WILLIAMA. SHEPPARD
131-1 [COXTRIBUTIOS No. 819 FROM
THE
Vol. 85
CENTRAL RESEARCH DEPARTMENT, EXPERIMESTAL STATION,E. I. WILMINGTON 98, DEL.]
DU
POXTDE SEMOURS AND C o . ,
The Effect of Fluorine Substitution on the Electronic Properties of Alkoxy, Alkylthio and Alkylsulfonyl Groups1 BY WILLIAMA. SHEPPARD RECEIVEDOCTOBER18, 1962 The effect of fluorine substitution on the properties of alkoxy, alkylthio and alkylsulfonyl groups has been determined quantitatively from pK, measurements on anilines, phenols and benzoic acids containing the substituents: OCFs, OCF2CF3, OCF?CF*H,SCFs, SCF2CF2Hand S02CF3. The OR( groups are like halogen and strongly deactivate by inductive electron withdrawal (urn,0.34 t o 0.48) but donate electrons by resonance ( u p , 0.21 to 0.35). The SRr groups deactivate by both inductive and resonance niechanisms (om,0.38 to 0.46; up, 0.47 t o 0.64). The origin of the +R character of the SRr groups is discussed in terms of d-orbital participation by sulfur: The SO?CF3 group was found to be the strongest neutral electron-withdrawing group known with urn 1.00 and up 1.65 (from pK,'s of anilinium ions). Dipole moment data for the OCF3 group are presented and discus&
Unusual properties result from accumulation of fluorine in organic molecules. However, no significant amount of quantitative data useful for explanation, prediction and development of theories has been collected for simple fluorinated substituents. One approach to this problem is determination, by classical methods, of substituent constants and dipole moments of fluorinated groups in aromatic systems. Such studies have been reported for the trifluoromethyl and, recently, the sulfur pentafluoride As part of a study of fluorinated systems, synthetic routes to aryl perfluoroalkyl ethers and sulfides have been examined and the new, improved syntheses dev e l ~ p e d . ' , ~As , ~ a consequence, the preparation of
literature.'^^*^,^ All compounds used for measurements were purified by distillation (center-cut from constant refractive index fractions) and/or recrystallized and sublimed to constant melting point. The compounds were shown t o contain no detectable amount of impurities by analysis, spectral studies (infrared, ultraviolet and F19and H1 n.m.r.), and, where possible, vapor phase chromatography. Ionization Constant Measurements.-Apparent ionization constants of m- and p-substituted benzoic acids were obtained a t 25' in 50% water-50% ethyl alcohol ( b y volume) as described by Roberts and co-workers** using a Beckman pH meter, model G, with glass electrodes. The measured pK,'s are reported in Table I and are the average of two to five separate determinations that agreed within f 0 . 0 2 unit (generally within fO.01). Ionization constants of anilinium ions ( p K , of the reaction KNH?+ F? K S H ? H+) were determined a t 25" in aqueous solution by spectrophotometric measurements in the ultraviolet
+
TABLE I
pK, MEASUREMENTS Substituent
( a ) Benzoic acids, water-ethanol, 2 5 . 0 O X C ~ H I C O ~ H X C ~ H I C O ?f~ H e ' '
x
PK,
io%
( b ) Anilines, water, 2.5' XC~H?SH@e X C ~ H ~ N H~~ @
*
+
hnx, mir .
P&
h
(c) Phenols, water, 25' XCsHaOH S XCsHa03 HBC haX, mir PKn
+
4 . 56d .. 9 . 91gd 2.44 .. .. iizeta 5 . 1 5 (5.19)' 3.25 287 9.02 paw 5.19 (5.26)' 3.83, 3.80' 282 9.35 .. 3.24 28i 8.95 OCF2CF3 metu paru , . 3.78 292 9.32 OCF*CFIH iizeta 5.22 3.38 .. .. para 5.34 3.98 .. SCF? nieta 5 . 1 3 (5.21)' 3.30 303 8.97 para 4 . 9 8 (5.17)' 2.80, 2.76' 260 8.66 SCFXFzH metu 5.15 3.39 , . .. puru 4.98 2.8i .. .. SO?CF3wzeta . . (4.54T 1 . '79 325h 7.87 para 4 . 3 5 (4.17)" 287 -0.Olfl 287 6.79 P = +1.467 (ref. 2 ) . * p = +2.767 (ref. 9 ) . p = 2.229 (ref. 10). Literature value; see respective footnotes a, b, c. e V a ~ u e s in parentheses reported in literature (ref. 6). f Duplicate measurement. fl Anilinium ion too acidic for standard buffers, value by m-Hydroxyphenyl trifluorornethyl sulfone was prepared by diazoextrapolation from series of HCI buffered solutions; see ref. 10. tization and hydrolysis of corresponding aniline (ref. i a ) ; m.p. '74.5-76.2". Anal. Calcd. for C7HjF303S: C, 37.2; H , 2.04; F, 25.2. Found: C, 3 i . 3 ; H, 2.04; F, 25.1, 25.2.
Hd OCFa ortho
5.iId ..
the compounds needed for measurements was greatly facilitated. When this work was essentially complete, pK, measurements on benzoic acids containing the OCF3, SCF3 and S02CF3groups were published,e but no analysis or discussion of these results was presented.
..
280 282 285 281 284 282 285 294 255 295 255 314
.
I
Materials.-The compounds employed in this study are described in other publications from this Laboratory or in the __
region employing a Cary recording spectrophotometer with thermostated cell holder. ,4detailed description of this procedure, which employs a series of buffered solutions, was recently given by B r y ~ o n . ~In making these measurements a t least three results, using buffer solutions within 0.4 pK, unit of the anilinium ion, were averaged, and this average value is given in Table I. Agreement of determinations in repeat experiments and of the pK,'s of reference anilines were within the f 0 . 0 3 pK, unit precision ascribed to the m e t h ~ d . ~ T h e ionization constants of the phenols in aqueous solution at 25' were determined by the spectrophotometric methodlo using
(1) This work was presented, in part, in a communication: W. A. ShepI>ard,J . A m . Chem. Soc., 8 3 , 4860 (1901). ( 2 ) J. 11. Roberts, R . L. Webb a n d E. A. McElhill, ibid., 72, 408 (1450). (3) W. A . Sheppard, ibid., 84, 3072 (1962). f-1) W. A. Sheppard, publications in preparation. ( 5 ) 11. C. England, I-. R . hlelby, M. A. Dietrich a n d R . V . Lindsey, Jr., ./. A m . C h e m SOL.,82, 5116 (19fiO). ( 6 ) l,, M. Yagupolskii and I,. hl. Yagupolskaya, Pvoc. Acad. Sci. U S S R (f':ngl. Transl.), 134, 1207 (1400)
(7) (a) 1,. M . Yagupolskii a n d M . S. Marenets, J . Gen. Chem. U S S R (Engl. Transl.), 24, 885 (1954); (b) L. M. Yagupolskii a n d M. S. Marenets. ibid., 26, 99 (1956); (c) L. M. Yagupolskii a n d B. E. Gruz, i b i d . , 31, 1219 (1961). ( 8 ) J. D . Roberts, E. A. McElhill a n d R . Armstrong, J . A m . Chem. S O L . , 71, 2923 (1949). (9) A. Bryson, ibid.,82, 4858 (1960). (10) A . E . Biggs a n d R . A. Robinson, J . Chem. Soc., 388 (1901).
Experimental ~~
ELECTRONIC PROPERTIES OF ALKOXY AND ALKYLTHIO GROUPS TABLE 11 DIELECTRICOONSTASTS, DESSITIES A N D POLARIZATIONS IN BESZESE AT 25' F? t D. Pz Phenyl trifluoromethyl ether (DZ5aj1.2218, n Z 51.4038) ~ 0 00000 2.2740 0.8722 ( P I2 6 . i q 3 ) " 020148 2 4316 ,8851 143,7 041031 2,5813 8950 141.5 ,9041 139.0 60363 2,7193 p-Sitrophenyl trifluoromethyl ether (Oz52& 1.4458, nZ5D1.4652) 0.021065 2,4156 0,8937 133.6 .041019 2.5399 ,9100 132.6 9271 131.0 061675 2.6683 p-Aminophenyl trifluoromethyl ether (D2%51.3212, n Z 5 D 1.4609) 0.020243 2.6954 0.8880 317,23 ,9010 294.48 040753 3.1064 282.66 3.5184 ,9125 059891 p-Bromophenyl trifluoromethyl ether 2,3000 0,8985 60,3i 2,3255 ,9237 61.58 040803 ,061026 2.3482 ,9469 61.71
i ) .019501
1,4-Bis-(trifluoromethoxy )-benzene 0.020333 2,2779 0.8951 51.95 2,2796 0,9151 53.80 0.042181 ' I Benzene standardization liquid ; literature values, used for each determination.
TABLE I11 MGL.4R KEFRACTIGS, POLARIZATION A N D DIPGLEMGMESTS P" Compound
MRo
(2.5')
32.42 1 4 6 . 0 3 9 . 6 0 135 2 3 6 . 7 7 331 0 40. TO 61,2% l,-l-Bis-(trifluoromethoxy)-benzene 3 9 . 5 7 52 87
Phenyl trifluoromethyl ether p-Siirophenyl trifluoromethyl ether p-Aminophenyl trifluoromethyl ether p-Bromophenyl trifluoromethyl ether
bi.l (Debye) I
2.36 2.16 3.79 1 00 0 87
borax-buffered solutions. T h e results are reported in Table I. h precision of 1 0 . 0 1 p K , unit is ascribed t o this method,'O and in duplicate t o quadruplicate measurements on the phenols this precision was in general attained. However, the measurements on the phenols with p-CF3S and ~ - C F ~ S O groups Y were only precise to f 0 . 0 3 pK, unit, possibly because of relatively poor hydrolytic stability of these phenols in the basic medium.*! Dipole Moments.-The dipole moments were determined by literature methods3 employing a \X7issenshaftlick Technische Werkestatten Dipolmeter type DM 01, Cell D F L I (20-ml. volume). The data are given in Tables I1 and 111. A precision of f 0 . 0 2 D. is ascribed to the dipole moment results given in Table 111.
Results and Discussion The Hammett u-constants for the perfluoroalkoxy, perfluoroalkylthio and trifluoromethylsulfonyl groups, as calculated from the ionization constant data, are given in Table IVa. To assist in analyzing the results, the ionization constant data were also used to calculate the inductive and resonance parameters, QI and U R , as proposed by Taft",I2 and the F and parameters described by Dewar.I3 For comparison, the substituent parameters from literature value^^^-'^ of selected well-known substituents and hydrocarbon analogs of the fluorinated groups are presented in Table IVb. 111) K \V, 'l'aft, J r , "Separation of Polar, Steric and Resonance ISffcct i n Reactions," in A I . S S e w m a n ( E d i t o r ) , "Steric Effects in Organic Chemist r y . ' John Wiley a n d Sons, I n c . . New York. N Y ,1950, (121 R. N'. T a f t , Jr.. a n d I . C. I,ewis. J . A i n . C h r m Sor.. 81. .5:34:3 ( 1 9 5 9 ) ; R . R'. l ' a f t , J r . , J . P h y s . ChPin , 64, 180.5 (19iiO). i 1 3 ) XI. J . S 1)ewar and P. J. Grisdale, J . A m Cheiii. Sor , 84, 3.%9 a n d 3348 (1962). (14) H. H. Jaffe, Cheiii, R e c , 63, 222 (1933) ( 1 5 ) F. G . Bordwell a n d P. J . Barton, J . A n t . C h e i i i . .Sot.. 7 8 , 85k (l!l5li); 1;. G . Brirdwell and G . C Copper, ibid , 74, 1 0 3 (3982). i l l ' , ) R. R. Beishline. J . O r y C k t m , 2 6 , 25:Cj ( I u r i l ) .
1315
From an examination of the u-parameters in Table IV, it is immediately apparent t h a t the perfluoroalkoxy groups behave like alkoxy groups and withdraw electrons inductively but supply them by resonance. 17 However, the perfluoroalkyl group enhances the inductive withdrawal of electrons by oxygen to the extent t h a t deactivation of the aromatic system occurs to the same, or greater, extent than for the halogens. Also, electron release by the resonance mechanism is diminished over that for an alkoxy group, again to an extent comparable to t h a t for the halogens. On this basis the perfluoroalkoxy groups may be classed as halogen-like (or even as ''super-halogen" since over-all they deactivate to a greater extent than halogens), It is also noted, from comparison of the a-parameters for the OCF3, OCF2CF3 and O C F Z C F ~ H groups, that the inductive effects of F and CF3 are almost the same but, as expected, significantly greater than that of a CF2H group. This result is predicted from the similar QI values for the F and CF3 groups. The perfluoroalkylthio groups have urn values essentially the same as the corresponding perfluoroalkoxy groups, but unexpectedly the up parameters have larger positive values with an added positive increment of 0.14 for the bp calculated from the anilinium ions over that from the benzoic acids. This result is indicative of a + R substituent. Recently, BeishlinelG analyzed the ionization constant data for the SCH3, 0
I1
SCCH3 and SCN groups in terms of Taft's U R parameters and presented arguments for expansion of the valence shell of sulfur in the thioacetate and thiocyanate groups.1B The QR parameters for the SCF3 and SCFzCFzHgroups have significantly larger values (0.14 to 0.22) than those for any other divalent sulfur groups.16 The major contributing resonance forms are
I I1 I11 and the u-parameter data provide strong evidence for large contribution of form I11 in the equilibrium acidity constant measurements. The contribution of resonance form I must be minor but becomes significant in the transition state for substitution of the ring by an electrophilic reagent since orientation is ortho-para and not r n e t ~ . ' ~For the SCH3 group, like OCH3, the major contribution must be from form I rather than form 111. Another possible series of contributing resonance forms is represented by IV which involves fluoride ion "no-bond'' structures analogous to the
~
_
_
IV
(17) This result is predicted f r o m t h e ortho-pava
directing influence of the OCFI group in electrophilic substitution; see ref. 4 a n d L. XI. Tiagupolskii a n d V. I . Troitskaya, J . Gen. C h e m C S S R , Engl. Transl., 31, 815 (1901). Dr. H . K . Hall, Jr., suggested t h a t there is n o positive evidence for electron donation b y O R ( groups in t h e ground states a n d t h a t urn is greater t h a n c p because of a direct field e f f e c t which falls off with distance. Complete lack of oitho substitution in nitration a n d bromination* could be cited a s evidence t o support this argument. (IS) T h e evidence for o u t e r shell expansion in sulfur was recently summarized: G. Cilento, Chem. Reo., 60, 14G (1960). (19) Nitration was reported t o give only ortho-pauo substitution a s determined b y classical methods of chemical analysis.7b This result was checked in this Laboratory using spectral (infrared a n d F19 n . m . r . ) methods, a n d n o mela substitution product could be detected, Gas chromatographic analysis was n o t succescful since synthetic mixtures of m- a n d p-nitrophenyl trifluriromethyl sulfides could n o t be separated on a wide range of columns normally useful for separating aromatic substituent isomers or fluorocarbons.
\VILLIAM A. SHEPPARD
1316
VOl. 85
TABLE IYa SUBSTITUENT PARAMETERS FOR FLUORINATED ALKOXY,ALKYLTHIOASD METHYLSULFOXYL GROUPS
OCF3
,--Hammett"-a*
UP
--Taft 01
B d 0.40 .47 P .40
0.35 .27 .26
0.51 .50 ,39
..
..
..
.48 .43
.28 .27
.52 .42
.25
.39 ,45
x B X P
OCFzCFa
-
DewarC
b----
-0.13 .23 - .15
F'
.M
17
OR
0.00
0.69
Ai'
0.i0
0.00
-
....
....
, .
..
....
-
.25 - .17
-
B A P
.34 .43
..
.21 .,
SCFr
B A P
.40 .46 .43
.50 .64 .5T
.31 .40 .39
.1i .22 .14
,69
1.09
. 71
1.42
SCFzCFzH
B A .
.38 .42
,47 .61
20 .21
,66
0.98
,68
1.26
P
. .
. .
.29 .3i ..
OCFzCF2H
.14
.59
-0.31
.58
-0.40
- .25
..
....
....
B
( .79) .93 .69 ,22 1.37 2.45 1.42 3.15 x 1.00 1.65 .84 .73 P 0.92 1.36 .84 ,19 Calculated using Hammett equation from ionization constant data and p-values given in Table I. b Calculated as described in ref. 12 using values of P I S l . 5 0 , a 0.42 for ionization of benzoic acids; P I +2.90, 01 0.15 for ionization of anilinium ions; and PI +2.36, CY 0.07 for ionization of phenols. Calculated as reported in ref. 13 using values of ~1343,714 2 ; Q I S 0, l/7; r13 +0.009, x u -0.102. Data calculated from ionization constants of B benzoic acids, A anilinium ions, P phenols.
SOzCF;
TABLE IVb LITERATURE VALUESOF SUBSTITUENT COXSTANTS FROM p K a Pm
CF3 SFj OSFI" F
c1 OCHa SCHI 0
0.43 .61 ..
WP
0.54 (0.65)* .68 ( 0 . 8 6 ) .44
.34 .37 .ll .14
.06 .23 - .2i - .01
38
.29
0.39
.55
.. .45 .12 .21 .22
OF
BENZOICACIDS ASD ANILINIUMION$
al?
17
0.12(0.18)b . l l (0.27) .... - .40
0.74 1 .05
a1
- .25
-
.4i .28
.. 0.58 .65 .20 .26 .68
.M
1.12 1. 0 i .... -1.61 -0.70
F'
0.77 1.08
' 1.46 1.39
M
..
....
-0.91
0.56 ,63 13 .24
-2.10 -0.91 -3.36 -1.19
-4.55
.56
-5.93
-2.58
il
OCCH,
0
Ii
SCCHI 70 0.91 .37 ,42 .32 .10 .68 0.70 1.83 S02CHa ,65 .73 (1.05) 1.40 1.08 .62 .16 (0.51) 1.04 1.26 1 48 1.23 1.14 KO2 .71 0.78(1.27) .68 .14 (0.67) Values taken from ref. 3, 11 to 16; see also footnote c. b Values in parentheses are from data on ionization of anilinium ions; all other values are calculated from the ionization of benzoic acids. J. R . Case, R . Price, N.H. Ray, H . L. Roberts and J . Wright, J . Chem. Soc., 2107 (1962). This group was shown to be para directing to electrophilic substitution. The value of urn was not measured but would be predicted to be about 0.50.
proposal advanced to explain the resonance effect of a CF3 group.? However, the almost identical u-parameters of the SCF3 and CF3 groups serve as evidence against any major contribution from form IV since appreciable diminution of the effect for the CF3 would be expected on transmittal through sulfur. (It is possible, but unlikely, t h a t the resonance effect of the CF3 group may be enhanced if steric interactions with hydrogens are removed when a sulfur separates the group from the ring.) Another argument is that the small decrease in the deactivating effect of a SCFzCFzH over the SCF3 group resides chiefly in the inductive but not in the resonance portion (as noted from comparison of u-parameters of SCFa and SCF2CF2H with those of OCF3 and O C F ~ C F Z Hand UR values for SCFzCFzH compared to SCF3). It is expected that this type of resonance would be decreased by loss Of one contributing resonance form through substitution of a CFzH group for F. As a final point in discussion of the d-orbital particiI'ation by the aspect be re\rieWed. Craig and hfagnusson2" have shown from a
study of overlap integrals t h a t the d-orbitals of sulfur and phosphorus in the free atoms are too weakly bound and diffuse to contribute significantly to bond energy. However, polarization resulting from substitution of the sulfur with electron-withdrawing groups contracts these orbitals and adapts them for better bonding. In particular, the bonding in SFs, where two d-orbitals are involved in hybrid bonds, was discussed. On the basis of these arguments it is reasonable to conclude that the strong electron-withdrawing character of the CF3 group induces polarization of the sulfur atom and contracts the d-orbitals SO that d-ir overlap can make a significant contribution to the bonding of sulfur in the molecule. The u-parameters determined for the SOzCFa group indicate that it is the strongest, neutral electronwithdrawing group that has been measured.21 The (20) D . P. Craig a n d I,. A , hfagnusson, J . C h e m . S O L . , 1895 (1956); see also I). W. J . Cruikshank, ibid., 8486 (1961), for a recent discussion of t h e role of 3 d-orbitals in bonding of sulfur t o oxygen or nitrogen. (21) T h e diazonium group, iYz@, is t h e only one reported to have larger positive a parameters, a,, = 1.8 + 0 . 5 a n d a,,, = 1.13f r o m ionization of t h e benzoic acid derivative; R value of approximately 3 was determined f r o m
1317
ELECTRONIC PROPERTIES OF ALKOXY ASD ALKYLTHIO GROUPS
May 5 , 1963
calculated values of UI and UR for this group suggest that, like the nitro group, the strong electron-withdrawing power originates from a significant resonance interaction as well as by the inductive mechanism. The S02CF3group should be a useful addition to the series of strong electron-withdrawing substituents; a possible advantage of this group is improved solubility in organic solvents compared to the corresponding nitroaromatics. On the other hand, it is anticipated that CF3S023 will be an excellent leaving group, although no indication of loss of CF3S023 was observed in the aromatic series under a range of oxidizing and reducing conditions or in strong bases or acids.22 The F and -If parameters were recently proposed by Dewar and Grisdale13 as a measure of the modes of transmission of the primary effects of a substituent to the reaction center. The F parameter is a measure of the primary inductive effect, which is attributed chiefly to a field effect operating through space, while the u- and a-inductive effects operating through the molecule make only minor contributions. The M parameter is a measure of the combined n-inductivemesomeric effect of the substituent. These parameters may be calculated from u-constants by use of the equations uij = F/rij u,j =
+ Mqi, +
F'/~ii
However, the -21 parameters for the SRf groups have relatively large positive values approaching those of the NO2, CF3 and SF5groups, again indicating that the resonance and inductive effects in the n-system operate in the same direction and reinforce the field effect. The -16 parameter for the SOzCF3 groups has a larger positive value than t h a t of any other substituent tabulated, l 3 indicating the very strong electron-withdrawing effect that this substituent exerts on the nsystem in conjunction with the large field effect. Another method of quantitatively comparing substituents is by electrophilic (.+) substituent constants as proposed by Brown and O k a m ~ t o . *Although ~ U+ values have not been determined for the perfluorinated groups, agreement between u + and u is expected for the ORf groups, but large discrepancies must occur between these parameters for the SRr groups (see ref. 19). The dipole moment of phenyl trifluoromethyl ether is 2.36 with the negative end of the dipole directed toward the OCF3 groups as determined for the moments of the bromo-, nitro- and aminophenyl trifluoromethyl ethers (see Table V). This moment is almost double that for anisole and very closely approaches that for benzotrifluoride. TABLE 1'
M'rrij
DIPOLE MOMESTSAT 25.0'
r1j is the distance between atoms i and j in a molecule q i j is the formal charge a t position j produced by attaching the group -CH& a t position i xij is the atom-atom polarizability of atoms i and j
In this treatment, the electromeric effect from resonance interaction is not considered. This effect is unimportant unless there is mutual conjugation between the substituent and the reaction center. Since this factor does not affect the properties of aromatic carboxylic acids but can be very important with a NH:! group, the F and M parameters are calculated only from the benzoic acid data. As expected, all of the fluorinated substituents exhibit a strong field effect and have large positive values of F (0.6 to 1.4). I n particular, it should be noted that the field effect for the ORr and SRf groups are essentially the same and are also of similar magnitude to that for CFa, the halogens, acetoxy and thioacetoxy groups. A significantly larger value of F is calculated for the SF5 group (1.05) and is similar to that for cyano (0.97) and methylsulfonyl (1.04). The F-value of 1.37 for the S O Z C F group ~ is much greater than that for nitro and, according to the compilation of F-values given by Dewar and Grisdale,I3 is exceeded only by groups bearing a formal positive charge, (CH3)&@ (1.52) and (CHB)BS@ (1.73). For the simple fluorinated substituents, the F-value per fluorine is relatively constant: CF3. 0.24; SF5, 0.21; OCF3, 0.22; SCF3, 0.22. This proportionality of F-values with number of fluorines is expected if the F parameter is chiefly a measure of field effect provided that the distances and electronegativity of the substituent atoms (carbon or sulfur) do not differ grossly. The ;li'-values for the ORr groups are close to zero or slightly negative indicating that the inductive and resonance effects operating through the a-system are in opposition and effectively counterbalance one another. ionization of t h e p-phenol a n d p-aniline derivatives; E. S. Lewis a n d M . U. Johnson, J . A n i . Chem. Soc., 81, 2070 (1959). T h e trinitromethyl group, C(NOd3, was previously reported t o have t h e largest c,, value (0.82) measured for a n y electrically neutral group. J. Hine and W . C . Bailey, Jr., J . O i g . Chem., 26, 2098 (1961). (22) P-Hydroxyphenyl trifluoromethyl sulfone is reported t o he hydrolyzed o n t r e a t m e n t with warm dilute base."' I n contrast, p-hydroxyphenyl verfluoroalkyl sulfides a n d ethers are more stable under these conditions.
( I N DEBYE) Calculated valuesVector sums with correction Algebraic for angular position of OCXa sum group (angle in parentheses)a
--
where
Compound
Measured value
2.36 2.60 1.52 C&BrC P-BrC6HaOCF3 1.00 CcHjNHpC 1.53 P-HzKCsHaOCFs 3.79 C6H5XOpC 3.95 2.16 ~-0~SC&0Cl'a P - ( C F B O ) P C ~ H I 0.81 1.2 CtjHjOCHs' ~ - 0 2 ~ C 6 H 4 0 C 1 & C4.75 ~ - ( C H ~ O ) B C & I ' 1.73
-
CeHsOCF3
C~H~CF~"
0.84
l.OO(163O)
3.88 1.62 0
2.15(153') 0 . 8 0 (160')
2.10
4 , 3 5 (90")
0
a Calculated assuming restricted rotation so that the OCX3 group lies in the plane of the ring; see discussion. * Ref. 2 . Ref. 24.
From the dipole moments of the dimethyl ether of hydroquinone and other substituted anisoles and phenols it has been shown that the OCH3 and OH groups lie a t approximately 90' from the axis through the plane of the ring (C-0-C or C-0-H angle).24 Since the angles must be taken into account, the OCH3 group cannot rotate freely and must lie in the plane of the ring. The coplanarity of the OCH3 group with the aromatic ring arises from resonance contribution by the quinoidal forms. %1
8
8
0
The moment of 0.81 11.for p-bis-(trifluoromethoxy)benzene shows that the C-0-C angle for the OCF3 group is also less than 180'. The smaller dipole value (23) H. C. Brown a n d Y. Okamoto, J . A m . Chem. Soc., 79, 1913 (1957); Y. Okamoto a n d H . C . Brown, J . Org. Chem., 22, 48.5 (1957). (24) (a) W. Hiickel, "Theoretical Principles of Organic Chemistry," Vol. 2 , Elsevier Press, Kew York, h-.Y., 1968, Chapter XII, pp. 127 and 137; (b) G. W. Wheland, "Resonance in Organic Chemistry," John Wiley and Sons, I n c . , New York, N. Y., 19.55, Chapter 5 , pp, 238-243.
131s
J. K . STILLE, P. CASSIDY A N D L. PLUMXER
compared to t h a t for the dimethyl ether of hydroquinone and also the size of the moments for p-bromophenyl and p-nitrophenyl trifluoromethyl ethers (compared to the values for vector sum and to those of the corresponding anisoles) can be explained in one of two ways: (1) The angle for the OCF3 group with the axis in the plane of the ring is closer to 150' than to 90'; or ( 2 ) the angle is still near 90' but the OCF3 group rotates almost freely so t h a t it is restricted to only a small extent to a coplanar position with the aromatic ring. This latter explanation is considered most reasonable since the resonance contribution by the quinoidal forms is grossly reduced by the electron-withdrawing
[COSTRIBUTION FROM THE
Vol. 85
effect of the CF3 group (note t h a t for OCF3 U R ranges from -0.13 to -0.23 while for OCH3, U R is -0.47). a'ith the precision of the data in mind, a tenuous conclusion can be drawn t h a t the @-nitro group behaves as expected and helps to stabilize the quinoidal form cornpar,-d to the p-bromo group. A more quantitative analysis of the dipole moment results is not appropriate in view of the problems discussed in ref. 24b. Acknowledgments.-n'e wish to acknowledge with thanks the assistance of Xiss Ellen It'allace and Mrs. Flora Youngken in carrying out the ionization constant measurements and of I l r . Charles It'ortz for the dipole moment measurements.
DEPARTMENT O F CHEMISTRY, USIVERSITY OF IOTVA,
IOIVA CITY, IOIYA]
Photodecomposition of p-Benzoquinone Diazides : Copolymerization with Tetrahydrofuran's2 BY J. K . STILLE, P. CASSIDYA N D L. PLUMIMER RECEIVED NOVEMBER 8, 1962 p-Benzoquinone diazide and 2,6-dimethyl-p-benzoquinonediazide decompose in various solvents in the presence of heat or light with the loss of nitrogen t o give different products. Whereas the light-catalyzed decomposition in many solvents affords substituted phenols, in tetrahydrofuran nearly a l : l Zopolymer with solvent is obtained. The polymer, which contains the polyether structure, is probably an alternating copolymer and is formed through a cationic ring opening of tetrahydrofuran.
Introduction The potential thermal stability of polymers containing the poly-(phenylene oxide) chain have made them the object of recent investigation with respect to the methods of formation of such a p~lyrner.~-'O Compounds containing the p-benzoquinone diazide structure decompose readily in the presence of light or heat and should lend themselves readily to the preparation of poly-(phenylene oxides), However, decomposition in the solvents benzene, anisole, and N,N-dimethylaniline provides the substituted hydroxybiphenyl derivatives, B'11,12 while the photodecomposition of solid p-benzoquinone gives insoluble polymer. l 1 The thermal decomposition of 2,B-dibromo-p-benzoquinone diazide in chlorobenzene affords a polymer containing the expected recurring unit as well as incorporated ~ o l v e n t . ~In this decomposition, a mildly electrophilic diradical has been proposed as the reactive intermediate, 3, '? Results The nionomers p-benzoquinone diazide (I) and 2,Gdimethyl-p-benzoquinone diazide (11) were prepared in anhydrous (red) and tetrahydrated (yellow) forms which could not be distinguished spectrally except for the fact t h a t the anhydrous forms have lower molar extinction coefficients (Table I ) . The anhydrous form of @-benzoquinonediazide and the tetrahydrate form of I l j This article was to have been published prior t o t h a t of Kunitake and Price (ref. I S ) , but publication was delayed due t o unforseen circumstances. (2) (a) Presented in part a t a symposium i n honor of C . S. hlarvel a t the University of Arizona, December 27-28, 1961, ( b ) This research was supported in part b y a grant from the Petroleum Research F u n d , administered by the American Chemical Society. Grateful acknowledgment is hereby made t o the donors of this f u n d . f 3 ) 51. J . S. Dewar and A. S .James, J . Cheiii. Sor., 917 (19%). ( 1 ) G . I ) . Staffin and C C . Price, J . A i n . C h e m So(., 82, 3 i 8 2 ( 1 Y f i C I ) . f;) C . J . Kurian and C . C. Price, J . P d y i r ? ~ iSci., . 49, 2ii7 119(il). (fi)H . S. Blanchard, H. I.. Finkbeiner and G . 4.Russell, ibid , 68, 4bil (1YfiZ). ( 7 ) 4.S. H a y , i b i d . , 68, 581 (1962). (8) B. 0. Lindgren, A d a Chem. S t a n d . , 14, 1203 (1960). (9) C . G. Haynes, A. H. Turner a n d UT,A . Waters, J . Cheiiz. SOC.,2823 11 9.5fi).
(10) E, Adler and R . Magnusson, A c l a Cheiii. Scaml., 13, 30,i (1%-19). (11) 0. Sus, K . hloller and H. Heiss, Artrz., 698, 123 (19,j(i) ( 1 2 ) Chi-Hila Wang, PYOC. ' T h e m S o f . , 309 (19ijl). l l : i ) R l J . S. I)ewar and A. S James, J . C h e i i ? . So
