[CONTRIBUTION FROM TBE CHEMICAL LABORATORY OF THE UNIVERSITY OF CALIFORNIA]
RESONANCE AND SOME PHYSICAL AND CHEMICAL PROPERTIES OF BIPHENYL TYPES MELVIN CALVIN Received March 1.4, 1939
If pure states of the type*
I are to contribute to the ground state of biphenyl it is necessary that the two rings be capable of assuming a coplanar configuration. When such is the case the C1-C1~ bond will acquire some double-bond character, and the distance C1--C1t will be decreased.l This has an important consequence for the rate of racemization of certain optically active biphenyls. The activation energy for the racemization of a biphenyl is almost entirely the energy required to overcome the repulsion between the valence saturated o-substituents in order to bring the rings into the coplanar position.2 Whether the racemization is effected by the actual interpenetration of the electronic charge distribution, or a change in bond angles is immaterial. The repulsive forces acting between already paired and shared electrons must be overcome. Since this repulsive force is a very sharp function of the distance (usually represented by c/r12 or ae-r/b,where r is the interatomic or intergroup distance, and a, b, and c are constant parameters) it may readily be understood how very small changes in r could effect huge changes in the repulsive potential of a pair of groups. The kinetics of the racemization of a number of optically active diphenyls have been studied both in solution and in the gas phase.3 The significant
* Pauling and Sherman [J.Chem. Phys., 1, 679 (1933)l used similar conjugated first excited canonical states but in a form which would be equivalent to the diradical since non-adjacent exchange integrals were neglected. Hiickel [Z.Physik., 83, 651 (1933)l includes the ionic states as here written. I n either case the bond CI-CI~ is double, but for the present purposes the ionic states are the more important. 1 ( a ) PENNEY, Proc. Roy. SOC.,A168, 306 (1937); (b) COULSON, Proc. Roy. Soc., A169, 413 (1939). 2 SHRINER, ADAMS,AND MARVEL, “Optical Isomerism,” p. 259 in Vol. 1of Gilman’s “Organic Chemistry.” ( a ) KISTIAKOWSKY AND SMITH, J. Am. Chem. SOC.,68, 1043 (1936); (b) KUHN AND ALBRECHT,Ann., 466, 272, 458, 221 (1027). 256
RESONANCE AND PROPERTIES OF BIPHENYL TYPES
257
differences in the first-order rate constants are due primarily to changes in the activation energy. Thus a t 0°C. a change of 3000 calories in the activation energy could change the half-life of a racemization from 1 sec. to 5 min.; ie. make the difference between a resolvable compound and a non-resolvable compound under ordinary experimental conditions. Using a Lennard-Jones potential function of the type4 V ( r ) = 4 c (1/R12-1/RB) where R = r/c, r = distance between group centers, = distance between group centers for V ( u )= 0, and E = the depth of the van der Waals minimum, and A?-1z represents the repulsion with R-6, the van der Waals attraction, it can be seen that decreases of the order of a few hundredths of an Angstrom would be sufficient to produce the required changes in potential. Just such :a case has been observede6 The 2,2'-dibrom04,4'-diaminobiphenyl (11) is not resolvable, while the 2,2'-dibromo-4,4'-dicarboxybiphenyl (111) is resolvable and has a half-life for racemization of 5-10 minutes a t 0°C. Br Br
I1 Br
o=c- /-\
I
Br
j-\
0
c/
/ vl.= \ = J -
HO
OH
I11 I n I1 the amino groups are incapable of accepting electrons and are of no assistance in lowering the energy of the ionic resonance state I and thus increasing its contribution to the ground state of the molecule. I n I11 however, although the carboxyl group functions more generally as an electron accepting group, it is not impossible that it function as a source of electrons (IV) as well and be of some assistance in promoting the contribution of the ionic state to the ground state of the molecule. Br +
Br
:o:c: .. * . :&::c:o: :0: .. H
:0: .. H
IV DEBOERAND MICHELS,Physica, 6, 97 (1939). ADAMS,et aE., J . Am. Chem. SOC.,66,2112 (1934);ibid., 66,1649 (1933).
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MELVIN CALVIN
The activation energy for the racemization of 111 (half-life 5 minutes at 0°C.) using the temperature-independent factor of loll is 17,500 cals. If this is reduced in I1 to 14,000 cals. the half-life at 0' would be 1 sec. and presumably the compound I1 would be unresolvable. This would require a change in each Br-H interaction of only 1,750 cals. and it is quite evident that only a small additional contribution of IV could easily account for the few hundredths of an I f change in the C1--C1t distance necessary. It is thus to be expected that 2,2'-dibromo-4-nitro-4'-aminobiphenyl (V) would be easily resolvable, and that the half-life of the racemization would be considerably longer than 10 minutes.
Br
Br
2oN&&NH2 V The effects of other meta and para substituents on the racemization rate of biphenyls may be accounted for at least in part by such a mechanism. Recently an attempt has been made to demonstrate the participation of I in the structure of diphenyls by a study of the catalysis of the racemizaBut tion by catalysts which affect the rate of cis-trans it is apparent from what has just been said that such an experiment should fail. In the first place the biphenyl which was used, VI,
NOz
VI was tri-o-substituted by large groups and hence the interaction of the pair of o-substituted groups would be sufficient to produce aDslowracemization even if the C1-C1# bond were as large as possible (1.53 A). In the second place the acid catalysts which produce cis-trans isomerization would probably themselves be incapable of reaching the ?r bond between the rings if E
GILBERT,TURKEYITCH, AND WALLIS,J. ORG.Cam., 3,611 (1939).
RESONANCE AND PROPERTIES OF BIPHENYL TYPES
259
it existed. Such a phenomenon has already been observed in some substjtuted stilbenes.' Another correlation which follows from these considerations appears in the absorption spectra of substituted biphenyl types in which the coplanar arrangement is prohibited. When this is the case the contribution of states of the type I to both the ground state and the excited states is reduced or eliminated, and the spectrum of such a molecule should be very close to that of the two independent groups.8 If, however, the coplanar configuration is possible, the individual fine structure is completely lost, and a great increase in intensity results.* The precise mechanism by which this takes place will be taken up in a later discussion of absorption spectra of organic molecules in general. It is sufficient for this discussion to note that such a correlation exists and that in molecules exhibiting this washed-out absorption spectrum conjugated structures of the type I do contribute to the actual state of the molecule. This leads to the expectation that molecules exhibiting this characteristic washed-out spectrum will have chemical properties associated with the conjugated resonating states which contribute to their structure. Such a property is the ability to undergo a Diels-Alder condensation which would be expected of a conjugated series a in which the structures b and c contribute appreciably.
c=c I c=c U
.._
c-c /I
C-Cf b
+
c--c /I c--c.. C
Such cases have recently been report'edgin which the correlation between absorption spectrum and the ability to undergo the diene condensation has been observed. The compounds 9,9'-biphenanthryl (VII) and 9cyclohexenylphenanthrene (VIII)
$0 VI11
8
9
-\
IX
TAYLOR AND MURRAY, J . Chem. SOC.,1938,2078.
* PICKETT, WALTER,AND FRANCE, J . Am. Chem. Soc., 68,2296 (1936).
' (a) HENRIAND BERQMANN, Nature, 143, 278 (1939); (b) BERQMANN AND B ~ R G
MANN,
J . Am. Chem. Sac., 69,1443 (1937).
260
MELVIN CALVIN
fail to react with maleic anhydride, and their absorption spectra are close to those of their components, whereas 9-cyclopentenylphenanthrene (IX), which does condense, has the washed-out spectrum. Thus the small contraction in the size of the cylcopentenyl group as compared with that of the cylcohexenyl is sufficient to permit the coplanar arrangement of the rings and hence all of its concomitant properties. Another case in point, for which the absorption spectra are not yet available, is that of the substituted 9-phenanthrylethylenes, X(a) and X(b).9b
x (4
X(b)
When R = CH, both X(a) and X(b) undergo the diene synthesis; when R = C a b , X(b) is incapable of this condensation. This may be due to one of two causes. The ortho C-H of the phenyl may interfere with Cs-H of the phenanthryl and be fixed in the trans-planar position opposite Clo-H so as to prevent the condensation by sheer spatial arrangement (pure steric effect), or the interference of the ortho C-H of the phenyl with Clo-H may be large enough to prevent the coplanar configuration in that direction as well and thus prevent the conjugation resonance. In the first case the absorption spectrum would show the conjugation broadening and intensity increase whereas in the second case it would not. We are thus led to the prediction that optically active compounds of the type VI1 and VI11 should be preparable and perhaps resolvable, and conversely unresolvable biphenyls of the type X I and XII'O
&om
COZH F
F XI
C1
HQOCH3CO OCHa XI1
should exhibit the conjugation absorption spectrum. It should be borne in mind that such conclusions as these based upon the 10
ADAMB, et al., J . Am. Chem. SOC.,66,4219 (1933); 66,4225 (1933); 64, 2973 (1932).
RESONANCE AND PROPERTIES OF BIPHENYL TYPES
261
failure to observe a given reaction are contingent upon reversals due to improvement in technic or the discovery of new experimental conditions. It should also be noted that the correlation between the conjugation absorption spectrum and the non-resolvability of a biphenyl or the ability to undergo a diene condensation need not be complete. There is as yet no theoretical criterion by which we can determine how much of a contribution of the conjugated states is required to produce the washed-out spectrum or how much of a contribution is required to prevent rotation through the coplanar position. So it may be possible to find cases in which the contribution is sufficient to produce one effect and not the other. For example, in I11 (IV) the conjugation may be sufficient to prevent the rapid racemization but the molecule in its equilibrium position may be insufficiently far removed from the coplanar configuration so that a sufficient contribution of the conjugated states exists to produce the washed-out spectrum. Also in XI and XI1 the equilibrium position may be sufficiently far removed from the coplanar one to prevent the conjugation spectrum and still permit rapid racemization. The only one of these borderline cases which seems at all likely to occur, however, is the one in which the racemization is slow but there is still sufficient contribution to produce the optical effects. It is clear that these are all matters of degree. SUMMARY
1. The requirement that the four bonds extending from a double bond
3
\ /
/
C=C
\
1 must be coplanar is applied to the contributing resonating
2 4 states of biphenyls. 2. 'The effect of non-o-substituents on the rate of racemization of certain biphenyls is discussed and it is predicted that 2,2'-dibromo-4-nitro-4'aminobiphenyl should be resolvable and have a racemization half-life greater than 10 minutes. 3. The effect of the possibility or impossibility of conjugated resonating states on the absorption spectrum of substituted biphenyls is considered and it is predicted that certain non-resolvable tetra-0-substituted biphenyls should show the conjugation absorption spectrum, whereas other tetra-,o-substituted biphenyls in which the coplanar arrangement of the rings is impossible will have an absorption spectrum very similar to the uncoupled parts. 4. It is predicted that optically active derivatives of 9 ,g'-biphenanthryl and of 9-cyclohexenylphenanthrene should be preparable. 5. The relationship between the contributing resonating states and the reactivity toward a diene condensation is discussed.
