Mr. Sayad Imran, Asst. Professor Y B C C P- A Bonding Parameters Mr. Sayad Imran, Asst. Professor Y B C C P- A SAYAD IMRAN YBCCP-A

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Bonding Parameters Bond Length Bond Order Bond Energy Bond Angle Steric Effect Resonance SAYAD IMRAN YBCCP-A

It is expressed in Angstrom units (Å) or picometers (pm). Bond Length Bond length is the average distance between the centers of the nuclei of two bonded atoms in a molecule. It is expressed in Angstrom units (Å) or picometers (pm). 1Å = 10-10m and 1pm = 10-12 m. It is determined by X-rays diffraction, electron diffraction and other spectroscopic methods. SAYAD IMRAN YBCCP-A

Vibrational motions in general diatomic and triatomic molecules. SAYAD IMRAN YBCCP-A

Covalent radius and Bond length One half the equilibrium distance between atomic nucleus of two covalent bonded atoms of same kind is called covalent radius. The sum of covalent radii of two atoms is usually the bond length. For example, the covalent radii of H and C are 37 and 77 pm respectively. The C-H bond is thus (37+77) 114 pm. SAYAD IMRAN YBCCP-A

Bond length and covalent radius. Internuclear distance (bond length) Covalent radius 72 pm Internuclear distance (bond length) Covalent radius 114 pm Internuclear distance (bond length) Covalent radius 100 pm Internuclear distance (bond length) Covalent radius 133 pm SAYAD IMRAN YBCCP-A

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Factors affecting Bond length Bond Order Hybridization state Size of the atom Electronegativity SAYAD IMRAN YBCCP-A

Bond length decreases with increase in bond order. The bond order is the number of electron pairs being shared by a given pair of atoms. A single bond consists of one bonding pair and has a bond order of 1. For a given pair of atoms, a higher bond order results in a shorter bond length and higher bond energy C≡C bond length is shorter than C=C bond which in turn is shorter than C-C. Similarly, N≡N bond length is shorter than N=N bond which in turn is shorter than N-N and O=O > O-O. SAYAD IMRAN YBCCP-A

The Relation of Bond Order, Bond Length, and Bond Energy SAYAD IMRAN YBCCP-A

nucleus than do p orbitals. Hybridization State The bond lengths are also affected by the hybridization states of the carbon atoms involved. ● The greater the s orbital character in one or both atoms, the shorter is the bond length. This is because s orbitals are spherical and have more electron density closer to the nucleus than do p orbitals. ● The greater the p orbital character in one or both atoms, the longer is the bond length. This is because p orbitals are lobe-shaped with electron density extending away from the nucleus. Hybridization % S % P sp 50 sp2 33 67 sp3 25 75 SAYAD IMRAN YBCCP-A

C—F (142 pm) < C—Cl (177 pm) < C—Br (191 pm) < C—I (213 pm). Size of the atom The bond length increases with increase in the size of the atom i.e. atomic number H—H < C—C < I—I This is because as the atomic number increase, the distance of the electrons from the nucleus increases successively with the addition of a new shell. Therefore the average distance between the bonding nuclei (bond length) increases. C—F (142 pm) < C—Cl (177 pm) < C—Br (191 pm) < C—I (213 pm). SAYAD IMRAN YBCCP-A

Electronegativity The order of electronegativity of hybrid orbitals is: sp > sp2 > sp3 i.e., the electronegativity of carbon is maximum in the sp-hybridized state and minimum in sp3 -hybridized state The a bond formed with a more electronegative atom will be shorter than that formed with a less electronegative atom. SAYAD IMRAN YBCCP-A

Bond Energy (Bond Dissociation Energy) The minimum amount of energy required to break a bond is called bond dissociation energy or simply bond energy. It gives us information about the strength of a bonding interaction. It is expressed in terms of kJ mol-1 or Kcal/mole. When a bond is formed between the atoms, energy is released. SAYAD IMRAN YBCCP-A

Bond dissociation energy depends upon: Size of the bonded atoms The smaller the size of the bonded atoms, the stronger is the bond and larger is the value of bond dissociation energy. For example, the bond dissociation energy of H-H bond in hydrogen molecules is 433 kJ mol-1. This is larger than the bond dissociation energy of Cl-Cl in Cl2, which is 242.5 kJ mol-1. Bond length Shorter the bond length, larger is the value of bond energy. For example, C-C bond length (154 pm) is larger than C=C bond length (134 pm). Consequently, the dissociation energy of C-C bond (348 kJ mol-1) is smaller than that of C=C bond (619 kJ mol-1). SAYAD IMRAN YBCCP-A

Important features of bond energy The magnitude of the bond energy depends on the type of bonding. Most of the covalent bonds have energy between 50 to 100 kcal mol–1. Strength of sigma bond is more than that of a π-bond. A double bond in a diatomic molecules has a higher bond energy than a single bond and a triple bond has a higher bond energy than a double bond between the same atoms. C ≡ C > C = C > C – C The magnitude of the bond energy depends on the size of the atoms forming the bond, i.e. bond length. Shorter the bond length, higher is the bond energy. Resonance in the molecule affects the bond energy. The bond energy decreases with increase in number of lone pairs on the bonded atom, due to electrostatic repulsion of lone pairs of electrons of the two bonded atoms. SAYAD IMRAN YBCCP-A

Homolytic and heterolytic fission involve different amounts of energies. Generally the values are low for homolytic fission of the bond in comparison to heterolytic fission. SAYAD IMRAN YBCCP-A

Yellow fill circle indicate center atom Bond Angle Lewis structures of molecules are two dimensional, giving only the image of bonding in the molecule. But, all molecules containing three or more atoms are three dimensional in nature. The shape of a particular molecule is determined by the specific arrangement of atoms in it and the bond angles. Polyatomic molecule contain at least two or more atom covalently bonded with central atom. Yellow fill circle indicate center atom The angle between any two covalent bond axes at central atom in a molecule is Bond angle SAYAD IMRAN YBCCP-A

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Bond angle measured by X-ray diffraction and spectroscopic methods. Because of the constant atomic vibrations, the bond angles thus measured are really average bond angles. SAYAD IMRAN YBCCP-A

Factor affecting Bond Angle Electronegativity of central atom The bond angle of a molecule having the same terminal atom decreases with decreasing electronegativity. H2O < H2S < H2Se [Hyderogen selenide] 104028’ 9205’ 910 Effect of Electronegativity or Size of terminal atom Bind angle increases as Electronegativity of terminal atom decreases. Bind angle increases as size of terminal atom increases. PF3 PCl3 PBr3 PI3 970 1000 101.50 1020 SAYAD IMRAN YBCCP-A

Number of regions of high electron density around central atom Molecular shapes and bond angles Number of regions of high electron density around central atom Arrangement of regions of high electron density in space Predicted bond angles Example Geometry of molecule 4 tetrahedral 109.5° CH4, methane NH3, ammonia pyramidal H2O, water bent 3 trigonal planar 120° H2CO, formaldehyde C2H4, ethylene planar SO2, sulfur dioxide 2 linear 180° CO2, carbon dioxide C2H2, acetylene SAYAD IMRAN YBCCP-A

Steric Effect The bulk or volume of an atom or group of atom on the reacting part of an organic species and their special arrangement has varied type of effect on stability, reactivity(acidity, basicity), rate of reaction, stereochemistry and yield of product these effect may be called Steric Effect. Type of Steric Effect i. Steric Strain ii. Steric Acceleration please refer Sachin Ghosh iii. Steric Retardation Page no176-178 iv. Steric Effect and electron availability } SAYAD IMRAN YBCCP-A

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