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INTRAMOLECULAR HYDROGEN BONDING TO T-ELECTRONS I N orthoSUBSTITUTED PHENOLS BY W. BECKERING Grand Forks Lignite Research Laboratory, Region 111, U . S . Bureau of Mines, Grand Forks, X. Dak. Received M a g 1.4. 1960
Intramolecular hydrogen bonds between hydroxyl groups and aromatic *-electrons are examined in phenols that have an aromatic group attached to the ortho-position. The strength of the hydrogen to *-electron bond is varied by placing different substituents on the primed aromatic ring. The strength of the intramolecular bond is related to the planarity of the two aromatic rings in a biphenyl system.
The formation of the hydrogen bond through intermolecular and intramolecular association has been investigated extensively in recent years. Intermolecular hydrogen bonding may occur either between solute and solvent or between two solute molecules. The latter is a concentrationdependent phenomenon. Intramolecular hydrogen bonding occurs in those phenolics that have a proton-acceptor group attached to the molecule. The steric configuration of the proton-acceptor group must be such that the internuclear distance between it and the proton will permit the formation of a hydrogen bond. This kind of bonding does not depend on the concentration of the solution. Luttke and ?cIecke8 classified those compounds that exhibit intramolecular hydrogenbonding into two types. I n type I compounds, the substituent X may be either a halogen, alkoxy, hydroxy, cyanide, aromatic or an olefinic group. These compounds generally display two melldefined O H bands. The long wave length band results from the cis-form of the molecule and the short wave length band from the trans-form. In H
solution an equilibrium exist’s bet>weenthese two structures. In type I1 compounds where X = XOZ, CHO, COOR or COR, the OH band displays a single, broad, diffuse absorption which is shift’ed considerably to longer wave lengths. In t’hese compounds, the increased st’rengthof the hydrogen bond can be at’tributed to t’he increased stability of the six-membered ring which is in contrast to the compounds found in type I, where a fivemembered ring is present. In type I compounds, the ortho substit’uent may be an electronegative atom or it may be a T electron system arising from an olefinic group or an aromattic ring. Recent,ly West,7 and Baker and Shulging have investigated type I compounds in (1) K. D. Coggeshall, J. A m . Chem. Soc., 69, 1H20 (1947). i2) W.Liittke and R. Meeke, 2. Elektrochem., 63,241 (1919). (3) N. D. Coggeshall and E. L. Saier, J. A m . Chem. Soc., 73, 5414 (1951). (4) F. A. Smith and E. C. Creitz, J . Research Natl. Bur. StarLdards, 46, 145 (1931). (5) R. A . Friedel, J . A m . Chem. S o c . , 73, 2881 (1951). (6) D. S. Trifan, 3 . L. IV‘einmann and L. P. Kuhn. ?bid., 79, 6566 (1957). (7) R. West, ibid., 81, 1614 (1959). (8) W.Liittke and R . LMecke, 2. physik. Chem., 196, 56 (1950). (9) A . W.Baker and A . T. Shulgin, ibrd., 80, 5358 (1958).
which the proton-acceptor group consisted of olefinic 9-electrons. I n this paper intramolecular hydrogen bonding in type I compounds will be discussed where the proton-acceptor group ortho to the hydroxyl group consists of aromatic T electrons. The effective concentration of the T electrons will be varied by placing different substituents on the aromatic ring.
Experimental A Perkin-Elmer model 112 double-pass spectrophotometer was used for determining the fundamental hydroxyl stretching frequencies in the 3,600 cm.? region. The instrument was equipped with calcium fluoride optics and had a slit width of 75, a t 3,600 cm.-l. The spectrophotometer waE purged with dry air prior to use. Calibration was accomplished b y drawing a smooth curve through the points obtained from the absorption peaks of ammonia and the atmosphere water vapor bands. Absorption peak values were taken from the 1iterature.lO All samples except 4’nitro-2 hydroxybiphenyl were run in reagent grade carbon tetrachloride at concentrations of 5.0 g./liter. The 4’nitro-2-hydroxybiphenyl compound was a saturated solution in CC1, of approximately 4.0 g./liter. The path length of the cell was 1.O mm. All spectra were recorded a t room temperature. The reproducibility of the spectra was f 2 cm. or better. The 4’-chloro-2-hydroxybiphenyl, 4’-bromo-2-hydroxybiphenyl, 4‘-iodo-2-hydroxybiphenyl and 4’-nitro-2-hydroxybiphenyl were synthesized in this Laboratory according to standard procedures recorded in the literature. 2’-Hydroxy-1-phenylnnphthalenewas synthesized a t the Bureau of Mines. 2-Propenylphenol, 2-phenylphenol and 2,2‘dihydroxybiphenyl were obtained from commercial sources. The above compounds were purified by either recrystallization, distillation or gas-liquid chromatography.
Results and Discussion Table I lists the hydroxyl frequencies of the cis and trans form. Phenol iT included in the table for comparison purposes. The high-frequency band is due to the unassociated OH-group found in the trans form of the molecule. and the lowfrequency band is produced by the cis configuration.’l There has been considerable discussion in the literature about the planarity of the two aromatic rings in a biphenyl ~ y s t e m . l ~ - ’Slthough ~ the exact angle between the two planes has not been established, the consensus is that the aromatic rings are neither coplanar nor perpendicular to (10) -4.R. Downie, &I. C. Magoon. T. Purcell and B. Crawford, Sr., J . O p t . Sac. Am., 43, 941 (1953). (1 1) L. Pauling, “The Nature of the Chemical Bond.” Cornel1 Unlv. Press, Ithaca, N.P., 2nd ed., 1918, pp. 320-27. (12) G W. Theland, “Resonance in Organic Chemlstry,” John iley and Sons, I n c , Kew Pork, N. P.. 1955 p. 1 i 7 ff. (13) I. L. Karie and I,. 0 Brockway, J . Am. Clrem Ssor. 66, 1974 (1944). (11) 1’. P. Kreiter, W. 4. Bonner and R. 11. Eastnian, zhld.. 76, 5770 (1954).
ISTRAMOLECULAR HYDROGEN BONDISG TO a-ELECTROSS ISPHESOLS
Feh., 1961
TABLE I ABSORPTION PEAK VALUESFOR THE cis A N D tran.s BANDS IX Cn1.-1 Comporind
cis band
trans band
2-Phenylphenol 2,2 ’-Dihydroxybip henyl 2 ’-Hydroxy-1-pheriylnaphthalene 4’-Chloro-2-hydro~:ybiphenyl 4’-Bromo-2-hydroxybiphenyl 4’-Iodo-2-hydroxybiphenyl 4’-Nitro-2-hydroxybiphenyl 2-Propenylphenol Phenol
3568 3556 3560 3575 3573 3572 3587 3550
3609 3599
..
207
2,Z’-Dihydroxybipheny1.-In this compound the two hydroxyl groups are favorably located so that both of the protons can “reach over” and form a hydrogen bond with the adjacent n-electrons of the aromatic ring. d twofold twisting effect is now present because both of the hydrogen to
.. 3610 3607 3609 3604 3607 3610
one another but rather are a t some intermediate angle. This angle can be increased by placing bulky groups in the 2- and 6-position, thus forcing the two rings to approach a perpendicular config~rationl~-~’; or it can be decreased by placing on the ring ceriain groups which teiid to promote conjugation between the two rings. As the amount of double-bond character of the 1-1’ carbon bond increases, the degree of coplanarity increases also. 2-Phenylphenol.-2-Phenylphenol will be treated first, because it is the simplest aromatic substituted phenol with nal functional groups on the primed ring. It displays a strong, sharp band at 3568 cm.-l which is due to the cis form of the hydroxyl
group. The absorption energy is 42 cm.-l lower than the OH-group in phenol. The proton-acceptor group in 2-phenylphenol is the aromatic a-electron systeni of the primed benzene ring. These a-electron orbitals are perpendicular to the plane of the aromatic ring. Therefore, one anticipates that the strength of the hydrogen to aelectron bond should increase as the degree of noncoplanarity increases. The increase is represented in Fig. 1. Correspondingly, the by the angle (rl
H-0
a-electron bonds tend to rotate the two aromatic rings perpendicular to each other. This rotation should produce a stronger hydrogen to a-electron bond than in 2-phenylphenol. The cis form has its absorption peak at 3556 cm.-l, which is a decrease in energy of 12 cm.-l from that of 2-phenylphenol. It may be questioned whether this decrease in energy of the absorption band can be ascribed solely to the increase in the non-coplanarity of the two rings. It must be recognized that there is also an increase in the a-electron charge of each of the rings because of the resonance effect of the non-bonding a-electrons on the oxygen atom with the a-electrons on the aromatic ring. This increase in the n-electron concentration also will favor a strengthening of the proton to a-electron bond which would result in a lowering of the OHfrequency. The absorption frequency of the trans form in 2,2’-dihydroxybiphenyl is 3599 cm.-l. This represents a drop of 11 cm.-’ below that of phenol and is the lowest frequency for the trans form of the compounds listed in Table I. An explanation for this drop can be found in the decreased basicity of the oxygen atom of the trans OH-group. The drop in basicity is produced by the hydrogen to a-electron bond of the cis OH-group. The proton to a-electron bond decreases the effective aelectron concentration of the ring containing the trans OH-group, which, in turn, transmits its effect to the oxygen atom through resonance. This lowers the force constant of the trans OHbond, and therefore, a drop in frequency results according to the equation
v=aG 1
Fig. 1.--w
is the, angle between the planes of the two aromatic rings in biphenyl.
strength of thle OH-bond should decrease with incredsing strength of the proton to a-electron bond. This stabilization of the molecule through the formation of the hydrogen bond will tend to increase the angle w in Fig. 1 from that of the unsubstituted biphenyl. The absorption frequency of the trans hydroxyl group in 2-phenylphenol is 3609 cm.-l, which is approximately the same as that of phenol. The intensity of the band in the trans form is considerably weaker than that of the cis form.I8 (15) E. Marcus, V i . 11. Lauer and R. T. drnold z b z d , 80, 3742 (1958). (16, E Williamson and W. H Rodebush, zbmd, 63, 3018 (1941) (17) G. H. Beaver, D M. Hall, M. S. Lesslie and E. E. Turner, ibzd.. 7 4 , 854 (1952)
-
where v is the frequency, k the force constant, and p the reduced mass of the atoms involved in the vibration. One is led to believe that any resonance that might exist between the two rings must be small because the hydroxyl group normally produces an increase in the electron density of the carbon atoms at the ortho and para positions. This increase in a-electron density should be transmitted across the 1-1’ carbon bond of the two rings and hence to the ortho carbon atom attached to the trans OH-group. The resulting increase in basicity of the oxygen atoms should increase the frequency of the trans OH-group. Sinre a drop in the frequency is observed, it can be roncluded either that (181 0. K.TVulf, t-.Liddel and S. B. Hendricks, J . Am. Chem. Soc.. 68, 2287 (19’36).
208
E. G. PROGT A N D P. J. HERLEY
1’01. 65
the inductive effect. The energy values indicate that the inductive effect outweighs the resonance effect, so that the net result is a decrease in the a-electron density compared to 2-phenylphenol. The OH-frequency of the cis form increases in the order C1 > Br > I, which is in accordance with the electronegativities of the halogens. there is little resonance between the two aromatic The trans OH-frequencies in the halo-suhstituted rings or that the effect of the electron depletion compounds are slightly below the value for phenol. due to the hydrogen to n-electron bond of the cis The bromo compound has the lowest value of the group counteracts any increase resulting from three compounds. conjugation between the two aromatic rings, so 4’-Nitro-Z-hydroxybiphenyl.-The nitro group that the over-all observed effect is a lowering of the serves as an electron sink for the 7r-electron density basicity of the oxygen atom. of the nitro-substituted aromatic ring. The result2 ’-Hydroxy-1-phenylnaphthalene.-Ultraviolet ing decrease in the n-electron concentration spectra of catacondensed aromatics show a red is reflected in the weak proton to n-electron bond shift with increasing size of the hydro~arbon,’~and a stronger cis OH-bond. The energy of the indicating a corresponding increase in the n- cis OH-bond in this compound is the strongest of electron system. The greater n-electron system in the compounds examined. With the weakened naphthalene compared to benzene should result proton to a-electron bond is a shift in the cis-trans in a stronger proton to n-electron bond in 2’-hy- equilibrium in favor of the trans form of the moledroxy-1-phenylnaphthalene than in 2-phenylphe- cule. This shift is observed in the equivalent nol. This difference is observed in the spectra of intensities of the two peaks-assuming equal 2’-hydroxy-l-phenylnaphthalene where a strong extinction coefficients. The other compounds, band occurs a t 3560 ern.-’, representing a drop of except 2‘-hydroxy-l-phenylnaphthalenewhich has 8 cm.-l from that of 2-phenylphenol. The absence no trans band, display a weak trans-absorption of the trans form in this compound probably is band and a strong cis-absorption band. This is in due to a shift in the thermal cis-trans equilibrium contrast to 2-allylphenolg and 2-pr~penylphenol~ because of the added stabilization of the cis form. which have weak cis-absorption bands and strong Other factors such as the size of the molecule un- trans-absorption bands. doubtedly enter also in order to account for the From the above results, a relationship is observed existence of the trans form in 2,2‘-dihydroxybi- between the strength of the hydrogen to n-electron phenyl. bond and the n-electron concentration on the aro4’-Halo-2-hydroxybiphenyl.-The halo-substi- matic ring. Furthermore, there seems to be a tuted compounds display absorption frequencies of relationship bet\Teen the degree of non-coplanarity the same order of magnitude; therefore, these three and the strength of the hydrogen-n-electron bond. compounds will be treated as a group. The values By a careful examination of the hydroxyl frequency, of the cis-frequencies are all higher than that of the type of bonding present in the molecule fre2-phenylphenol. The halogens can release elec- quently can be determined. trons by resonance and can withdraw electrons by (9) A . W. Baker and A . T. Shulgin, J . Am. Chem. SOC.,80, 5358 (1958). (19, J. R. Platt, J. Chem. Phys., 17, 484 (1949).
THE THERMAL DECOMPOSITION OF BARIUM PERIXAKGANATE BY E. G. PROKT .4ND P.J. HERLEY Rhodes University, Graharnstown, South Africu Received M a v $4, 1960
The thermal decomposition of whole and ground crystals of Ba(MnO4)z has been investigated in the temperature range 140-190’. Activation energies for the acceleratory and decay coefficients have been measured. Reaction during the main acceleratory period is presumed to proceed by the mechanism proposed for the decomposition of KMnO4. The decay period follows the unimolecular Isw. Pre-irradiation in a -,-ray source accelerates the thermal decomposition and the results are interpreted by a mechanism similar to that suggested for pre-irradiated KMnOa.
Roginski!, et ab.,l studied the topography of the thermal decomposition of barium permanganate and suggested that decomposition commenced from a number of surface nuclei and formed an amorphous film which spread into the crystal by may of multiplication of grains of product. Roginski!, et ~ l .also , ~ showed that the kinetic curves are (1) S.Z. RoginskiI. E. I. Shmuk and M. Kushnerev, Izuest. Akad. S a u k S.S.S.R., Otdel. K h i n . Nauk, 573 (1950). (2) S.8. Roginskil. S. Y.Elovieh and E. I. Smuk, ibid., 469 (1950).
sigmoid and that the acceleration of the reaction velocity conforms to Xt*/8,where X i is the mass of solid product a t time I , and the deceleration to mt2/8,where mt is the mass of unchanged permang:iiiute. The reaction is presumed to be autocatalytic. However no fully developed theory for the decomposition of Ba(Aln04)z has been presented and no activation energies determined. The topographical examination suggests that the decomposition
