Page 36 - Mechanism and Theory in Organic Chemistry

Figure 1.15 Formation of sp hybrids from an s and one p orbital.
The reader should now complete the bonding model for CH, by construct-
ing a bonding-antibonding pair for each of the other three interacting pairs of
atomic orbitals.
Other Types of Hybridization
A hybridization scheme can be constructed for each of the various possible geo-
metries about the central atom. The sp3 hybridization discussed above gives
hybrids oriented at 109.5" angles to each other, and is appropriate to tetrahedral
atoms. For trigonal atoms, the two P orbitals lying in the plane containing the
nuclei are combined with the s to yield three sp2 hybrids, as shown in Figure 1.14.
For a linear geometry, the appropriate hybridization is sp (Figure 1.15).
The relative _-- contri4-Lutir>ns.of~andp __ orbitals to - the..hpbrids_isidiffer-g-nt fo_~
the different types of hybridizati0.n. An sp3 hybrid contains a larger proportion of
p and a smaller proportion of s than an sp2, which in turn contains more p and
less s than an sp. Since,-s electrons can penetratecloser to. the-nu-cld-s. than p
electrons which - have a node -- at_the-nu_cl~s,_s_electrons are held more tightly.
-
..
..
.
-
-
Y
Therefore an atom is effectively more electronegative in bonds that use a larger
p.roPo~ti~n of s. Appendix 1 to this chapter gives a systematic procedure for
specifying the proportions of s and P, and also shows how the s and P contribu-
tions change with changing geometry. __- - Hybrid Orbitals 25
a And T Bonding. Ethylene
The ethylene molecule will illustrate construction of a model containing both o
and n bonding. The Lewis structure (22) shows that each carbon should be
approximately trigonal. Therefore we need sp2 hybrids on each carbon. Figure