For more FREE Organic Chemistry Material visit www.examville.com...
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 ( σ - 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 are molecules in which the bond is polar but the molecule is not polar. In CO2, each bond is polar but the whole molecule is not polar. It doesn't have an end (or a side) which is more negative and the other one which is more positive. There is no overall separation of charge from top to bottom, or from left to right although each of its bonds is polar.
When one end of the molecule has more positive charge and the other end has more negative charge, the molecule is called a polar molecule, meaning that it has poles formed by opposite charge Otherwise; it is called a non-polar molecule. A non polar molecule does not mean that it has non polar bond A dipole has two poles, one positive and other negative formed by positive and negative charges. It can be present in a molecule or in a bond. All the positive charges come from proton present in nucleus and all the negative charges come from electron.
There are several centers of positive and negative charges present in a molecule. These positive and negative charges are distributed through out the molecule The total positive and negative charges are equal so that the molecule is neutral.( ions have net charge) A dipole is formed in a molecule when center of mass of all positive charges and all negative charges do not coincide. All polar molecule and polar bond have dipoles.(As discussed above a non polar molecule may have several polar bond) A neutral molecule may also have a dipole, while an ion in spite of having charges may not have a dipole
The essential condition of formation of dipole is that positive charge center and negative charge center in a molecule or ion should be situated at a different location The dipole moment is defined as the product of charge and distance between the charges
( is dipole moment, q is magnitude of charge and r is the distance between the + ve and – ve charge) The dipole moment is a vector quantity. It has a direction from + ve to – ve side and indicated by an arrow. Its unit is debye.
A molecule with dipole moment is called a polar and without dipole moment is called a non polar molecule. A molecule with polar bond may be non polar. As the dipole moment is a vector quantity, the sum of dipole moments of all polar bonds may add up to zero making the molecule non polar.
Induced dipoles occur when one polar molecule with a permanent dipole repels another non polar molecule's electrons, "inducing" a momentry dipole moment in that molecule.The polar molecule produces a electric field which pushes
the electron and nuclie of non polar molecule in opposite direction seperating center of + ve and – ve charge and thus creating a dipole. Induced dipoles are weaker than permanent dipols. Instantaneous dipoles (also called London dispersion forces) occur due to chance when electrons happen to be more concentrated in one place than another in a molecule, creating a temporary dipole. It is possible when at a given instant electron may not be distributed evenly between bonding atoms. Instantaneous dipoles are weaker than induced dipoles, short lived and can induce a dipole in the neighbouring atom Intermolecular forces. are forces that act between stable molecules or between functional groups.. These non-covalent forces, are generally much weaker than the bonding forces. Nevertheless, intermolecular forces are responsible for a wide range of physical, chemical, and biological proerties These forces occur only due to dipole moments. The + ve charge in one molecule attract – ve charge and reppel + charge in other molecule. All these forces jointly called Van der Waals forces A hydrogen bond is a special type of attractive interaction ( a type of dipole-dipole bond) that exists between an electronegative atom and a hydrogen atom This type of bond always involves a hydrogen atom, Hence the name. Hydrogen bond is always between either nitrogen,oxygen or floriene.and hydrogen forming a large dipole moment.as all these element are highly electronegative Hydrogen bonds can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly). The typical hydrogen bond is stronger than Van der Waals forces but weaker than covalent or ionic bonds.Hydrogen bond is resposible for high boiling point of the molecule ELECTRON DELOCALIZATION Delocalization is systems of electron in which bonding electrons are not localised between two bonding atoms forming a single bond but are spread (delocalized) over the whole group. These electrons are called delocalized electron. Delocalization generally (not always) occurs with pi – electron present in pi – bond. Resonance structures are the structures of the molecules which have pi – electrons delocalized. These molecules can not be represented by one structure and are represented by combination of two or more structure called resonance structure. The characteristic properties of the molecule are represented by average properties of all the resonating structure. Resonance energy is the difference in energy of real molecule and the energy of most stable resonating structure of the molecule. The energy of real
molecule is always lower than the energy of any resonating structure. The energy of all resonating structure is not same but differs from each other. Real molecule is different from any of the resonating structure. It is not one of the resonating structures but has average properties of all contributing structure.
Steps to draw Resonating structure: Taking the example of nitrate ion NO3 – 1. Count the total number of valence electron. Nitrogen has five and oxygen has six electrons and one for ion Total number of valence electron = (1Η5) + (3Η6) + 1 = 24 2 Draw the valence bond structure fig a 3 Add octet electrons to the atoms attached to central atom fig b 4 Place any left over electron on the central atom (24 - 24 = 0) 5 Check if central atom octet is complete It has six electrons 6 Add a multiple bond (first try with a double bond) to complete the octet of central atom fig c 7 Check if central atom has completed its octet If yes shift the multiple bonds on other atom to complete the resonating structure fig d
Precautions to be taken while drawing resonance structure • • • • •
Only proper Lewis dot structure should be drawn. All the atoms must lie in the same plane. Only electron should move Atoms should not move and should remain stationary. Number of unpaired electrons should remain unchanged.
The contribution of any structure to actual molecule is roughly proportional to stability of that structure. Higher stable molecule makes higher contribution, equally stable makes equal contribution and lowest stable will make least contribution. A molecule must have a conjugated unsaturated system to exhibit resonance phenomenon. A molecule or an ion is conjugate when it has an atom either with ‘p’ orbital or an unshared pair of electron. And this atom must be attached to another atom by a single bond. The .p. orbital may contain zero one or two electrons. This conditions occur in ϑ - bond. The ‘p’ orbital permits adjacent pi – bond to extend and encompass more than two nuclei. Ring structure showing resonance properties must satisfy Huckle’s rule. The compound which follows Huckle’s rule are aromatic The rule state that a planer monocyclic molecule must have 4n+2 pi – electron (where n = 0,1,2,….. etc) if the molecule is aromatic (show resonance)