N C DENO,C U PITTMAN, JR,
4370
stantially faster rate than Grignard compound addition. Prior coordination of the ketone by MgX2 followed by R2Mg addition to the polarized carbonyl group is not probable on the basis that coordination by MgX2 in the presence of RMgX and RzMg is unlikely (for the reasons discussed previously). Also the intermediate product of such an addition (R2’RC--OMgX) does not explain the reduced reactivity of the Grignard after 5(lyoreaction. Finally, it appears that Grignard compound addition to ketones can also be explained in terms of attacking ionic species. 2RMgX
+ MgX?
KyMg
KMg+
+ R M g ’slowd
R’-C-O
/
+ KMgXZ-
K’-CB-OMgR
(6) (18)
/
K’
R‘
fast + RMgX2- +
R’-C-OMgR
4”
R
K‘
0
R‘
\/\
R’-C
\
+R’-C-OMgR
Mg-R
.e
x
R1
(19)
/
K
\e / Mg
\
+ MgXz
X
The same arguments justifying RMgX addition also hold for attacking ionic species. In summary, the composition of Grignard compounds in ether solution appears to be a function of the solvent, as well as the nature of the halogen involved. The composition of “EtMgBr” in tetrahydrofuran, for example, is adequately described by the equilibrium
[CONTRIBUTIOS F R O M
THE
AKL,
hl J ~VISOTSKY 2RMgX
Yo1 h 0 RZMg
+ MgX?
In diethyl ether, however, the composition is expanded to include dimeric species
x R-hlg
/ \
Mg--K
‘\X
2IihIgS
_r
/ R
x
1Vhen X = Br and I, association increases with concentration. so that a t 0 05 I f the species present in solution are predominantly monomeric, whereas a t I 1 3 ,If the species present in solution are about one-hdf monomeric and one-half dimeric \Then X = C1, dimeric species predominate even a t (1 05 -If Conductance data indicate that ionic species such as RMg+, RAlgXZ-, and ion triplets exist in solution, but only to a minor extent -4 new mechanism describing the addition of Grignard compounds to ketones is presented in terms of an attacking monomeric or dimeric RMgX species A11 of the pertinent facts reported to date, concerning Grignard compound addition to ketones, can be accounted for in terms of a mechanism involving, in the rate-deterniining step, ketone displacement of a solvated ether molecule from the attacking Grignard species Acknowledgment.--The encouragement of Professor H C Brown of Purdue University a t the beginning of‘ this study, and helpful discussions with Professor H hI Xeumann of the Georgia Institute of Technology during the preparation of the manuscript, are gratcfully acknowledged
DEPARTMEST O F CHEMISTRY, b’HITMORE BUILDISG,THEPESSSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PENSSYLVANIA]
Carbonium Ions. XVII. The Direct Observation of Saturated and Unsaturated Acyl Cations and Their Equilibria with Protonated Acids BY
x. c. DENO,CIIARLES u.PITTMAN. JR.,
.4ND M . 4 X
J. WISOTSKY
RECEIVED JCSE 10, 1964 The equilibria between RCOOH and RCOOH?+ and between RCOOH2+ and R C O + have been studied in 0-1007c aqueous HrSO; and in 0-80% SO3 in HrS04. The simple aliphatic carboxylic acids have [RCOOH?+] equal to [RCO-] in 10-255; SOa and the shift from >905;. RCOOH!- to >SOT, RCO’ occurs within narrow (-47; j ranges of SOa concentrations. I t is suggested t h a t the equilibriu3 s’lifts between R C O - and RCOOH,+ can be used to evaluate changes in the activity of H?O. The acyl cations, R C O + , may now be regarded as commonplace and familiar chemical species. Their availability i n wide variety will be of synthetic interest.
h representative selection of 11 carboxylic acids have beer examined by nuclear magnetic resonance ( n . m , r , ) spectroscopy in O--1007c aqueous H2S04 and (!-SC!YcSOa in H2S04. The shift of equilibria from free acid (RCOOH) t o protonated acid (RCOOHz+) and the further shift from RCOOHz+ to the acyl cation (KCO-) were evident. These results, along with the work of Olah, reduce acyl cations to conimonplace chemical species and reinforce the current viewpoint that such acyl cations have real existence. Figure 1 shows the changes in n.m.r. band position for solutions of acetic acid in varying concentrations
of S03-H2S04-H20. Only the band position shifts, indicating that the changes are entirely confined to the carboxyl group. The downfield shift of 0.X p.p.111. between 30 aTid SticG H2S04is of the magnitude expected for simple protonation. More convincing, the shift is hali-completed a t Ti?; H,SOI (,‘33”),agreeing with ultraviolet studies which estimated acetic acid to be half-protonated in 74yGH2S04(25”). I Similarly. the n.m.r. data for propionic acid indicate it to be halfprotonated in SO% H2SO4(3.7”) in agreement with the ( 1 ) A . R. Goldfarb, A . M e l e , and S . Gutstein, J . A m . C h e n i 61!11 (lQL53j.
.So( , 7 7 ,
ACYLCATIONEQUILIBRIUM WITH PROTONATED ACIDS
Oct. 20, 1964
TABLE I POSITIONS OF EQUILIBRIA AT -35' FOR RCOOH A N D RCOOH,' G RCO' % H~SOI, [RCOOHI = [RCOOHz+l
Acid
Acetic Propionic Isobutyric Cyclopropanecarboxylic Cl-clobutanecarboxylic Cyc1ohexanecarbox)-lic 2-Butenoic (crotonic) 2-hlethl-l-2-butenoic (tiglic) 3-Methyl-2-butenoic
i3
KCOOHn+
5%
SO3 in HzS01, [RCOOHz+] = [ RCO -1
15% so3 23% SO3 16% SO3 23% SOs 2 5 q SO3 -12'); so3 14% SO8 11% so3 0% so, (100% HzS04) 7?G so2
77 80 82 80
76 71
2,4-Hexadienoic (sorbic) (-71)= Chloroacetic 100 h a Estimated from studies of the ultraviolet spectrum as a function of % H2SOd. These studies will be reported in detail in the The protonated acid was Ph.D. Thesis of C. U. Pittman, Jr. still dominant in 65% SOI.
'
4371
resulting band position of 3.93 p.p.m. (downfield from tetramethylsJane) agrees with the value of 4.02 reported for CHaCO+in SbFj.4sb Similarly, for propionic acid and its acyl cation, CH3CH2CO+, the methyl triplet ( J = 7 c.p.s.)centers a t 1.91 and the methylene quartet centers a t 3.OG p.p,m. in good agreement with the ranges of 1.8-2.1 and 4.0-4.4 found for CH3CH2CO+ in different media.4,j Less direct evidence is t h a t the band positions of the a-hydrogens are like those of the a-hydrogens in alkenyl cations (2.5-3.5) and (CH3)4N+(3.10).6 Figure 1 is typical of the curves found for the carboxylic acids. Noteworthy is the narrow range of SOa concentrations within which the equilibria shift from >90% KCOOH2+ t o >90% RCO+. This is generally over a range of 4yc in SO3 concentration. T h e 91, H2S04 or yGSOs a t which [RCOOH] is equal to [RCOOHl+] and [RCOOH2+] is equal t o [RCO+] are summarized in Table I. The n.m.r. band positions
TABLE I1 N.M.R.BANDPOSITIONS OF RCOOH, RCOOHz+, AND R C O f -Band RCOOH in 50% HzS0.i
Acid
Aceticb Propionic
a-H
B-H Isobutyric Cyclopropanecarboxylic
Cyclobutanecarboxylic
CY-H @-H CY-H 8-H a-H
8-H Cyclohexanecarboxylic 2-Butenoic (crotonic)
(Y-H CY-H B-H Y-H 2-Methyl-2-butenoic (tiglic) a-CH3 P-H Y-H
2.11 2.41 1.09 2.67 1.30 1.5 1.02 3.17 2.17 5.70 7 15 1.90 1.72
1.79
positions in p . p . n ~ . ~ RCOOHzT R C O + i n 60% in 96-7, SOa-40% HiSOa HiSOa
2 6i 2.94 1.37 3.18 1.49 1.8
3.93 4.06 1.91 4.23 1 90 2.8
1.78 3.56 2.42 2.82 6.26 7.57 2.28 2.00 7.84
2 . 76 4.62 3.13 4.24 6.64 8.95 2.68 2.33 9 08
2.08
2.56
Character of band
Quartet, J = 7 C.P.S. Triplet, J = 7 C.P.S. Septet, J = 7 C.P.S. Doublet, J = 7 C.P.S. Multiplet Doublet, J = 6 C.P.S. Broad band Poorly resolved quartet, J = 8.5 C . P . S . Complex multiplet, position of most intense peak listed Multiplet J = 1 and 16 C.P.S. J = 7.5 and 16 C.P.S. J = 1 and 7.5 C . P . S . Doublet, J = 1 C.P.S. Quartet, J = 7.5 c.P.s., each component split t o a doublet, T. = 1 C.P.S. Doublet, J = 7.5 c.P.s., each component split t o a doublet, J = 1 C.P.S.
6.20 6.56 2.44 2.81 2.30 2.75 2,4-Hexadienoic (sorbic) c-H 1.9e (4,31)" Doublet, J = 5.5 C.P.S. ' ?;.m.r. spectra were recorded on a Varian A-60 Mc. instrument. Sample temperature was 35". The internal standard was (CH3)(ri+ a t 3.10 p.p.m. below ( C H s ) S (ref. 6). Before each set of runs, a sample of CHCl3 plus (CH3)dSi was used to check the scale calibration of the instrument. There was a significant broadening of the hybrid band in 1 5 1 7 % SOs suggesting t h a t the equilibrium was nearly slow enough to resolve the bands of CH3CO+and CH3COOH2+. This was measured in 9 (or 13)y0 SO3 because the acyl cation was unstable in larger SO, concentrations. Although the n.m.r. spectrum has not been completely interpreted, the fact t h a t the acid can be recovered unchanged from solution in 13% so3 justifies its inclusion in the table. T h e spectrum of the protonated acid in 967, H2S04 and the acyl cation in 9 % SO3 are shown in Fig. 2. 3-Methyl-2-butenoic
CY-H 7-H
5.81 2.11 1.94
value 79% H2S04 (25') estimated from ultraviolet studies2 The n.m.r. studies show both acetic and propionic acids to be dominantly protonated in 1007, H2S04 in agreement with cryoscopic studies. For acetic acid, a further downfield shift of 1.26 p.p,m. occurs very sharply between 13 and 17% SO3 in H&04 (13 t o 177, oleum). This is interpreted to be the result of the conversion of CHaCOOHz+ to CH3CO+. The most convincing evidence is that the (2) J. T. Edward and I. C. Wang, Can. J . Chem., 40, 966 (1962). (3) L. P. Hammett and A . J. Deyrop, J . A m . Chem. Soc., 66, 1900 (1933).
for all three species are briefly summarized in Table 11. Substituents that stabilize carbonium ions also stabilize RCOOH2+ with respect to KCOOH. This is understandable in terms of the carbonium representation of RCOOHz+, 11, which makes a small contribution to the resonance hybrid. T h e dominant contribution is of course made by I. The stabilizing effect (4) G. A . Olah, W. S. Tolgyesi, S. J . K u h n , M. E. Moffatt, I. J. Bastien, and E. B. Baker, ibid., 86, 1328 (1963). ( 5 ) G. A. Olah, S. J. K u h n , W. S. Tolgyesi, and E. B. Baker, ibid., 8 4 , 2733 (1962). (6) N. Deno, H . G. Richey, Jr., N . Friedman, J. D . Hodge, J. J. Houser, and C . U. Pittman, J r , , ibid., 86, 2991 (1963).
N. C. DENO,C. U. PITTMAN, JR.,
4372 I
I
I
I
F-
I
AND
M. J. WISOTSKY
VOl. 86
I
3.5
I
Ei
2 U 3
2
n
3.0
2c Fig. 2.- S . m . r . spectra of solutions of 2,4-hexadienoic (sorbic) acid in 8$4 SO3 in H?SOI (RCO-, upper curve) and 96';/, H L S O ~ (RCOOH,-, lower curve).
2.5
I
I
b
R-C
4 \.
+
R-C
I
I
L
+/
\
OH
OH
I
I1
These same substituents t h a t stabilize carbonium ions also stabilize RCO+ relative to RCOOH*+. This requires an explanation because the equilibrium formally involves only loss of water RCOOH*+
KCO
+
+ H20
(1)
T h e most obvious interpretation is that more of the positive charge is on carbon in RCO+ than in RCOOHz +. Incidentally, we suggest that the shifts in eq. 1 will be a satisfactory method of estimating activity of HzO in S03-H2S04and similar systems. The acyl cations are probably in equilibrium with ketenes RCH?COi
If KCH=C=O + H +
(2)
and carbonium ions (the Koch reaction) RCO+
IT R C + CO
(3)
Hydrogen-deuterium exchange studies will elucidate eq. 2 . As for eq. 3 , similar to the results in SbFj, (CH,),CO+ in 65% so3 rapidly loses CO. After 5 min. a t 25', CO evolution has subsided. The solution a t this time shows no trace of the 4.35 p.p.m. band characteristic of (CHs)sC+.7 What is present are thc q.m.r, bands of the set of cyclopentenyl cations (the ubications) formed from (CH3)&+ in oleum.8 Of the acids listed in Table I , only cyclohexanecarboxylic and 2,4hexadienoic noticeably decomposed in 20 min. a t 23". Chloroacetic acid solutions exhibited a single band in the n.m.r. spectrum. The position shifted from 4.16 in ( 7 ) G. A . Olah. E. B. Baker, J . C . E v a n > , W. S. T d g y e s i , J , S. 5IcIntvt-e, and I . J. Bastien, J 4 7 n C h r m .Soc , 86, 1360 (1U61). ( 8 ) K . Deno, D. B. Boyd, J . D. Hodge, C. U . P i t t m a n , J r . , and 1. 0. Turner, ibid., 86, li45 (1964).
H20 to 4.22 in 50% to 4.31 in 90% HzS04. The change is small and so gradual t h a t it is interpreted as attributed t o changes in hydrogen bonding energy uf the free acid. Between 967, H2S0a and 5% SOa the band shifts from 4.37 t o 4.59. This would seem to be the result of protonation, since from 5 to 65Yo SO3 there is only a small further shift from 4.59 to 4.67 p.p.m, Concentrations of acyl cations of 25% by weight are readily prepared and i t is evident that these have synthetic possibilities. Already we have found that Newman's method of e s t e r i f i ~ a t i o naddition ,~ of a solution of the acid in 100yc H2SO4to alcohol, formerly thought to be restricted to mesitoic acid and its congeners, is generally extensible to the majority of carboxylic acids by the simple expedient of using 25% SO3 in H2S03in place of 1007c HzS04. Practically, it is necessary to add RCOOH to -70% SOa, and a simple calculation of the HzO produced will serve to estimate the ratios required to result in an effective SOs concentration of 25-3OCj,. The addition of SOXH z S 0 4systems to alcohol causes no trouble providing rapid dispersal methods are used such as we have championed for the addition of H z S 0 4to 107, aqueous NaOH.6~X A solitary example of equilibria between KCO+ and RCOOH2+ had been reported previously. 2,4,6Trimethylbenzoic (mesitoic) acid was known to form the acyl cation in 100yGH2S04from cryoscopic studies3 and Schubert and co-workers found [RCO-]= [RCOOHz+]in 97% H ~ S 0 4 . l ~ The current n.m.r. study confirms this value. In 60% SOa, the band positions of the acyl cation are 2.73, 2.60, and 7.41 for 0- and p methyls and m-hydrogens. The analogous values for the protonated acid in 92% were 2.43, 2.34. and 7.08. The equilibration of RCO+ and RCOOHz+ is slow enough with this acid that the n.m.r. bands of KCO+ and RCOOHz+ are independently observed from 93-99,57; H2SO4. The n.m.r. spectra indicate RCO+ to be half-formed a t 987, H2SO4and 35' Acknowledgment.-lye are grateful to the Petroleum Research Fund of the *%mericanChemical Society for a grant in support of this work and to the National Science Foundation for a grant supporting the purchase of the Varian X-60 n.m.r. instrument. 51. J . W. received a fellowship supported by the Esso Research and Engineering Co. (9) 51. S. S e w m a n , ibid., 63, 2431 (1941). (10) W .S. Schubert, J . Donohue, and J. D. Gardner, ibid.. 7 6 , 9 (1954)
