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Stereoelectronic Effects and General Trends in Hyperconjugative Acceptor Ability of Bonds

Summary: Stereoelectronic Effects and General Trends in
Hyperconjugative Acceptor Ability of Bonds
Igor V. Alabugin* and Tarek A. Zeidan
Contribution from the Department of Chemistry and Biochemistry, Florida State UniVersity,
Tallahassee, Florida 32306-4390
Received November 30, 2001
Abstract: A systematic study of general trends in acceptor properties of C-X bonds where X is a main
group element from groups IVa-IIa is presented. The acceptor ability of the C-X bonds in monosubstituted
ethanes increases when going to the end of a period and down a group. Enhancement of acceptor ability
of C-X bonds as one moves from left to right in periods parallels the increase in electronegativity of X,
whereas augmentation of acceptor ability in groups is opposite to the changes in electronegativity of X and
in the C-X bond polarization, following instead the decrease in the energy of *C-X orbitals when one
moves from the top to the bottom within a group. This simple picture of acceptor ability of bonds being
controlled by electronegativity in periods and by * orbital energy in groups is changed in monosubstituted
ethenes where the role of electronegativity of the substituent X becomes more important due to increased
overlap between orbitals. The combination of several effects of similar magnitude influences acceptor
ability of bonds in monosubstituted ethenes in a complex way. As a result, the acceptor ability of bonds
can be significantly modified by substitution and is conformer dependent. Stereoelectronic effects displayed
by C-X bonds with X from second and third periods are highly anisotropic. For example, C-chalcogen
bonds are excellent acceptors at the carbon end but poor acceptors at the chalcogen end. This effect


Source: Alabugin, Igor - Department of Chemistry and Biochemistry, Florida State University


Collections: Chemistry; Biology and Medicine