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Infrared Intensities as a Quantitative Measure of Intramolecular Interactions. XV.’ Monosubstituted Ethylenesz A. R. Katritzky, R. F. Pinzelli, M. V. Sinnott, and R. D. Topsom Contributionfrom the School of Chemical Sciences, University of East Anglia, Norwich England, and the School of Physical Sciences, La Trobe University, Melbourne, Australia. Received January 14, 1970 Abstract: Integrated intensities for the VC-C mode of 62 monosubstituted ethylenes are tabulated, together with UR’ values for the substituents, derived by the relation: UR’ = [ ( A - 80)/27,100]1’2. The resulting ult’ (irE) values are in good agreement with the corresponding UR’ (irPh) values from the integrated intensities of monosubstituted benzenes, but are more reliable for values of small magnitude.
T
he study of electronic interactions in aromatic species has been the subject of much a t t e n t i ~ n . ~ The major approaches to the effect of a substituent on a benzene ring have been derived either from the Hammett equation or from MO treatment. The separation of Hammett substituent constants (a) into inductive field (aI) and resonance components (uR:) by Taft4 has proved fruitful ; in eq 1 the uI component is treated as a constant, but uR changes when variable amounts of direct resonance interaction with the reaction site are involved. = UI UR (1) For cases where no additional resonance interaction occurs over that normally found between the substituent and the benzene ring, the resonance component is denoted uR0. The quantities uI and uR0 measure the tendency of a substituent to interact, and they should be applicable to other classes of compounds in addition to benzenes. Indeed, the relationship of uI to inductive constants derived from aliphatic systems, such as CT* is well known. However, much less attention has been given to intramolecular electrical interacticln in substituted ethylenes. In particular it is not evident if the tendency of a substituent to conjugate with a benzene ring, as measured by its uR0 value, is also indicative of its conjugation with a carbon-carbon double bond. The u R 0 values for substituents attached to benzene can be obtainedb from nmr and infrared measurements as well as from reaction rate and equilibrium data and we first briefly consider analogous information on substituted ethylenes. A limited amount of work has been done on the acidities of substituted acrylic acids.6 The p K , values of the trans acids give a good linear Harrimett plot against normal aromatic para-substituent constants. This suggests that the substituents conjugate with the double bond in an analogous way to that when they are substituted in benzene. Similar results are found7
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(1) Part XIV: A. R. Katritzky, H . J. Keogh, S . Ohlenrott, and R. D. Topsom, J . Amer. Chem. Soc., 92, 6855 (1970). (2) Preliminary communication of part of this work: J. M. Angelelli, A. R. Katritzky, R. F. Pinzelli, M. V. Sinnott, and R. D. Topsom,
ibid., 91, 4013 (1969). (3) (a) C. D. Ritchie and W. F. Sager, Progr. Phys. Org. Chem., 2, 323 (1964); (b) S. Ehrenson, ibid., 2, 195 (1964). (4) R. W. Taft, J. Phys. Chem., 64, 1805 (1960). ( 5 ) A. R. Katritzky and R. D. Topsom, Angew. Chem., Int. Ed. Engl., 9, 87 (1970). (6) M. Charton and H. Meislich, J . Amer. Chem. Soc., 80, 5940 (1958).
for the rates of esterification of trans-substituted acrylic acids.
x\,c=c
/HB
HA
“c I
The pmr spectra of substituted ethylenes (I) also provides information about electron densities. Thus the quantity 6* - 1/2(6B ,a) where 6 is the substituent chemical shift, has been showng to follow Taft’s resonance parameterlo for the substituent X. The shifts of protons B and C can also be related to the group dipole moment of Xstll or to Hammett u constants.12 A similar relationship holds l 2 for 1,2-trans-disubstituted ethylenes where one substituent is invariant. The geminal proton-proton coupling constants seem to give a measure of the substituent effect8rL3and shouldI4 be capable of giving information of .ir-electron densities. Changes in vicinal proton-proton coupling constants are saidlj to be dominated by cr electron effects. The trans J“ values have also been suggested to depend linearly on the substituent electronegativity in mono- and 1,2-disubstitutedl6 ethylenes. Very little works has been done on I3C and 19Fshifts, both of which have proved very useful probes in the investigation3n5of aromatic systems. a!. 3C-H coupling constants in monosubstituted ethylenes do, however, correlate linearly with those of the corresponding monosubstituted methanes l7 which suggest that they would provide little guide to r-electron effects. The P 13C-H coupling values seernl7to be very little affected by change in substituent. There have been a number of results published on dipole moments18 and ultraviolet spectralg,2O which
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(7) W. C. Bailey and J. Hine, ibid., 81, 2075 (1959). (8) J. W. Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 11, Pergamon Press, Oxford, 1966. (9) C. N. Banwell and N. Sheppard, Mol. Phys., 3,351 (1960). (10) R. W. Taft, J . Amer. Chem. SOC.,79, 1045 (1957). (11) J. H. Goldstein, L. Mandell, and G.S . Reddy, ibid., 83, 1300 (1961). (12) J. Niwa, Bull. Chem. SOC.Jap., 40, 1512 (1967). (13) S . J. S . Formoshinho-Simoes and V. M. S . Gil, Mol. Phq’s., 15, 639 (1968). (14) H. Giinther, Tetrahedron Letr., 2967 (1967). (15) R. Ditchfield and J. N. Murrell, Mol. Phys., 15, 533 (1968). (16) J. Niwa, Bull. Chem. SOC.Jap., 40, 2192 (1967). (17) L. Lunazzi and F. Taddei, Spectrochim. Acto, Port A , 25, 533 (1 969). (18) C. P. Smythe, “Dielectric Behaviour and Structure,” McGrawHill, New York, N. Y.,1955.
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indicate contributions from resonance effects of substituents joined directly to the double bond in substituted ethylenes. However in neither case has it been possible to produce a more quantitative result. Many investigations have been concerned with the ability of an olefinic linkage t o transmit electric effects when conjugated with a substituted benzene ring. Results such as ionization constants, hydrolysis rates,22 and pmr shifts23 of cinnamic acid and derivatives, and prnr8jz4 and 13C shifts8,*; of substituted styrenes, suggest that conjugative effects between a substituent and a benzene ring are fairly effectively transmitted t o an atom or group (probe) situated trans to an ethylenic linkage. The infrared spectra of substituted ethylenes have been well studied but mainly with regard to frequency measurements; z6-3n thus the stretching frequency of the carbon-carbon double bond at approximately 1640 cm-l is altered by both the degree and geometry of substitution of the ethylene and the electrical and steric nature of the substituent itself. This has t o date mainly been used to diagnose the number and geometrical arrangement of substituents relative t o the double bond, particularly in hydrocarbons. It is iniportant2’b3I t o minimize variations in bond angles and substituent masses if the results are to reflect charge distribution effects. In cases where this is done the frequency has been relatedz7 to the electronegativity of the substituent group. Further the frequency can be related t o the heats of hydrogenation of the double bond. Variations of vOH in compounds where a hydroxy group is hydrogen bonded to ethylene T electrons are also dependent on the nature of the substituents on the ethylene,z7 as are the frequencies of CH? wagging and C=C twisting vibrations in substituted ethylenes. 3 z , 3 3 The frequency of the twisting vibration seemed t o be related to inductive effects of the substituent while the wagging mode apparently gave a measure of T-electron effects. The identities derived have been useda4to assess Hammett constants (19) C. N. R. Rao, “Ultra-violet and Visible Spectroscopy,” 2nd ed, Butterworths, London, 1967. (20) C. A. Coulson and E. Stewart in “The Chemistry of Alkenes,” S. Patai, Ed., Interscience, London, 1964. (21) R. Fuchs and J. J. Bloomfield, J . Org. Chem., 31, 3423 (1966); D. N . Kursanov, Z. N. Parnes, and V. I. Zdanovich, Dokl. Akud. Nuuk S S S R , 165, 566 (1965). (22) J. J . Bloomfield and R . Fuchs, J . Org. Chem., 26, 2991 (1961); A. B. Thigpen mid R . Fuchs, ibid., 34, 505 (1969). (23) T. A. Wittstruck and E. N. Tachtenberg, J . Anier. Chenc. Soc., 89, 3803 (1967); F. W. Wehrli, E. Pretsch, and W. Simon, Helc. Chirn. A c m , 50, 2189 (1967). (24) Gurudata, J. B. Stothers, and J. D. Talmau, Can. J . Chem., 45, 731 (1967); G. I
