Download Contemporary Problems in Carbonium Ion Chemistry I/II by Vladimir Alexandrovich Barkhash (auth.), Ch. Rees (eds.) PDF

By Vladimir Alexandrovich Barkhash (auth.), Ch. Rees (eds.)

In 1965 a booklet via P. Bartlett seemed below the name "The Nonclassical Ions" 1). The e-book is a suite of papers reprinted from numerous journals. the various experiences that experience seemed given that 2-22) are both antiquated (the publication released in 1972 12) covers the literature in most cases prior to 1968) or really biased (e.g., 3.4,10» on short 2, 7,11). This evaluation makes an attempt to debate some of the issues of view at the "nonclassical" carbocations. the most element is to set up the relative function of "nonclassical" and "classical" ions in numerous chemical approaches. the writer has P. Bartlett's recommendation 1) that once atmosphere forth the achievements of the human brain one may still see how we got here to the trendy knowing of a given challenge (" ... how we all know what we know"). the speculation of "nonclassical" ions deals a proof of many designated chemical, stereochemical and kinetic peculiarities of bicyclic compounds. It has accelerated our wisdom on chemical bonds in carbocations by means of introducing electron-deficient bonds (as in boron hydrides). It has accounted for plenty of rearrangements of reliable cations. As a "side" consequence our wisdom has been prolonged approximately ionization techniques in an answer, in addition to approximately stereochemical equipment. 2 major phrases of Nonclassical Carbocations In 1939 Hevell, Salas and Wilson 23) assumed an intermediate, "bridge" ion 2 to be shaped while camphene hydrochloride 1 is rearranged into isobornyl chloride three. This occurred 17 years after Meerwein first postulated the intermediate formation of "carbonium" ions in chemical reactions.

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Thus, Schleyer's data cannot convince of the rapid equilibrium of classical 2-norbornyl ions. The tertiary carbocation having a p-anisyl group as a substituent can be assumed to be classical; however, the data reported do not preclude a nonclassical structure either. 49 Vladimir Alexandrovich Barkhash Corey and co-workers 150) have shown that solvolysis of optically active 2-exonorbornyl m-carboxybenzenesulphonate yields a racemic product. If the reaction proceeded via a classical ion, then since the interaction between the carbocation and the carboxylate-anion must be high compared with the Wagner-Meerwein rearrangement (this is not always true, see 151»), some optical activity would be retained.

Both ions 5 and 71 are nonclassical since the positive charge is 43 Vladimir Alexandrovich Barkhash de localized 2-electron 3-centre bonding (2e 3C); the carbon atoms participating in the delocalization of the positive charge are penta- and tetra-coordinated. Ion 5 differs from ion 71 in its geometry; secondly, ion 5 is homoaromatic (see p. 98) while ion 71 is not. Finally, in ion 5 atomic orbitals interact with atoms uncombined with the a-bond (C 2 and C6 ), in ion 71 this is not the case. OTs @3 ..

7 kcaljmole). 2 "The Windshield Wiper Effect" Schleyer and co-workers have shown (49) that the ions formed from 2-p-anisyl- and I ,2-di-p-anisyl-endo-2-norbornanols have the same ultraviolet spectra (H 2 S04 ) which they believe to speak in favour of the rapid equilibration of classical cations I ~ II; if the developing ion had a nonclassical structure III then both aromatic nuclei in case (b) would be simultaneously conjugated with cation centres. 4 A (according to models). Therefore in a nonclassical ion the effective conjugation of the two aromatic rings will be decreased and the electronic absorption spectrum may accidentally coincide withh that of the mono aryl-substituted ion.

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