Baker–Nathan effect, hyperconjugation and polarizability effects in isolated molecules

Abstract

The Baker–Nathan effect in alkyl-substituted benzene derivatives has been reinvestigated on the basis of the AM1 calculated enthalpies of formation, correlations of the known gas-phase basicities and ab initio calculated geometries. In neutral molecules any stabilization is absent; protonated species are strongly stabilized, depending on the alkyl polarizability. This dependence was revealed very clearly when the substituent CH₂Bu^t was added to the traditional series, Me, Et, Prⁱ, Bu^t. Any hyperconjugative order of alkyl reactivities was not observed in the gas-phase equilibria. Geometrical structures of benzyl, 4-methylbenzyl and 4-tert-butylbenzyl cations, calculated at the HF/6-31Gδ** and MP2/6-31G** levels, revealed small angle distortions within the CH₃ group attributable to hyperconjugation of the α-hydrogen atoms. However, this effect is much smaller than the total interaction of either Me or Bu^t with the benzene nucleus seen particularly on the shortened C_ar–C_alk bond and classified formally as a mesomeric effect. One must conclude that the term hyperconjugation has a very restricted meaning in the case of substituted benzyl cations. The well-known limiting formula reflects to a small degree the real structure but hyperconjugation has a negligible effect on energy quantities of these compounds. The Baker–Nathan effect reported in solution kinetics is thus evidently caused by steric hindrance to solvation; in isolated molecules it is not observable.

References

1. C. K. Ingold, Structure and Mechanism in Organic Chemistry, Cornell University Press, Ithaca, NY, 1953; ch. 46e.
2. O. Exner, Org. Reactivity (Tartu), 1995, 29, 1.
3. M. Decouzon, P. Ertl, O. Exner, J.-F. Gal and P.-C. Maria, J. Am. Chem. Soc., 1993, 115, 12 071.
4. M. Decouzon, O. Exnor, J.-F. Gal and P.-C. Maria, J. Chem. Soc., Perkin Trans. 2, 1996, 475.
5. M. Decouzon, O. Exner, J.-F. Gal and P.-C. Maria, J. Phys. Org. Chem., 1994, 7, 615.
6. S. Böhm, M. Decouzon, O. Exner, J.-F. Gal and P.-C. Maria, J. Org. Chem., 1994, 59, 8127.
7. J. W. Baker and W. S. Nathan, J. Chem. Soc., 1935, 1841 and 1844.
8. L. Radom, Prog. Theor. Org. Chem., 1982, 3, 1.
9. E. Berliner, Tetrahedron, 1959, 5, 202.
10. J. W. Baker and M. L. Hemming, J. Chem. Soc., 1942, 191.
11. R. F. W. Bader, T. S. Slee, D. Cremer and E. Kraka, J. Am. Chem. Soc., 1983, 105, 5061.
12. W. M. Schubert, R. B. Murphy and J. Robins, Tetrahedron, 1962, 17, 199.
13. J. M. W. Scott, Tetrahedron Lett., 1962, 373.
14. M. Charton and B. I. Charton, J. Org. Chem., 1982, 47, 8.
15. R. W. Taft and R. D. Topsom, Prog. Phys. Org. Chem., 1987, 16, 1.
16. (a) M. Bureš, R. Holub, J. Leitner and P. Voňka, Thermochemické veličiny organických sloučenin (Thermochemical Quantities of Organic Compounds), Institute of Chemical Technology, Prague, 1992; (b) E. S. Domalski and E. D. Hearing, J. Phys. Chem. Ref. Data, 1993, 22, 805.
17. M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. Soc., 1985, 107, 3902.
18. J. J. P. Stewart, J. Comput. Chem., 1989, 10, 209.
19. R. Karaman, J.-L. Huang and L. Fry, J. Comput. Chem., 1990, 11, 1009.
20. M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. J. P. Stewart, M. Head-Gordon, C. Gonzales and J. A. Pople, GAUSSIAN94, Revision C.3, Gaussian, Inc., Pittsburgh, PA, 1995.
21. O. Exner, Prog. Phys. Org. Chem., 1990, 17, 129.
22. N. D. Epiotis, J. Mol. Struct. (Theochem), 1991, 75, 205.
23. W. J. Hehre, C.-F. Pau, A. D. Headley and R. W. Taft, J. Am. Chem. Soc., 1986, 108, 1711.
24. B. T. Cooney and D. A. R. Happer, Aust. J. Chem., 1987, 40, 1537.
25. O. Exner, M. Ingr and P. Čársky, J. Mol. Struct. (Theochem), in press.
26. A. Domenicano and A. Vaciago, Acta Crystallogr. Sect. B, 1979, 35, 1382.
27. M. Cyrański and T. M. Krygowski, Pol. J. Chem., 1995, 69, 1080.
28. A. Domenicano and P. Murray-Rust, Tetrahedron Lett., 1979, 24, 2283.