Orbital overlap in single and multiple bonds

A close look at the bonds shows two modes of orbital overlap, which correspond to the two types of covalent bonds  

Now we can answer a question brought up in our discussion of VSEPR theory in Chapter 10 why do the two electron pairs in a double bond act as one electron group that is, why don t they push each other apart The answer is that each electron pair occupies a distinct orbital, a specific region of electron density, so repulsions are reduced. 

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Orbital Overlap in Single and Multiple Bonds Orbital Overlap and Molecular Rotation... 

Conjugation (Chapter 14 introduction) A series of overlapping p orbitals, usually in alternating single and multiple bonds. For example, 1,3-butadiene is a conjugated diene, 3-buten-2-one is a conjugated enone, and benzene is a cyclic conjugated triene. 

An extra amount of stability or instability is associated with a compound that has a cyclic array of continuously overlapping p orbitals. Such a compound may have a ring with alternating single and multiple bonds, or the ring may contain both alternating 7r bonds and one atom with a lone pair or an empty orbital. If there is an odd number of electron pairs in the cyclic array of orbitals, then the compound is especially stable (as compared with the corresponding acyclic system with two additional H atoms), and it is said to be aromatic. If there is an even number of... 

As mentioned before, carbon and silicon mostly differ in their ability to from multiple p,-p, bonds E=Y with suitable partners (E = C, Si Y = element of group 14 to 16 ). While the p orbital overlap in compounds >C=Y is sufficient to yield stable multiple bonded species, this overlap is strongly reduced in the case of silicon (classical double bond rule of Pitzer and Mulliken). Consequently, under comparable conditions the equivalents of many unsaturated monomeric compounds of carbon, such as H2C=CH2, R2C=0 or CO2 are silicon single bonded polymeric products, e g. polysilanes (-H2Si-SiH2-)n, silicones (-R2Si-0-) and silicon dioxide (Si02)n... 

Orbitals overlap directly in sigma bonds. Parallel orbitals overlap in pi bonds. A single covalent bond is a sigma bond but multiple covalent bonds are made of both sigma and pi bonds. 

A situation in which two multiple bonds are separated by a single bond. Alternatively, a series of overlapping 2/) orbitals. 1,3-Butadiene, for example, is a conjugated diene, and 3-butene-2-one is a conjugated ketone. 

Bond types and multiple bonds Single bonds are always sigma bonds, bonds in which the electron pair is shared in an area centered between the two atoms. Such bonds can form when the bonding orbital is created by overlap of two s orbitals, an s and a p orbital, or two p orbitals. 

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