This document provides an overview of molecular orbital theory (MOT) as it applies to organic compounds. It introduces MOT and the requirements for molecular orbital formation, including compatibility of atomic orbital symmetries and equal numbers of atomic and molecular orbitals. It discusses MOT of ethylene, allyl cation/anion, butadiene, hexatriene, and benzene. It explains how conjugation affects stability and reactivity, and how benzene is more stable than 1,3,5-hexatriene due to aromaticity from conjugation of its pi orbitals.
2. OVERVIEW
Why MOT was introduced?
Molecular orbital theory
Requirements for formation of molecular orbital.
Mot of organic compounds
Stability of benzene over 1,3,5-hexatriene
Effect of conjugation on stability and reactivity
4. VSEPR - By Sidgwick
Provides a simple procedure to
Methane (CH4)
predict
Lewis structure:
the shapes of covalent molecules.
• The shape of a molecule depends Central atom: carbon
upon the number of valence shell Valence 4
electron pairs (bonded or electrons on
nonbonded) around the central central atom:
atom. 4 H each 4
• Pairs of electrons in the valence contribute 1
shell repel one another since their electron:
electron clouds are negatively
Total: 8
charged.
Divide by 2 to 4
• These pairs of electrons tend to
give electron
occupy such positions in space that
pairs
minimise repulsion and thus
4 electron tetrahedral for the
maximise distance between them.
pairs: four shape-
determining
electron pairs
5. VBT
By Heitler and London
For example, consider CH4
Carbon
Electronic configuration:1s22s22p2
G.S
E.S
Forms 4 sp3 hybridised orbitals
6.
7. Molecular orbital theory
Molecular orbital is a mathematical function which describes wave like
character of an e-. The wave character of the electron tells about the
distribution of e- density among the molecular orbitals.
8. Introduced by Friedrich Hund and Robert S. Mulliken
1)The electrons in a molecule are present
in the various molecular orbitals as the
electrons of atoms are present in the
various atomic orbitals.
2)In a molecular orbital e-s iare influenced by two
or more nuclei depending upon the
number of atoms in the molecule.
3)The electron probability distribution around
a group of nuclei in a molecule is given
by a molecular orbital.
4)The molecular orbitals are filled in
accordance with the aufbau principle
obeying the Pauli’s exclusion principle
and the Hund’s rule.
9. Requirements For Formation Of Molecular
Orbital
1) There should be compatibility between the
symmetry of atomic orbitals, if not, there will
not be a formation of a molecular orbital.
12. 2) The number of atomic orbitals undergoing
bonding interactions should produce a same
number of molecular orbitals.
13. For example, consider H2
H has 1e-in its valence shell
Needs 1e- to acquire stable state.
So 1s orbital of 1H atom interacts with 1s orbital of another H
atom
This gives 2 mos.
14. number of atomic orbitals before the bonding
interactions = number of molecular orbitals
after the bonding interactions
15. Comparison of energy between atomic & molecular orbital
Energy level of Energy level of
>>>>
atomic orbitals molecular
orbitals
16. MOs Of Organic Compounds
Pi –Mos of ethylene: LUMO
σ= a- b
2
a b
##The electrons in the
BMO possess less
σ= a+ b energy than the
1 individual atomic orbitals
HOMO
