3.5 0.36 -2.4 3.2 -3.6 7.0 0.72 -2.8 10.7 -4.7 11.7 1.44 -3.4 13.2 -5.3 -6 46.6 6.3 -5.4 14.0 100 .. 14,2" a Average of two signals. b S . F. Hall and W. F. Spengeman, ,I. A m . Chem. SOC., 62,2487 (1940).
From the known p K R + = 4.18 for triphenylcarbinol (Deno, Jaruzelski and Schriesheim, ref. 8) the value of Jo for any of these acid solutions can be calculated from the n.m.r. data. The ratio CR+/ C R ~ was H obtained from the observed shift according to __ CR+ - _ _6_ _-- - ~ R O H (6) CROH
6 ~+ 6
where 6 is the observed chemical shift and 6~ t, ~ R . O H are the chemical shifts for triphenylcarbonium 1011 and carbinol, respectively. The J o values calculated from the chemical shift data are given in Table VI where they are compared to available H O values for this system. As shown in Fig. 3 , J O parallels the Ho data although J O increases more rapidly. A similar feature has beeii found in the sulfuric acid-water system. 1 2 , 1 3 Deuterium Exchange.-The triphenylcarbonium ion dissolved in 80% D2S04 (99.5% deuterated) showed no evidence of isotope exchange in eight hours at 25". The intensity of the n.m.r. signal obtained from the phenyl protons remained unchanged and no increase of the acid H resonance x a s detected. This result is in agreement with the work of Den0 and Evans,l4 and may be taken to (12) N. C. Deno, J. J. Jaruzelski and A. Bchrlesheim, J . A m f'hem SOC.,7 7 , 3044 (1955). (13) M.A. Paul and F. A. Long, Cham. Reus.,67. 1 (1957). (14) N. C. Deno and Wm. L. Evans, J . A m . Chem. Soc., 7 8 , 589
(1'256).
Oct
, 1960
KINETICS O F METHYLCYCLOHEX.INE DEHYDROGENA4TIOSOVER
indicate that hyperconjugation bond” structures such as
involving “no
/-I
H+&C,
P
does not play an important role in stabilization of this ion. Experimental Reagents.-Triphenylcarbinol, m.p. 162-163’, waa prepared by recrystallization of Eastman White Label material.
I’T--~L?OI
1539
Diphenylethylene ( n 1.6093) ~ was prepared by distillation of Aldrich Chemical Co. DPE. Deuteriosulfuric acid was prepared by deuteriolysis of anhydrous sulfur trioxide using D20 (99.5%) from Stewart Oxygen Co. Measurements.-N.m.r. measurements were made with a Varian Associates model V-4300A spectrometer and 12 inch electromagnet. Chemical shifts were measured by the audio sideband technique with the use of a frequency counter. The chemical shifts of Tables I, I1 and VI were measured by use of trimethylsilane in a capillary placed in the tube which contained the material under study.
Acknowledgment.-The portion of this work carried out a t the Mellon Institute was sponsored by the Gulf Research & Development Company as a part of the research program of the Multiple Fellowship on Petroleum.
KINETICS O F METHYLCYCLOHEXAXE DEHYDROGEYATION OVER PTBY J. H. SINFELT, H. HURWITZ AND R.il. SHULMAN ESSOResearch and Engineering Company, Linden, -1-J . Receiuod .!fag 1 8 , 1360
The kinetics of methylcyclohexane dehydrogenation over a platinum-on-alumina catalyst were investigated over the temperature range 315 to 372O, a t methylcyclohexane partial pressures ranging from 0.07 to 2.2 atmospheres and hydrogen pressures ranging from 1.1to 4.1 atmospheres. The reaction was found to be nearly zero order with respect to methylcyclohexane and hydrogen over the range of conditions studied. The activation energy for the reaction was found to be 33 kcal./ mole. The near zero order behavior of the reaction suggests that the active catalyst sites are heavily covered with adsprbed molecules or radicals at reaction conditions. It is suggested that adsorption equilibria are not established at the condJtions used in this study. .4 simple kinetic scheme, according to which the reaction rate corresponds to the rate of desorption of toluene, is proposed t o account for the observed kinetics.
Introduction The formation of aromatics by the catalytic dehydrogenation of saturated six-membered ring hydrocarbons is a well-known reaction. The transition metals and their oxides are active catalysts for such dehydrogenation reactions. Supported metal catalysts, such as platinum or palladium-on-alumina, are particularly active. It has been shown by hydrogen chemisorption measurements that freshly prepared platinum on alumina catalysts are characterized by extremely high dispersion of the platinum on the support.’ The high dispersion of the platinum is an important factor contributing to the very high activity of these catalysts. Although a number of investigations on the dehydrogenation of cyclohexanes over supported platinum catalysts hare been r e p ~ r t e d ~the - ~ kinetics of the reaction have not been extensively investigated. Therefore, it was decided to study the kinetics of methylcyclohexane dehydrogenation over a platinum-on-alumina catalyst to gain some insight into the nature of the surface phenomena involved in the reaction. (1) L. Spenadel and 1RI Boudart, THISJOURYAL,64, 204 (1960). (2) V. Haensel and G R Donaldqon, I n d Eng Chem 43, 2102 (1951).
(3) W. P. Hettingei, C D Keith, J L Gring and J. W Teter, z b z d , 47, 719 (19j6). (4) 9. I. M. Keulernans and H. H. Vope, THISJOURNAL,63, 476 (1959).
Experimental Procedure.-Reaction rates were measured in a flow system in the presence of added hydrogen using a l / z inch i.d. stainless steel reactor with a volume of about 20 cc. The reactor was surrounded by an electrically heated aluminum block to maintain isothermal operation. The runs were made with a catalyst charge of 6 g. diluted with inert ceramic beads to fill the reactor volume. Prior to introducing the methylcyclohexane feed, the catalyst was pretreated with flowing hydrogen for three hours at 527”. Reaction products were analyzed by a inch i.d. chromatographic column coupled directly to the outlet of the reactor. The column was 4 meters long, packed with firebrick impregnated with polyethylene glycol, and operated at a temperature of 90’. This gave excellent resolution of the methylcyclohexane, toluene and other aromatics used in this study. Reaction periods of 30 minutes were employed in all of the runs to ensure the attainment of steady-state conditions prior to sampling the reaction products. Several reaction temperatures were used in this work, ranging from 315 to 372 Total pressure was varied from 1.4to 6.3 atmospheres and hydrogen to methylcyclohexane mole ratio from 2 to 20. Molal space velocities ranged from 0.2 to 1.0 g. mole of methylcyclohexane per hour per g. of catalyst, depending on the reaction temperature. Materials.-Phillips pure grade methylcyclohexane (> 99 mole % purity) was used in all the experiments. The methylcyclohexane was dried with Drierite ( CaS04) to less than 5 p.p.m. by weight of water before using. The benzene and meta-xylene which were used in binary mixtures with methylcyclohexane in some of the experiments were also Phillips pure grade and were dried in the same way. The hydrogen was passed through a Deoxo cylinder containing palladium catalyst to convert trace amounts of oxygen t o water, and then dried over Linde 5A molecular sieves. The catalyst used in this study contained 0.3 w t . yo platinum
.
