TAFT,PRICE,Fox, LEWIS,ANDERSEN, AND DAVIS
3146
sufficiently large concentration to make the rearrangement competitive with chain reactions of the bromine atom. The effect of the removal of bromine atoms from the reaction mixture by complexation with olefin (either as a u- Or T-complex)~couldshift the mechanism for bromination to One a 'lower chain process with succinimidyl radicals acting as chain carriers. 27
[COSTRIBUTION FROM
THE
Vol. 85
Acknowledgment.-This research was supported in part by a grant from the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is hereby made to the donors of these funds. (27) This is consistent with the observation t h a t the addition of 2 % of ncrbornene greatly inhibits t h e benzylic bromination of a mixture of toluenes in benzene solution, unpublished work of Joseph Incremona
COLLEGE O F CHEMISTRY AND PHYSICS, THE
PENNSYLVAXIA S T A T E
UNIVERSITY, c S I V E R S I T Y PARK, PA.]
Fluorine Nuclear Magnetic Resonance Shielding in p-Substituted Fluorobenzenes. fluence of Structure and Solvent on Resonance Effects1
The In-
w.TAFT,ELTONPRICE, IRWINR . F O X , 2 IRWINc. LEWIS,K. K. ANDERSEN,3A N D GEORGET. DAVIS
B Y ROBERT
RECEIVEDM A Y7. 1963 Fluorine n.m.r. intramolecular shielding effects of 63 p-substituents in fluorobenzene have been determined a t high dilution in a wide variety of solvents. The correlation has been extended between the shielding parameter for a p-substituted fluorobenzene relative to its m-isomer as internal standard and the reactivity resonance effect parameter, q0.The relationship is used both to obtain URO values for new substituents and to assess the trends and magnitudes of solvent effects on this parameter. Approximate MO *-electron charge density distributions are also derived. A plot of the shielding parameter for p-substituted fluorobenzene relative to internal fluorobenzene, f ~ p - vs. ~ ,the corresponding parameter for the m-isomer, J B ~ - ~ provides , new insight into relationships between the resonance and inductive effects of substituents. Linear U R vs. UI relationships are shown to be quite special rather than general. A special and well defined class of substituents is recognized by a linear f ~ p as.- f R~ ~ plot. ~ These substituents are designated as UAFPD (united atom-like first row pair donor) substituents. The effects of UAFPD substituents on appropriate chemical and physical properties are shown to be quite precisely linear with U I in aromatic (or in general, unsaturated) systems, provided that there are essentially no contributing effects of direct charge delocalization interactions between the substituent and the functional group. These remarkably precise relationships, although limited by the number of UAFPD substituents, greatly extend the range of applicability of therelationship log ( k l k o ) = Clur ,C2: The effects of polar solvents on fluorine intramolecular shieldings are interpreted as indicating rather distinct categories of solvent dependence on the contributions to the ground electronic state of various types of dipolar resonance forms. Small or indetectable solvent effects are indicated for the contributions of dipolar resonance forms for which at least one of the atomic centers of formal charge is buried within the molecular cavity. On the other hand, with increasing solvent polarity the contributions are generally appreciably increased for those dipolar direct interaction forms in which both of the formal charge centers are on the periphery of the molecule. Some applications to chemical reactivities are considered.
+
Intramolecular FIg n.m.r. shielding in the special case of p-substituted fluorobenzenes apparently can be directly related, a t least approximately, to the Pelectron charge density on the fluorine atom4,jor its bonded carbon at0rn.j Extensive studies6 of the solvent effects on the fluorine shielding in m-substituted fluorobenzenes suggest that the intramolecular shielding effect of a p-substituent in fluorobenzene can be obtained a t high dilution in any solvent from the measurement of shielding referred to the standard internal fluorobenzene. The substituted and unsubstituted fluorobenzene present to their environment fluorine atoms which are sufficiently similar that intermolecular shielding cancels with considerable precision in the shielding parameter of the substituted relative to the unsubstituted internal fluorobenzene. Fluorine shielding is so sensitive to the very small (absolute) intramolecular perturbations in the fluorine atom *-charge density produced by a distant p-substituent t h a t this measurement ranks as the most sensitive probe (relative to the experimental error) currently available to investigate such interactions. *-' (1) This work was supported in part by the Sational Science Foundation ( 2 ) Deceased: N S F CooperaLive Graduate Fellow, 1939-1960 Taken in part from the P h . D . Thesis of Irwin R Fox, Pennsylvania State University, August, 1961 (3) Public Health Service Postdoctoral Research Fellow, 1939-1960 ( 4 ) hI. Karplus and T P. Das, J . C h e m . P h y s , 3 4 , 1683 (1961) ( 5 ) (a) F . Prosser and I < , Goodman, i b i d . , 38, 374 (1963), ib) R W. T a f t , F Prosser. I, G o o d m a n and G . T Davis, i b i d , 3 8 , 380 (1963); ( c ) P C . Lauterbur. Teliahrdion L e l l e i s , 8 , 274 (1961), (d) H Spiesecke and W G Schneider, J . Chew P h y s , 36. 731 (1961); (e) K . I t o , K . Inukai, and T Isobe, Buil. C h e m Soc J a p a n , 33, 315 (1960). (6) R W . T a f t , E Price, I R Fox. 1.C Lewis, K K . Andersen, and G . T Davis, J . A m . C h e m S O L 86, , 709 (1963). ( 7 ) (a) H S.Gutowsky, D W ?*IcCall, B . R. McGarvey, and L . H.
Using this distant but very sensitive probe, new and highly instructive investigations of both the effects of substituent structure and of solvent-substituent interactions on the intramolecular shielding effect of psubstituents are made possible. In the present paper we report the results of extensive studies of both types. Solvent-substituent interactions previously investigated by this technique include proton transfer equilibria,8 Lewis acid-base equilibria,6 hydrogen bonding,6 and polar interactions.fi This paper reports additional results of the last two kinds, especially the latter. The results can apparently be interpreted in terms of the effects of polar solvents on the relative contributions of various types of dipolar resonance forms to the ground electronic state of the fluorobenzene. The effects of structure on the intramolecular shielding of - R p-substituents have been correlated with the u~~ resonance effect parameter from chemical rea~tivities.'~The correlation is extended with the present results and is utilized both to obtain ~ R values O for new substituent groups and to assess the magnitude of solvent effects on UR values. Comparison of the present results with the previously reported corresponding m-substituent effect shielding parametersfi provides new insight into relationships between the inductive and resonance effects of substituents. Experimental Solvents.-Purification viously described.O
of solvents was carried out as pre-
hleyer, i b i d . . 74, 4809 (1952); (b) R. W. T a f t , S. Ehrenson. I . C l,ewis, and R E . Glick, i b i d . , 81,5253 (1959), (c) R . W. T a f t , J P h y s C h r m , 64, 1805 (1960) ( 8 ) (a) I . R Fox, P L. Levins, and R . W T a f t , Tetrahedron Lelters, 7, 249 (1961); (b) R W T a f t and P. L Levins, A n d C h e m , 34, 436 (4982)
MAGNETIC RESONANCE IN p-SUBSTITUTED FLUOROBENZENES
Oct. 20, 1963
SHIELDING PARAMETERS,
f Hp-x,
TABLE I - R p-SUBSTITUTED FLUOROBENZENES'
FOR
Solvent
OCHa
11.70 Cyclohexane Benzene 11.45 Carbon tetrachloride 11.50 Diethyl ether 11.65 ... Tetrahydrofuran 11.45 Dioxane 11.40 Diglyme 11.50 Diethyl maleate 11.40 Acetic anhydride 11.45 Acetone ... Pyridine 11.50 Dimeth ylformamide 11.55 Meth ylformamide 11.45 Benzonitrile 11.55 Nitrobenzene 11.40 Nitromethane 11,35 Dimethyl sulfoxide 11.45 Methanol 11.20 i57c (vol.) a q methanol 10.70d Formic acid 6 95d Trifluoroacetic acid 11.45 Mean value f O . 08 1 .4v dev. a In p.p.m. relative t o fluorobenzene; exptl. error in obtaining mean value.
OCsHs
#-Substituent SCHa CHs
F
C6Ha
6.80 5.40 4.40 3.00 6.60 5.40 6.80 5.40 4 30 2 90 4 80 6.65 5.45 20 4 75 6.40 5.40 25 4 95 2 90 5.50 6.60 4 75 95 6.35 5.30 6.30 25 4 70 5.40 6.25 4 70 6 95 5.45 6.30 .. 2 70 6 90 6.35 5.45 4 75 7 00 4 95 6.20 .. .. 6.35 .. 6.35 4.80 2.10 6.30 7 05 5.55 .. 6.25 6.15 5.30 6 85 4.50 2.80 6.40 7 25 5.55 4.40 I 6.40 Ib 5.60 .. 6.35 .. 6.40 5 20d 4.70 2.75 6.40 7 15 5.45 f0.16 f0.22 0 16 10.13 3t0.07 f = 1 0 . 0 8 . * I designatesinsufficient solubility. e 1.55 in
7 7 7 7 7 7 6 7
3147
45 15 40 25
F1uorobenzenes.-p-Fluorotoluene, p-fluoroaniline, p-fluorophenol, p-chlorofluorobenzene, p-bromofluorobenzene, p-fluorobenzaldehyde, p-fluorobenzotrifluoride, p-fluorobiphenyl, and p-fluoroacetanilide were obtained as previously r e p ~ r t e d . ' ~ The following compounds were obtained from the Pierce Chemical Co., Rockford, 111.: p-nitrofluorobenzene, b . p . 95' (34 m m . ) , n 2 j ~1.5298 (reported'" b.p. 86.6" (14 mm.), 1.5316); pfluoroacetophenone, WZ5D 1.5120 (reported" n Z i r1.5081); ~ and pfluorobenzyl chloride. T h e following compounds were obtained from L . Light and Co., L t d . , Colnbrook, E n g . : p-difluorobenzene, b.p. 87-88" (735 m m . ) , n Z 5 ~1.4388 (reportedll b.p. 88.4' (767 mm.), 1.4423); ethyl p-fluorobenzoate, n z 5 1.4842 ~ (reported13 nZ5'D 1.4707); p-fluoro-a,a,a-trifluoroacetophenone, b.p. 89" (100 m m . ) nZ5D1.4571; p-fluorophenylisothiocyanate, b.p. 66" ( 3 m m . ) (reported14 b.p. 228' (760 m m . ) , m.p. 12'); p-fluorobenzenediazonium fluoroborate, m . p . 162' dec. ; 2methyl-4-fluoro-X,S-dimethylaniline, b.p. 78-80" (17 mm.), which was prepared by formyl methylation of 3-fluoro-&aminotoluene. White Label p-fluorobenzoic acid, m.p. 181-183' (reported15m.p. 182'); p-fluorobenzonitrile, m.p. 35-36' (reported16 m.p. 34.8'); and p-fluoroanisole, n Z 5 1.4851 ~ (reported12 nZ3D 1.4862) were obtained from Eastman Organic Chemicals. The following compounds were prepared as previously depscribed: p-fluorostyrene," p-fluor~phenylmethylcarbinol,~~ fluorophenylhydrazine,18 p-fl~orothiophenol,'~ p-fluorophenyl methyl sulfide,zo p-fluorophenyl methyl sulfone,2o p-fluorobenzenesulfonyl c h l o r ~ d e , 'p-fluorobenzenesulfonic ~ acid,'$ p-fluorobenzenes~lfonamide,'~p-fluoroazobenzene,21 p-fluoroethylbenzene .22 The preparation of the following compounds was carried out similarly t o t h a t reportedg for the m-isomer: p-fluoronitrosobenzene, m.p. 37.5-39.0' (reported1* m.p. 39') in 16% yield; p-fluoro-S,S-dimethylaniline, b . p . 85-86' (18 m m . ) (reported23 86-87' (18 m m . ) ) ; p-fluorotrimethylammonium iodide, m.p. 215216"; p-fluorophenylacetate, b . p . 86" (26 m m . ) ; p-fluoro(9) Reference 6 and references listed therein. (IO) Beilstein, "Handbook of Organic Chemistry," Vol. 5, Band 11, p . 180. (11) M W. Renoll, J . A m . Chem. S o c , 68, 1159 (1946). (12) G Schiemann. Z . physik. C h e m . , 166A, 397 (1931). (13) Beilstein, Vol. 9, Band 11, p . 221 (14) D W Browne and G M . Dyson, J . Chem. S o c . , 3285 (1931); P. Buu-Hoi, D . Xuong, and H . " a m , ibid., 1573 (1955) (15) Beilsrein, Vol 9, Band I , p . 137. (16) Beilstein, Vol 9, Band I. p 138. (17) L A. Brooks, J A m . Chem. S O L 66, , 1295 (1944) (18) G OILh, A . PavlBth, and I. K u h n , Acta Chim , 7 , 65 (1955). ( l Y ) G Olah and R . PavlLth, ibid., 4, 89 (1954). (20) K Zahn and H . Zuber, B e r . , 86, 172 (1953). (21) J . Lichtenberger and R . Thermet, Bull soc. chim , (51 18, 318 (1951) (22) 0 R . Quayle and E E Reid, J . A m . Chem Soc., 47, 2357 (1925). ( 2 3 ) N J Leonard and L. E Sulton, ibid., T O , 1564 (1948).
Br
2 2 2 2 2 2 2 2 2 2 2 2
60 50 50 35 20 50 15 00 00 05 25 00
2 20
--OCFa
2 2 2 1
25 10 10 85
2 10
1 55 1 60 1 75
1 85 e
2 00 2 10 2 10
1 55
2 25 2 1 2 25 1 0 O 16 acetonitrile.
00 85 21 Excluded
diphenyl ether, b.p. 127' (17 m m . ) (reported2' b.p. 254-255' (860 m m . ) ) ; p-fluoroiodobenzene, b.p. 65-68" (13 m m . ) (re; portedz467-69" (11 m m . ) ) ; p-fluorobenzopl chloride, b . p . 85-86 (23 mm.) (reported% b.p. 104' (38 m m . ) ) ; p-fluorobenzamide, m.p. 155-156'; p-fluorophenyl benzoate, m.p. 63.564.5° ; p-fluorophenyl methyl sulfoxide, b.p. 114-116' ( 3 m m . ) ; ethyl p-fluorobenzenesulfonate, b.p. 123' (2 m m . ) ) ; p-fluoroaniline hydrochloride; p-fluorobenzylamine hydrochloride, m . p . 279281'; p-fluorobenzyl cyanide, b.p. 112-114' (14 m m . ) ; p fluorophenylacetic acid, m . p . 83" (reportedz6 m.p. 85'); pfluorobenzyl alcohol, b.p. 95" (12 m m . ) ; p-fluorophenyl sulfur pentafluoride, b.p. 103' (760 m m . ) ; p-fluorobenzoyl fluoride, b.p. 148-148.5' (750 m m . ) ; p-fluorobenzenesulfonyl fluoride, b.p. 179-180" (746 mm .); 1-(p-fluorophenyl)-2-nitroethene,m . p . 100-101". Anal. Calcd. for C8H6FNO?: C, 57.47; €3, 3.62. Found: C, 57.23; H , 3.37. p-Fluorophenyl trifluoroacetate was prepared by Dr. P. E . Peterson from the reaction of equimolar quantities of p-fluorophenol and trifluoroacetic anhydride.l' The phenol was heated alone at 100" and the anhydride was added from a dropping funnel cooled with Dry Ice to prevent evaporation of the anhydride. The reaction mixture was stirred and heated t o rapid reflux. The trifluoroacetic acid formed was distilled from the reaction mixture and the product fractionated under vacuum in a spinning band column; b.p. 83" (116 mm.). The infrared spectrum in CClr showed t h e typical strong band a t 1780 cm.-l. p-Fluorophenylacetylene .-Bromine was added dropwise to a solution of p-fluorostyrene in carbon tetrachloride until the bromine color persisted. The solution was concentrated in vacuo t o a n oil. Alcoholic potassium hydroxide was added and the mixture refluxed for 1 hr. The mixture was poured onto ice and extracted with three 50-ml. portions of hexane. The organic extracts were washed with a saturated salt solution and dried over Drierite. Solvent was removed and t h e residue distille: a t reduced pressure to give a 199; yield of product, b.p. 61-65 (50 m m . ) . p-Fluorophenyltrimethylsilanewas kindly prepared by Dr. A . Konstam from t h e reaction of the Grignard reagent of p-fluorobromobenzene with trimethylsilyl chloride p-Fluoro-a,a,a-trifluoroanisolewas kindly provided by D r . W . A . Sheppard. p-Fluorobenzyltrimethylsilanewas kindly provided by Dr . A . Konstam. p-Fluorophenylboronic acid was kindly provided by Prof. H. G. Kuivila. Procedure.--Xll measurements (unless otherwise noted) were made with approximately 57; (vol.) solutions of p-fluorobenzene. The shielding parameters reported are relative t o fluorobenzene (24) (25) (26) (27)
K . W. F. Kohlrausrh and G . P Ypsilanti, Monalsh , 66, 7 8 5 (1936) J . B Cohen, J Chem. Soc., 1058 (1911) G. Olah. e2 al., J . Org. Chem., as, 879 (1957) R . F. Clark and J . H Simons, J . A m Chrm Soc , 7 6 , 630.5 (1953)
:I: 14s
TAFT,PRICE,F O X , LEWIS,ANDERSEN,AND DAVIS
Vol. 85
TABLE I1 SHIELDING PARAMETERS, Solvent
CF3
FOR
SFK-CH=CHXOz
3-Methylpentane 5 20 4 95 Cyclohexane 5 05 5 35 Benzene 5 15 Carbon tetrachloride 5 15 5 50 Dioxane 5 15 5 50 Diethyl ether 5 45 5 90 Tetrahydrofuran 5 65 6 00 Ethyl acetate Diglynie 5 75 Chlorobenzene 5 50 5 95 Acetic acid Diethyl maleate 5 85 6 35 Methanol 5 90 6 35 Acetone 6 35 5 80 Acetic anhydride 5 80 Pyridine 5 70 6 15 Dimeth ylformamide 6 40 5 90 ,l~onomethylformamide 6 25 5 70 Beiizonitrile Nitrobenzene 5 70 6 25 .%cetonitrile 5 90 Sitromethane 5 75 6 35 Dimethyl sulfoxide 5 90 6 40 Methylene chloride Chloroform Formarnide Formic acid 5 35 6 00 ( o r b (vol.) aq. methanol 6 05 Ib %,2,3.3-Tetrafluoropropanol 5 45 6 05 5 35 Trifluoroacetic acid 5 95 I n p,p,rn. relative t o fluorobenzene; exptl. error occurs
--
- fH p - - x ,
fR
COCHB
P-SUBSTITUTED CN
8 8 9 9 8 9 9 9 9 9
FI.UOROBENZENES~ NO
NOz
COF
9.00 9.20 10 50 5.80 11 50 9.45 .. 11 10 9.55 5.40 9.10 11 60 .. 9.65 11 35 , . 9.75 12 00 5.80 9.85 12 00 .. 10.00 .. 10.05 11 95 .. ... 10.00 12 25 6.10 6.75 10.25 12 40 9 TO 6.20 7.65 10 25 12 45 10,35 5.95 6.55 9 70 10.10 12 45 .. 6.80 9 75 10.35 12 65 6.10 6.80 9 80 10,25 12 55 6.70 9 80 5.95 10.30 12 85 .. 7.00 9 90 10.30 12 75 .. 9 90 6.90 10.45 12 65 6.40 9 90 6.85 10.50 12 65 6.15 ... 9 90 10.35 12 80 6.30 6.95 10.55 9 95 12 90 5.95 6.80 9 85 13 20 10.30 .. ,.. 10.85 12 90 .. ... 11.05 12 95 , . 7.65 10 45 13 05 11.oo 6.80 11 15 9.55 11 .00 Rb .. 8.50 11 00 11.20 13 70 .. 9.30 11 60 11,85 14 80 12.80 13 75 14.05 R = f0.08. I designates insufficient solubility; R .. ..
6.05 6.10 6.50 6.60 6.15 6.40 6.35 6.50 6.55 6.70
as an internal standard. Under these conditions t h e shielding parameters correspond t o the intramolecular effect of the p substituent a t essentially infinite dilution in the indicated solvent.g Detailed procedures and relationships are the same as t h a t previously r e p ~ r t e d . ~
65 95 05 20 95 20 45 45 45 60
COCFs
11.15 11.35 11.40 ... . . .
... 11.70 ... , . .
11.70 12.00 12.10 11 85 11.95 11.85 11.95 ...
, . .
12.00 12.45 12.35 11.80 12.65
12.20 12.35 12.50 12.20 12.70 12.90 13.00
, . .
12.85 ... ... 13.00 13.25 K 12.95 13.15 12.90 12.95
... ...
...
12.05 12.00 12.15 I< ... 12.50
SO*F
...
, . .
... ...
13.05 13.40 13.50 13.35 13.45 13.
