![]() ![]() When we draw tetrahedral geometries of sp3 carbons like that found in methane it is conventional to draw two bonds in the plane of the page (straight or solid lines), one bond behind the plane as a dashed line, and the fourth bond as a shaded triangle coming out of the plane. Tetrahedral geometry creates a tetrahedron which is a four-faced triangular pyramid with bond angles of 109.5 degrees between each of the hydrogens. To minimize these repulsions between the hydrogens the methane adopts a tetrahedral geometry. These hydrogen atoms each have electron clouds around them which are negative and repel each other. An sp3 hybridized carbon like methane has four bonds each going to a single hydrogen atom. And a dashed line means a bond going away from you into the plane of the page. A shaded triangle (or wedge) means a bond coming toward you out the plane of the page. A straight line (or solid line) represents a bond that is part of the plane of the page. It's meant to show the 3-D shape of bonds in molecules like the sp3 hybridized bonds in methane. They can overlap, but in different ways, and the bonds thus formed are not called sigma bonds but pi bonds. Since the other orbitals are not oriented along the bond axis, they cannot overlap "head on". pz orbital, along the z axis, and any s orbital (which is spherical) can overlap to form a sigma bond. If the z-axis is taken as the bond axis, only orbitals with the central axis along z-axis can form sigma bond. Head on overlap is actually a layman's term to specify the requirement of specific symmetries in combining atomic orbitals. A sigma bond involves head on overlap of atomic orbitals. The second question can be much more satisfactorily answered. In that concept, there is no explanation as to why we do not include the inner orbitals, but by not including them we get the right answers, and hence that became a so called "rule" of hybrid orbitals. Actually, there are no hybrid orbitals and hybridisation concept, introduced by Pauling is now obsolete and replaced with the superior molecular orbital model, which answers all the shortcomings of hybridisation, one of which you just mentioned. This advocates for the increased role of theoretical methods in analysis of stereoelectronic effects.A god question, but unfortunately no simple answer. While the general trends revealed in this work should be useful for the qualitative understanding of stereoelectronic effects, one should bear in mind that the magnitude of hyperconjugative effects is extremely sensitive to small variations in structure and in substitution. This effect can be relied upon in the design of molecular diodes with sigma bridges with unidirectional electron conductivity. For example, C-chalcogen bonds are excellent sigma acceptors at the carbon end but poor sigma acceptors at the chalcogen end. ![]() Stereoelectronic effects displayed by C-X bonds with X from second and third periods are highly anisotropic. As a result, the acceptor ability of sigma bonds can be significantly modified by substitution and is conformer dependent. The combination of several effects of similar magnitude influences acceptor ability of sigma bonds in monosubstituted ethenes in a complex way. This simple picture of acceptor ability of sigma bonds being controlled by electronegativity in periods and by sigma 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 sigma orbitals. Enhancement of acceptor ability of C-X sigma 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 sigma(C)(-)(X) orbitals when one moves from the top to the bottom within a group. The acceptor ability of the C-X sigma bonds in monosubstituted ethanes increases when going to the end of a period and down a group. A systematic study of general trends in sigma acceptor properties of C-X bonds where X is a main group element from groups IVa-IIa is presented. ![]()
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