Overlap sigma

Covalent bonds are formed when atomic orbitals overlap. The overlap of atomic orbitals is called hybridization, and the resulting atomic orbitals are called hybrid orbitals. There are two types of orbital overlap, which form sigma (cr) and pi (tt) bonds. Pi bonds never occur alone without the bonded atoms also being joined by a ct bond. Therefore, a double bond consists of a O bond and a tt bond, whereas a triple bond consists of a ct bond and two tt bonds. A sigma overlap occurs when there is one bonding interaction that results from the overlap of two s orbitals or an s orbital overlaps a p orbital or two p orbitals overlap head to head. A tt overlap occurs only when two bonding interactions result from the sideways overlap of two parallel p... 

To generalize. Figure 4.8 shows three ways that the s ma overlap of a lone pair, a pi bond, or a sigma bond can stabilize a carbocation. The most effective sigma-overlap stabilization comes from overlap of the empty orbital of the carbocation with a lone pair orbital forming an onium ion for example, if X is Br, then it is called a bromonium ion. For comparison. Figure 4.5 gave the more common pi overlap stabilization of a carbocation by a lone pair, a pi bond, and a CH bond. 

Valence bond theory. Valence bond (VB) theory assumes that atoms form covalent bonds as they share pairs of electrons via overlapping valence shell orbitals. A single covalent bond forms when two atoms share a pair of electrons via the sigma overlap of two atomic orbitals—a valence orbital from each atom. A double bond forms when two atoms share two pairs of electrons, one pair via a sigma overlap of two atomic orbitals and one via a pi overlap. A triple bond forms by three sets of orbital overlap, one of the sigma type and two of the pi type, accompanied by the sharing of three pairs of electrons via those overlaps. (When a pair of valence shell electrons is localized at only one atom, that is, when the pair is not shared between atoms, it is called a lone or nonbonding pair.)... 

In the boat conformations (a)-(c), the sigma overlap of the shaded jr-orbitais at C4 and C5 can only occur if concerted rotations about the C3, C4 and C5, C6 axes take place as indicated. Such rotations, which are of particular significance in discussing electrocyclic reactions, either occur in the same relative direction as in (c), or are opposed as in (a) and (b). These types of concerted rotational motion have been called conrotation and disrotation by Woodward and Hoffmann (1965, 1968, 1969) see Section 3.3(b). 

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