Chapter 1:- Structure and Reactivity
B.sc 1st year Book Organic Chemistry (Page 3)
Bond Parameters
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Bond Lengths :
When in a molecule or ionic crystal, atoms are brought close to each other, the attraction takes place and therefore the potential energy of the system keeps on decreasing till a particular distance. If the atoms are further brought close to each other, the repulsion starts and the potential energy of the system begins to increase. When attractive and repulsive forces are equally balanced at a state of the stable equilibrium position, the potential energy of the system is minimum. At this state of equilibrium, the average distance between the centers of the nuclei of the two bonded atoms is called ‘bond length‘.
                                       Bond length is measured in either A° or pm(1Aº=10−10m and 1pm=10−12) by x-ray diffraction methods or spectroscopic methods. In an ionic crystal, the bond length is the sum of their ionic radii and in a covalent compound, it is the sum of the covalent radii of the bonded atoms.
Factors Governing the Bond length:
(i) Size of the atoms:
(ii) Multiplicity of the bonds:
(iii) Type of hybridization:
(iv) Type of compounds:
(v) Resonance effects:
Bond | Length (A∘) | Bond | Length (A∘) | Bond | Length (A∘) |
C−C | 1.54 | C−F | 1.42 | C−N | 1.47 |
CC | 1.40 | G−Cl | 1.77 | C−O | 1.43 |
C=C | 1.21 | C−Br | 1.91 | C=O | 1.20 |
C−H | 1.12 | C−I | 2.13 | C−S | 1.82 |
N−H | 1.03 | O−H | 0.97 |
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Bond Energies:
They are expressed in kJmol−1 and are two types :
(i) Bond dissociation energy and
(ii) Bond energy
(i) Bond dissociation energy :
It is designated by D. This is the energy required to break a particular bond in a polyatomic molecule, in the gaseous phase into neutral separate atoms. Greater the bond dissociation energy, the stronger the bond. Since a particular type of bond present in different molecules(O-H bond in H2O, alcohols, phenols, and carboxylic acids) or even in the same molecule(eg. O-H bond in H2O and C−H bond in CH4 ) do not possess the same bond energies, bond energies are usually the average value.
(ii) Bond energy :
It is designated by E. In polyatomic molecules, e.g., in CH4, all the four C−H bonds are equivalent, but the energy required to break the first bond (CH4⟶C˙H3+H˙) is not the same as that for the second bond ( C˙H3⟶C˙H2+H˙ ) and so on for the fourth bond as it is clear from table 1.04.
The table shows that the values of the dissociation energies of the C−H bond vary in the first four compounds. Thus, the energy of a bond depends on the nature of the rest of the molecule. In practice, it is usual to take the average of all the different values, and this average value is called bond energy. For diatomic molecules, D and E are identical.
Factors Governing the Bond energy:
Various factors are responsible for the differences in the energy of a given bond in different compounds.
(i) Size of the atoms:
Greater the size of the atoms, the greater the bond length and the smaller the bond energy.
(ii) Multiplicity of the bonds:
(iii) Angular strain:
Bond: | C−C | :N−N: | :O:−:O: | :F::−::F: |
Lon Pair of electrons: | 0 | 1 | 2 | 3 |
Bond Energy(kcal): | 83.0 | 39.0 | 35.0 | 36.0 |
Another factor that is also responsible for influencing the value of bond energy is steric effects.
3. Bond angles:

Hybridization: | sp3 | sp2 | sp |
Bond Angle: | 109.5°  < | 120°  < | 180° |
(iii) Multiplicity of the bonds:
(iv) Size of the central atoms:
Bond angle: 104.5∘ 92.5∘ 91.0∘ 89.5∘