A. L. MCCLELLAN AND S.W. NICKSIC
446
Simultaneous, Independent Hydrogen-Bonding Equilibria and Self-Association in Some Halomethanes and Haloethanes
by A. L. McClellan and S. W. Nicksic California Research Corporation, Richmond Laboratory, Richmond, California
(Received J u l y 80, 1964)
The n.m.r. shifts of 18 chloro- and bromomethanes and -ethanes and 8 related compounds have been measured for the pure compounds and for 5% solutions in dimethyl sulfoxide, cyclohexane, and CC1,. A more extensive study of some l,l,Ztrihaloethanes in dimethyl sulfoxide is reported. The various results can be summarized by three general statements. (a) Hydrogen bonding between 1,1,2-trihaloethanes and dimethyl sulfoxide can be adequately accounted for by assuming that each type of hydrogen acts independently to form a 1 : l complex with the sulfoxide. (b) All the halogenated compounds are selfassociated to about the same extent when grouped according to the number of protons per carbon atom. (c) The hydrogen-bond shift for the various compounds in dimethyl sulfoxide solution is roughly proportional to the diamagnetic shielding, as measured by their resonance frequencies in cyclohexane solution.
Y
Introduction Following our study of chloroform-dimethyl sulfoxide,l we looked at all the chloromethanes and -ethanes, a number of the bromo analogs, and a few similar compounds, to see if there were any unusual effects. I n particular, we were looking for large self-associations or strong interactions with proton acceptors. All the observed data are presented first; then the case of independent equilibria is considered.
Experimental The same general procedure and equipment were used as in our previous work.’ The irradiating frequency was 60 3Ic.p.s. I n these present experiments, the reagents were generally Eastman White Label quality used without further purification. Tetramethylsilane was used as an internal standard so magnetic susceptibility correction was not needed.
Results Table I gives the results. Figure 1 shows a plot of the cheniical shift for these halogenated hydrocarbons In cyclohexane (considered inert) us. the hydrogenbond shift in dimethyl sulfoxide. The line in the figure is the least-squares best fit for compounds 1-20 and 25. Its equation is The Journal of Physical Chemistry
=
A
+ 0.23X
where Y = hydrogen-bonding shift = resonance in dimethyl sulfoxide minus the resonance in cyclohexane, in c.P.s.; X = resonance in cyclohexane, in c.P.s.; A = -27.8 C.P.S. There is a rough correlation between these variables. Those resonances which occur farthest from the tetramethylsilane reference point show the largest hydrogenbond shift. One interpretation is that these compounds have an electron configuration in which the electrons around the proton are considerably displaced. As the dimethyl sulfoxide approaches such a “polarized” proton, it then exerts a large effect on it, and a large hydrogen-bonding shift is observed. The third and eighth columns in Table I show that, although the proton resonances occur over a wide region (120440 c.P.s.), the shift on dilution in cyclohexane is roughly the same for all. The data in Figure 2, although rather widely scattered, suggest a trend for both the self-association and hydrogen-bonding shifts to decrease as the number of protons per carbon goes from 1 to 3. This is a rough way to allow for the successively greater activation of the protons as addi(1) A. L. McClellan, 9.W. Nicksic, and J. C. Guffy, J . M o l . Spectry.. 11, 340 (1963).
HYDROGEN-BONDING EQUILIBRIA IN SOME HALOMETHANES AND HALOETHANES
447
Table I : N.m.r. Shifts (from Tetramethylsilane) a t 27 f 2” -Proton resonance, c.p.s.-Compd. no.
1
2 3 4a 4b 5a 5b 6 7 Sa 8b 9 10 11 12 13 14 15 16a 16b 17 18 19 20 21a 21b 21c 22a 22b 23 24 25a 25b 26a 26b
Compound
Chloromethane Dichloromethane Chloroform Chloroethane, H,b Chloroethane, HbC 1,l-llichloroethane, H, 1,l-Dichloroethane, Hb lJ2-llichloroethane 1,1,l-Trichloroethane 1,1,2-Trichloroethane, H, l,l,Z-Trichloroethane, Hb lJl,1,2-Tetrachloroethane lJ1,2,2-Tetrachloroethane Pent achloroethane 1,1,2-Trichloroethylene Bromochloromethane Dibromomethane Bromoform Bromoethane, H, Bromoethane, Hi, 1,2-llibromoethane 1,1,2,2-Tetrabromoethane 1-Bromo-2-chloroethylene 1,2-1libromoethylene lJ2-Dibromopropane, CH, l,2-DibromopropaneJ CH2 1,2-DibromopropaneJ C H 1,3-Dibromopropane“ lJ3-llibromopropane’ Dibromoacetonitrile Dichloroacetonitrile 1,1,2-TribromoethaneJ H. 1,1,2-Tribromoethane, Hb 1-Chloro-2-phenylethaneb 1-Chloro-2-phenylethane’
Pure
* 321 436 86 208 122.5 357 226 163.8 239 0 350.6 258.5 360 367 386 314 300 409 96 205 221 368 223 409 108 222 250 140.5 213 355 385 249 346 167 204
--
95%
95%
DMSO
C6Hii
95% CClr
183 344 5 498 85 220 120 8 385 8 234 2 171 253 2 394 287 5 416 5 449 5 440 332 324 462 103 211 228 414 232 438 104 229 270 138 218 418’ 446O 259 384 181 5 231
172 309 1 426 83 205 118 3 345 6 214 5 158 3 228 9 336 8 249 5 347 358 5 380 301 288 404 96 196 210 356 212 391 112 213 242 142 207 342
186 318.2 436 90 21 1 134.2 352.2 221 7 165 236 344 256.5 355.5 367 388 309 297 410 100 205 216 363 221 409 110 222 246 142 212.5 349
238 333 177 212 5
245 339 181 217
DMSO C6H12
11 35.4 72.0 2 15 2.5 40.2 19 7 12.7 24.3 57.2 38 69.5 91 60 31 36 58 7 15 18 58 20 47 -8 16 8 -4 11 76
21 51 4.5 18.5
--Shift. c.p.s.Pure C6H12
11.9 10.0 3 3 4.2d 11.4 11.5 5.5d 10.1 13.8 9.0 13 8.5 6 13 12 5 0 9 11 12 11 18 -4 9 28 -1.5 6 13 11 13 - 10 -8.5
PureCell
3 0 -4 -3 - 12 5 4 -1 3 7 2 5 0 -2
5 3 -1 -4
0 5 5 2 0 -2 0 4 -1 0 6 4 7 - 14 - 13
* H, refers to the proton on the carbon having more protons. Hb refers to the proton on the carbon a Gas a t room temperature. Proton nearer halogen atom. 10% in DMSO. Proton nearer having fewer protons. Methyl protons. Proton on carbon 2. phenyl group.
’
tional electronegative substituents are added to the molecule. The same trend is observed for bromo and chloro derivatives. Such curves indicate that chloroforiii (compound 3) is not particularly outstanding as a proton donor. A similar conclusion was reached by Allerhand2 for infrared studies of C-H stretching frequency. For example, the relative hydrogen-bonding shifts for CHCI,, C1,CHCS, and Br2CHCX were 29, 66, 80 cni.-‘ when the compounds were very much diluted in dimethyl sulfoxide-& N.ni.r. showed a smaller variation of hydrogen-bond shifts, namely 72, 61, 76 C.P.S. in diinethyl sulfoxide. The data of Table I allow a further comparison of
the differing effects of dilution in CC14 or cyclohexane. We might expect cyclohexane to be inert, nierely breaking up association by dilution, while CCI, would offer chlorine atoms for hydrogen bonding siniilar to the self-association present in the pure compound. If these expectations are realized, the shift on dilution with cyclohexane should be larger than that i n CCl,. A coniparison of the last two colunins in Table I shows that usually such is the case. Occasionally, a negative value of shift is found. According to the iutcrpretation above, a negative number means that bonding to the added solvent is greater than self-association. (2) A. Allerhand, Dissertation, Prlnceton Universitl, 1963, p. 94
Volume 69, “Fumber 2
February 1965
A. L. MCCLELLAN AND S. W. NICKSIC
448
0" 0 CHLORO COMPOUNDS b
80
BROMO COMPOUNDS NUMBERS REFER TO TABLE
I
0
TRICHLORO
b
TRIBROMO
A2,
21*2=c
'C
/
0'0
I:
0
f c
so
Al I. -10 IO0
I
I
I
200
300
400
RESONANCE POSITION IN CYCLOHEXANE, CPS FROM TMS
08
CHLORO COMPOUNDS BROMO COMPOUNDS 2 ? f 2.c
2
I
PROTONS PER C ATOM
Figure 2. Hydrogen-bonding and self-association shifts vs. protons per carbon atom.
The Journal of Phyeical Chemistry
0.2
0.4
0.6
0.8
1.0
Figure 3. Hydrogen-bonding shifts for 1,1,2-trihaloethane protons in dimethyl sulfoxide.
80
0
0
MOLE FRACTION I, I, 2-TRIHALOETHANE
Figure 1. Hydrogen-bond shift US. resonance position in cyclohexane.
OD
230
3
Compounds with Two Types of Protons. Chloro-, 1,1dichloro-, and 1,1,2-trichloroethane and the corresponding bromo compounds each have two types of protons with different electronic environments, and the n.ni.r. spectra reflect this by showing two proton resonances. The dilution shifts of the two different protons in cyclohexane still fit the trend shown in Figure 2. A similar effect on hydrogen bonding is also noted. The results of a more extensive study of 1,1,2-trihaloethane-diniethyl sulfoxide mixtures a t room temperature are shown as the circles and triangles in Figure 3. The data are corrected for self-association as in our previous work. The lines represent trends calculated as subsequently discussed. Preliiiiinary calculations were based on the assuniption, suggested by data of the type in Figure 2, that the H b proton would form the stronger hydrogen bond, The equilibriuin between sulfoxide and the H, proton is imagined as soniewhat weaker. The resulting equations, derived as outlined earlier, l were complicated and did not give the observed dependence of n.m.r. resonance position upon concentration. After considering and rejecting various schemes, involving as many as five equilibria, we found that the observed
HYDROGEN-BONDING EQUILIBRIA IN SOMEHALOMETHANES A N D HALOETHANES
shifts can be adequately explained by assuming that there are two complexing equilibria each forming a 1: 1 complex. If we symbolize the haloethane as H,CHb to emphasize the two different protons arid dimethyl sulfoxide as S, then we can write kl
H,CHb -k
s = H8CHb . . . s
s + H,CHb
k?
=
s..
*
(1)
,HCHb
(2)
With the further assumption that these equilibria are satisfied independently, we can calculate shifts following our previous work. The equilibrium constants used to make these calculations, as well as some on 1,1,2,2tetrachloroethane, are given in Table I1 for which Yvalues were found by the method of Huggins, Pimentel, and Shoo1e1-y.~
Table I1 : Summary of Equilibrium Calculations a t 27 f. 2" "free, 0.p.s.
0.p.s.
10'k, M -1
426 238 346.5 250.1 347.4 360
516 261.5 409 272.2 397.8 416
30 1.4 1.4 5.6 5.6 5.6
vbonded,
Chloroform (from ref. 1) 1,1,2-Trichloroethane, H. 1,1,2-Trichloroethane, Hb 1,1,2-Tribromoethane, H, 1,1,2-Tribromoethane1 Hb 1, lI2,2-Tetrachloroethane
We see the same variation in k as shown for hydrogenbonding shift in Figure 1. Chloroform forms a considerably stronger hydrogen bond than the haloethanes. The uncertaint,y of the k values is moderate, being perhaps f1 M -l. There still remains the intuitive feeling that the two types of proton should have different hydrogen-bonding equilibrium constants. Why is this not found in the calculations? One answer may be simply that the bonds are so weak that small differences are not detected by the technique used. In addition, the data and calculations do not exclude another possibility. That is that the two different protons form hydrogen bonds a t the same time to the oxygen atom of the sulfoxide group. Such a complex could be shown diagramniatically as
c1 .&
CH,
I
\ ,/,,' \c-e s=o I
CH,
"\*\
"8
H,
449
For this complex, we would expect weak bonding since the C-H . . . 0 angle is high (about 100'). Pinientel and McClellan4 discuss the problem of bent hydrogen-bond configurations, and, for the very simple view adopted there, bending is calculated to have little effect. As they point out, no great weight can be attached to their computation, but it is perhaps indicative that such complexirig is possible. In a similar way, we can argue that the finding of the same k value for H, and Hb in the same molecule is compatible with, but not a compelling argument for, simultaneous formation of two hydrogen bonds. Nuclear magnetic resonance spectra were recorded for the 1,1,2-trichloroethane-diniethylsulfoxide mixtures at 2.5', the lowest temperature which did not freeze some of the samples. The data for H, shift lies essentially on the line observed a t 27' (Figure 3). For HI, the points are higher by about 2 c.P.s., an amount just outside experimental error. The trend of the Hb line is the same as a t 27' so we cannot detect a change in the k values. This behavior would be expected for the cyclic 1 :1 complex, lending some additional support for this proposed structure. N.m.r. spectra were not measured a t higher temperatures because the k values, which are already small a t room temperature, would be still smaller, and differences could not be detected. Perhaps it is best to say that chemical intuition suggests the two types of protons should have different equilibrium constants, but our observations are fit more easily by the assuniption that they do not. The difference may be too small for observation, or a 1 : 1 complex with two hydrogen bonds from the sanie haloethane molecule may somehow "average out" the properties of the two protons.
Summary We have observed an example in which two types of protons in the same molecules form hydrogen bonds independently in the same inixtures of 1,l,Ztrihaloethane-dimethyl sulfoxide. We have noted hydrogenbond shifts of 26 halogenated hydrocarbons in dimethyl sulfoxide, cyclohexane, and CCl, and have shown a relation between self-association shifts and t,he number of protons per carbon atom.
Acknowledgments. We appreciate the thoughtful discussions with Dr. L. L. Ferstandig and the permission of California Research Corp. to publish this work. (3) C. M. Huggins, G. C. Pimentel, and J. N . Shoolery, J . Chem. Phus., 23, 1244 (1955). (4) G. C. Pimentel and A. L. McClellan, " T h e Hydrogen Bond," W. H. Freeman and Co., Snn Francisco. Calif., 1960, p. 243.
T'olurne 6 9 , Number 2
February 1965
