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Inorganic Pharmaceutical Chemistry 6

الكلية كلية العلوم للبنات     القسم قسم الكيمياء     المرحلة 3
أستاذ المادة محمد حامد سعيد الدهيمي       22/12/2012 04:23:17
Inorganic Pharmaceutical Chemistry
Lecture No. 6 Date : 22/11 /2012
Dr. Mohammed Hamed
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Hybrid Orbitals
Hybridization was introduced to explain molecular structure when the valence bond theory failed to correctly predict them. It is experimentally observed that bond angles in organic compounds are close to 109o, 120o, or 180o. According to Valence Shell Electron Pair Repulsion (VSEPR) theory, electron pairs repel each other and the bonds and lone pairs around a central atom are generally separated by the largest possible angles.
Based on the valence bond theory, carbon would only be able to form two covalent bonds, making CH2. However, as you will find out, we know that this is not true and that in reality, it makes CH4. The hybridization of orbitals is also greatly favored because hybridized orbitals are lower in energy compared to their separated, unhybridized counterparts. This results in more stable compounds when hybridization occurs. Also, major parts of the hybridized orbitals, or the frontal lobes, overlap better than the lobes of unhybridized orbitals. This leads to better bonding.
Carbon is a perfect example showing the need for hybrid orbitals. As you know, Carbon s ground state configuration is:
6 C 1S2 2S2 2P2

According to the valence bond theory, carbon should form two covalent bonds, resulting in a CH2. However, tests show that CH2 is highly reactive and cannot exist outside of a reaction. Therefore, this does not explain how CH4 can exist. However, you can excite a 2s electron and bump it into one of the 2p orbitals. This would give you the following configuration:

While this would allow us to have four covalent bonds, resulting in CH4, it also implies that the C-H covalent bonds would have different energies due to the different levels of orbital overlap. However, with testing, it has been proven that in CH4, any hydrogen can be removed with the same amount of energy. This means that every C-H covalent bond should have equal energies. Once again, this means that the valence bond theory fails to explain the existence of CH4. The only way it can be explained is if when we had the exited state above, the 2s and the 3 2p orbitals fused together to make four, equal energy sp3 hybrid orbitals. That would give us the following configuration:



This explains how a carbon can have four equal energy bonds. The next section will explain the various types of hybridization and how each type helps explain the structure of certain molecules.
For more information open pdf file

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