Bonding pi bonds

All single bonds between two atoms are sigma (o) bonds. Pi bonds can only be formed after a sigma bond has already been formed. Therefore a double bond contains one o and one n bond, and a triple bond contains one o and two k bonds. 

Pi bond When atomic orbitals overlap side to side in such away that the resulting molecular orbital is symmetric with the bond axis in only one plane, chemists say that a 7t bond (pi bond) is formed. 

Sigma bonds form when s or p orbitals overlap in a head-on manner. Single bonds cire usually sigma bonds. Pi bonds cire usually double or triple bonds. Figure 5-9 depicts these situations. 

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... 

The other type of bond that can form is a pi (it) bond. Pi bonds are the type of bonds that make up multiple bonds and are formed when p orbitals on neighboring atoms align with one another in a parallel fashion. The electrons in the p orbitals distribute themselves above and below the axis (where the s bond has occurred). Pi bonds are weaker than sigma bonds. Atoms that have only single available p orbitals can form a single tt bond, where atoms with two available p orbitals can form two tt bonds. The formation of pi bonds prevents molecules from rotating around the internuclear axis in Figure 7.23 ... 

The most important place to watch orbital overlap is in the formation of pi bonds. Pi bonds that have about a 30 twist can be made but are reactive. Since substitution competes with elimination, it does not take very much of a twist to tilt the balance away from elimination. There are many examples to show that changing the leaving group from approximately coplanar with the hydrogen to 60 out of alignment shuts off the E2 elimination process entirely. Be especially careful with eliminations in rigid systems. 

Addition is the characteristic reaction of alkenes and alkynes. Since the carbons of a double or triple bond do not have the maximum number of attached atoms, they can add additional groups or atoms. Double bonds undergo addition once and triple bonds can undergo addition twice. The reactivity of alkenes and alkynes is due to the presence of pi-bonds. Unlike sigma bonds, pi-bonds are directed away from the carbons the electrons are loosely held, very accessible, and quite attractive electron-deficient species (electrophiles) seeking an electron source. 

Apply Urea, whose structure is shown below, is a compound used in manufacturing plastics and fertilizers. Identify the sigma bond, pi bonds, and lone pairs present in a molecule of urea. 

In contrast, the bond between the planes of h-BN is very weak and even weaker than that of graphite. It is readily broken and layers can be cleaved with a knife like an onion skin. However this bond is electronically different from that in graphite. In graphite, it stems from the hybridized fourth valence electron which is paired with another delocalized electron of the adjacent plane by a weak van der Wools bond (pi bond). The high electrical conductivity of graphite is attributed to these delocalized electrons. 

Bonds formed by the sideways overlap of p orbitals are called 7T bonds (pi bonds). A tt bond is not symmetrical about the axes joining the nuclei of the atoms forming the bond. Figure 4.24 shows how a 7t bond is formed from two p orbitals overlapping sideways. 

Electron that is part of a double bond (pi bond)... 

Here P, and D, represent the degree of imbalance of valence electrons in the p and d orbitals on atom 4. P, may be written in terms of orbital populations or in terms of qualitative constants such as ionicity of the bond, pi bond order, degree of s character ... 

The oxygen molecule has the formula O2. In this molecule, a total of four electrons are shared by the two atoms. What type of bond is this single, double, or triple Is the bond made up of sigma bonds, pi bonds, or both ... 

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