Chapter 28.3, Problem 5P

(a)

Interpretation Introduction

Interpretation: MOs for the given conjugated systems has to be drawn and the HOMO has to be predicted as symmetric or antisymmetric.

Concept introduction:

Conjugated system: A system of connected p-orbitals with delocalized electrons with alternating single and multiple bonds and the compound may be cyclic, linear or mixed.

Molecular orbital theory suggests that atomic orbitals of different atoms combines to create molecular orbitals.

Molecular orbitals can be constructed from linear combination of atomic orbitals.

Bonding orbotals are formed by the additive combination of atomic orbitals and the antibonding orbitals are formed by the substractive combination of atomic orbitals.

Antibonding orbital is a molecular orbital that results when two parallel atomic orbitals with opposite phases interact.

Antibonding orbitals have higher energy than the bonding molecular orbitals.

Ground state and and exited states are the positions with lower and higher energy respectively.

HOMO is a molecular orbital which is the abbrevation of Highest Occupied Molecular Orbital.

LUMO is also a molecular orbital which is the short form of Lowest Unoccupied Molecular Orbital.

If the lobes at the ends of the MO are in phase, then the MO is symmetric.

If the two lobes are out phase then the MO is antisymmetric.

Woodward –Hoffmann rules are the set of rules used to vindicate or predict certain aspects of the stereo chemical outcome and activation energy of pericyclic reactions.

Woodward – Hoffmann rules for Electrocyclic reactions are listed below

$\begin{array}{ccc}\text{Number}\text{of}\text{conjugated}\text{Π}\text{bonds}& \text{Reaction}\text{conditions}& \text{Allowed}\text{mode}\text{of}\text{rong}\text{closure}\\ \text{Even}\text{number}& \text{Thermal}& \text{Conrotatory}\\ & \text{Photochemical}& \text{Disrotatory}\\ \text{Odd}\text{number}& \text{Thermal}& \text{Disrotatory}\\ & \text{Photochemical}& \text{Conrotatory}\end{array}$

(b)

Interpretation Introduction

Interpretation: The validity of Woodward – Hoffmann rule for an electrocyclic reaction has to be checked using the MOs of given systems.

Concept introduction:

Woodward –Hoffmann rules are the set of rules used to vindicate or predict certain aspects of the stereo chemical outcome and activation energy of pericyclic reactions.

Woodward – Hoffmann rules for Electrocyclic reactions are listed below

$\begin{array}{ccc}\text{Number}\text{of}\text{conjugated}\text{Π}\text{bonds}& \text{Reaction}\text{conditions}& \text{Allowed}\text{mode}\text{of}\text{rong}\text{closure}\\ \text{Even}\text{number}& \text{Thermal}& \text{Conrotatory}\\ & \text{Photochemical}& \text{Disrotatory}\\ \text{Odd}\text{number}& \text{Thermal}& \text{Disrotatory}\\ & \text{Photochemical}& \text{Conrotatory}\end{array}$