INDUCTIVE AND MESOMERIC EFFECTS IN SUBSTITUTED FULVENE AND PYRIDINE DERIVATIVES

Transcription

1 INDUCTIVE AND MESOMERIC EFFECTS IN SUBSTITUTED FULVENE AND PYRIDINE DERIVATIVES J. P. CARTIER XXD C. SANDORFY Afintvkal, Que. Dkpartelizent de Chimie, Cnioersitk de ~Wontrkal, Received May 21, 1963 ABSTRACT The electronic charge distribution in substituted fulvene and pyridine derivatives is examined using the simple Huckel n~olecular orbital method. Parameters characterizing the electronegativit) of the substituent and the carbon to TI-hich it is linlced are varied in order to co\er actually existing substituents. It is found that the nature of the inductive and nlesomeric effects is not basically different from that in the case of benzene. The most negative sites in fulvene show a certain ~ariation with the place of substitution. In p3ridine they are the ortho and para carbons for meta substitution and the lneta carbons for ortho and para substitution. ISTRODUCTION The inductive and mesomeric effect of substituents on the electroilic charge distribution in benzene has been the subject of many theoretical treatments. In the simple Huckel approximation the extent of the mesomeric effect due to the addition of two more 7-electrons to the system, as it occurs in phenol or aililine for example, depends on the difference between the Coulomb integral a, associated with the substituent, R, and the Coulolnb integral ac of a carbon atoin. This difference can be taken to be proportional to the corresponding difference in electronegativities (1, 2). The smaller it is the easier the penetration of the electrons of the substituent into the benzene T-orbitals. The inductive effect is due to the alteration of the potential field in the benzene ring because of the presence of the substituent. The inain consequence of this is an increase in the electronegativity of the carbon linked to the substituent (or decrease if the electronegativity of the substituent is lower than that of carbon) which may be represented by a supplement ac, to its Couloinb integral, generally taken as a fraction of ar. One of the present authors published a series of diagrams of electronic charge distributions in inonosubstituted benzene derivatives of the phenol or aniline type (3). The quantity (ar-ac) was given values of -2, - 1, 0, 1, 2, and 3 in units of the carboncarbon resonance The value of ac, was talien to be 1/10 of that of the respective value of (a,-ac). IVhen a,, is taken as zero we obtain a series of diagrams representing the purely rneso~neric effect. The purely inductive effect was obtained by cutting off the substituent froin the ring by setting PC-, equal to zero but giving acr values equal to ar/lo. The combined effect of the two was then colrlputed with the appropriate values of a, and acr and with PC-, = P The substituents -IJHZ, -OH, -F are examples for the positive values of ar; -ph2, -AsHZ for the negative ones. Recent developments in quantum chemistry (4-6) have given new justification to the simple Huckel method through its resemblance to the Pariser and Parr method (7). This fact encouraged us to undertake similar work on substituted fulvene and pyridine molecules. The former is an isomer of benzene although it contains a five-membered ring while the latter results from the substitution of one of the benzene carbon atoms by the Inore electronegative nitrogen atom. Comparisons with benzene derivatives were expected to be interesting. Canadian Journal of Chemistry. Volume 41 (1963)

2 2760 CANADIAN JOURKAL OF CHEMISTRY. VOL. 41, 1963 The computational work for both molecules is more considerable than for benzene since for each of them there are three non-equivalent places for substitution. The simple Huckel approximation was used throughout with all resonance integrals taken as equal and all overlap integrals neglected. In the case of pyridine the supplemental Coulomb integral for the nitrogen atom, a~--cu~, was taken equal to /3 and for the two ortho carbon atoms, ac,-ac = These values are not indicated in the tables, which only show the variable parameters due to substitution on the pyridine molecule. RESCLTS AND DISCUSSION Fulvene Tables I, 11, and I11 contain the electronic charge densities (q) and bond orders (p) for fulvene as used by Coulson and Longuet-Higgins (8). (See also earlier papers by Wheland and Pauling (9) and by Coulson (lo).) In the first columns of these tables R, 1, 2, 3, and 4 correspond to the charges, R-1, 1-2, 2-3, 3-4, and 4-5 to bond orders. The tables refer to substitution in 1, 3, and 4 respectively. In drawing conclusions from these tables we must remember that in fulvene itself the electronic charge densities are different from unity, all the ring carbons having negative charges and the extracyclic carbon a large positive charge. The numerical values of these charges were shown to be much too high and lead to a dipole moment b~, far exceeding the experimental value (11-14). It is assumed, however, that this does llot affect our comparative conclusions. The charges and bond orders of fulvene itserf are in the column corresponding to acr = 0 among the columns related to the purely inductive effect. The + and - signs after the numbers indicate that the charge density was decreased or increased respectively with respect to fulvene. When the variation is less than no signs were given. All the observations follow directly from the tables and therefore only some general features will be outlined. The largest variations of q occur on the substituent (R) itself. For ar = -2, from the two electronic charges originally on R there only remains 0.15 to 0.30 according to the place of substitution. This great positive charge decreases rapidly with increasing ar and for ax = +3 we have about 1.90 or remaining from This behavior is very similar to that obtained for benzene. The corresponding variations for the carbon linked to the substituent also follow a pattern as in the case of benzene. The inductive effect moderately diminishes q for negative parameters and increases it for positive parameters. The mesorneric effect increases q very substantially for negative parameters, then it decreases it slightly for positive parameters so that the two effects always act in opposite directions. The combined effect is not obtained by addition of the two but it still is intermediate between them. This is so whether the substitution takes place in 1, 3, or 4. The quaternary carbon 2 is significantly affected by substitution in 1 but the effect of substitution in 3 or 4 on carbon 2 is negligible. Substitution in 1 increases the negative charge in both 3 and 4 for all values of the parameters. The combined effect is close to the mesomeric effect except for large positive parameters when it is about half way between the inductive and the mesomeric effects. Substitution in 3 or 4 has an interesting effect on the charge density on carbon 1, which is the extracyclic carbon atom. The inductive effect is very slight for all parameters but the mesomeric effect, which is almost identical with the combined effect, sends large negative charges to 1 for negative values of a, and slight negative charges for positive values of a,.

3 CARTIER AND SANDORFY: INDUCTIVE AND MESOMERIC EFFECTS TABLE I Electronic charge densities and bond orders of 1-substituted fulvene derivatives A. Inductive effect a1-0.2p -0.lP 0 0.lP P R R f f f 1.033f B. R/Iesomeric effect ar - 2P - IP 0 IP 2P 3P C. Combined effect Substitution in 3 sends additional negative charges into 4 and 6, the combined effect being only roughly additive. Carbon 5 is only slightly affected. Substitution in 4 sends additional negative charges into 3. Except for negative values of the parameters, 5 and 6 are less affected. Other regularities may be found on examination of the tables. The "meta" positions (the next-nearest carbons to the place of substitution) are less affected than the others for substitution in 1 and 3. For substitution in 4, 5 and 6 are

4 CANADIAN JOURNAL OF CHEMISTRY. VOL TABLE I1 Electronic charge densities and bond orders of 3-substituted fulvene derivatives A. Inductive effect a3-0.2p -0.lp p 0.3p B. hiesomeric effect C. Combined effect

5 CARTIER AND SANDORFY: INDUCTIVE AND MESOMERIC EFFECTS TABLE I11 Electronic charge densities and bond orders of 4-substituted fulvene derivatives A. Inductive effect B. Mesomeric effect C. Combined effect OR - 2p - I@ 0 1P 2P 3P a4-0 2p -0.lp p 0.3p

6 2764 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963 comparable from this point of view. As in the case of benzene, for negative parameters the charge distribution undergoes more profound changes than for positive ones. In conclusion we can say that the influence of inductive and mesomeric effects on the electronic charge distribution in the case of fulvene is not substantially different from what is found for benzene. Looking at the absolute values of the charges in Tables I, 11, and 111, it is seen that in fulvene itself the most negative sites are the fields of carbons 3 and 6, 4 and 5 being somewhat less negative. This is still so after substitution in 1 with the apparent exception of ar = lp. For substitution in 3 the most negative sites are 4 and 6 for a, = 0, 1, and 2, which would correspond to the cases of substituents occurring in practice. For other values of a, we refer to Table 11. Substitution in 4 shows an even greater variety and only 3 is always negatively charged (Table 111). In general the negative charges in the ring become larger as a consequence of substitution. For negative parameters the extracyclic carbon is the most negative site. Pyridine Our work on pyridine was less extensive. Only ar = 0, 1, and 2 were used with the corresponding inductive parameters. The respective charge distributions have to be compared with that of pyridine itself, which is found in the third line of Tables I17A, B, and C. Substitution in 2 (ortho to the nitrogen atom) increases the negative charge on the nitrogen appreciably and sends more negative charges to all the carbons. Carbon 5 will be the most negative, then carbon 3. The meta positions are the least positive in pyridine itself but now we have net negative charges there. The inductive effect diminishes q on N, 3, 5, and 6, increases it on 2, and leaves it unchanged on 4. The mesomeric effect increases it on N, 3, 4, 5, and 6 and decreases it on 2. The combined effect is seen to be intermediate between the two. Substitution in 3 (meta) only affects q~ slightly, increases charges, rendering them negative in 2, 4, and 6 whereas in 3 and 5 it causes only a slight increase. This is a complete reversal in the charge distribution of the ring, the most negative sites being now the ortho and para carbons, not the meta. The inductive effect increases q on 3, decreases it on 2, 4, and 6, and leaves it practically unchanged on n' and 5. The mesomeric effect increases charges in 2, 4, and 6, decreases them in 3, and leaves them unchanged on N and 5. The combined effect is again intermediate. Substitution in 4 (para) increases q~ appreciably and also increases q in 2, 3, 4, 5, and 6; 3 and 5 become the most negative sites. This is a situation similar to that of the substitution in 2. The inductive effect slightly decreases qn, increases q on 4, decreases it on 3 and 5, and leaves it unchanged on 2 and 6. The mesomeric effect increases q, slightly, also increases q in 3 and 5, decreases it in 4, and leaves it unchanged in 2 and 6. Again the combined effect is intermediate. We may summarize our observations by pointing out that the inductive and mesomeric effects are opposed for positive values of the parameters whatever the place of the substitution. Furthermore, whereas in pyridine itself the charge is positive on all carbons, in the substituted derivatives there are negative sites, namely the meta carbons for ortho or para substitution and the ortho and para carbons for meta substitution. This is likely to lead to acceleration of electrophilic reactions with respect to pyridine itself for all three types of substitution. No attempt is made in this paper to make comparisons with experimental data. lye only refer to reviews by Day (15) and by Bergmann (16) and to the work of Thiec and

7 CARTIER AND SAKDORFY: INDUCTIVE AND MESOMERIC EFFECTS TABLE IV Electronic charge densities of substituted pyridine derivatives A. Substitution on carbon 2 CYR '22 qn q2 P3 P4 q5 46 q7 - B. Substitution on carbon 4 C. Substitution on carbon 3 Wiemann (17) on fulvene derivatives and to the review found in Streitwieser's book (18) on pyridine derivatives. It is hoped that the results presented in this paper will prove useful in establishing correlations between the electronic charge distribution in fulvene and pyridine derivatives and kinetic data related to these molecules. The spectra of fulvene and pyridine derivatives were extensively studied (19-24) as were the dielectric properties of the latter (25, 26). ACKNOWLEDGMENTS The eigenvalues and eigenvectors of the molecules treated in this paper were computed with the help of the Computation Centre, 5lcLennan Laboratory of the University of Toronto. Our sincere thanks are due to Dr. C. C. Gotlieb and Mr. I. Farkas for their valuable assistance. One of us (J. P. C.) was a holder of a scholarship from the Province of Quebec for which he expresses his gratitude.

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