Conference paper

Interactions in Carbocations and Huckel's 4n + 2 Rule

D. E. Sunko ; Laboratory of Organic Chemistry and Biochemistry, Faculty of Natural Sciences and Mathematics, University of Zagreb, Zagreb, Croatia, Yugoslavia

Abstract

The molecular orbital model of hyperconjugation was utilized
in rationalizing some long range interactions in carbocations. Conveniently
such interactions can be detected by measuring secondary ·
kinetic deuterium isotope effects. Hilckel's 4n + 2 rule can be
extended to interactions between the cationic center and pseudo-:n:
orbitals of CH3, CH2 and CH groups respectively. Thus, in analogy
to hyperconjugation one can envisage homohyperconjugation as
the probable cause of normal y-deuterium isotope effects. Results
are presented of studies of these effects in the adamantane and
norbornane system. The stability of the 1-adamantyl cation can be
ascribed to a favorable 4n + 2 configuration of a-electrons comprising
the ~,y-carbon carbon and/or y-carbon hydrogen bonds.
It is however not possible to distinguish between these alternatives
since both could explain the observed y-isotope effects. In contrast,
the four CH2 groups flanking C7 in the 7-norbornyl cation form
an antiaromatic 4n pseudo-:n: system which could explain the
exceptional instability of this cation. Interactions of the empty
p-orbital with the subjacent 4B2 pseudo-:n: orbitals give rise to
small normal y-deuterium isotope effects of homohyperconjugative
origin. In this system, due to symmetry constraints hyperconjugative
interaction with the ribbon orbitals of the a-framework
is not possible.

Keywords

Hrčak ID:

194449

URI

https://hrcak.srce.hr/194449

Publication date:

25.1.1981.

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