HYBRIDIZATION CONCEPT
It is the phenomenon of imaginary intermixing of individual orbital. The new orbital formed have equal energy and equivalent shape. The new orbitals are called hybrid orbitals. The properties of original orbitals forming hybrid orbitals are shared proportionally. The hybrid orbitals are formed because their formation not only lower the energy of atomic orbital but also give better direction to form more bond. The energy of the entire hybrid orbital is equal hence electron - electron repulsion keeps them at equal distance and at equal angle forming a symmetrical shape. This gives better directionability Lowering of energy is achieved by a) Minimization of electron pair repulsion b) Formation of strong bond c) Maximum overlapping of atomic orbital d) More bonds can be formed SP3 HYBRIDIZATION
In organic chemistry hybridization of carbon is most important. Carbon has four electrons in the outer most valence shell. The energy level of these electrons in ground state and hybridized state of carbon atom is shown
SP3 Hybridized orbital are formed by promoting one ‘2S’ electron to vacant ‘P’ orbital and mixing all four orbital to form four hybrid orbital of equal energy, capable of forming four bonds. Each orbital has 25% ‘S’ character and 75% ‘P’ character. The shape of orbital is like distorted dumbbell. Each orbital has one electron and can form a strong bond with another atom. Electron pair repels each other and best shaped formed is tetrahedral with each electron at each corner. SP3 hybridized orbital Tetrahedral shape of CH4
The close look at the structure of methane indicate that all the four bond formed are single bond or sigma – bond ( - bond). The four bond angles and bond length are equals. The structure is also symmetrical. These facts prove that all the four orbital has equal energy.
TYPE OF HYBRIDIZATION There are several other types of hybridized orbitals formed such as sp2, sp, dsp3, d2sp3, etc. However in organic compound most of hybridization is limited to sp3, sp2 and sp. In sp2 each orbital has 33.33% ‘s’ character and 66.66% ‘p’ character while in sp orbital ‘s’ and ‘p’ character is 50% each The type of hybridization on carbon, nitrogen and can easily be identified by using the following table.
Hybridization is applicable to a particular atom and not to a molecule. Different atom in a molecule may have different type of hybridization, e.g. carbon and oxygen in carbon dioxide has SP and SP2 hybridized respectively
In general hybridization can be found out by adding the number of sigma bond and lone pair on the atom on which hybridization is to be found out. (Consider only the sigma bond and do not account any pi – bond ) Match this number with number of orbitals involved starting with ‘S’ orbital. ‘S’ orbital can form one hybrid orbital. ‘P’ can form maximum three orbitals and ‘d’ can form maximum up to five orbital. Write these number as superscript on the symbol of orbital. No superscript is given if no. of orbital is one. Let us take an example of phosphorus pentachloride (PCl5). Central atom in the molecule is
Number of sigma bond formed = 5 Number of lone pair = 0 Total = 5
Now to find out type of hybridization number five has to be matched with various orbital superscripts. Number of ‘s’ orbital is one, number of ‘p’ orbital is three and number ‘d ‘ orbital is one. Therefore type of hybridization is sp3d Molecular shape, and bond angle depend upon type of hybridization. Each has its own shape and angle
The shape of molecules also depends upon lone pair of electron. Lone pairs, pi – electron and strain on the ring can distort the standard shape. This distortion occurs because lone pairs and as well as pi – electron needs more space e.g.lone pair on water makes the bond angle as 104.50
DIPOLE MOMENT Concept of dipole moment can best be understood by knowledge of electronegtivity, polar molecules and bond, formation of dipole and its characteristic. Electronegetivity is a measure of the tendency of an atom to attract a bonding pair of electron Consider a bond between two atoms, A and B.
If the two atoms are equally electronegative, both will have the same tendency to attract the bonding pair of electrons, and so the pair will be found half way between the two atoms, e.g. O2, N2, F2 If ‘B’ is more electronegative than ‘A’ then ‘B’ will attract the electron pair rather more than A. That means that the B end of the bond more negative. At the same time, the A end (short of electrons) becomes more positive.
This type of bond is called a polar bond. A polar bond is a covalent bond in which there is a separation of charge between one end and the other. In other words one end is more positive and the other more negative. e.g. hydrogen-oxygen bonds in water, hydrogen – iodine bond in HI, nitrogen – hydrogen bond in ammonia etc. In a simple molecule like HCl, if the bond is polar, the whole molecule is polar. But it is always not so. There