Chapter 28, Problem 43P

Interpretation Introduction

Interpretation:

It should be explained that why compound A will not undergo a ring – opening reaction under thermal conditions, but compound B will.

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

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 pericyclic reactions are listed below

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

Woodward – Hoffmann rules for the configuration of pericyclic reactions are,

$\begin{array}{ccc}\text{Substituents}\text{in}\text{the}\text{reactant}& \text{Mode}\text{of}\text{ring}\text{closure}& \text{Configuration}\text{of}\text{the}\text{product}\\ \text{Point}\text{in}\text{opposite}\text{directions}& \text{Disrotatory}& \text{cis}\\ & \text{Conrotatory}& \text{trans}\\ \text{Point}\text{in}\text{the}\text{same}\text{directions}& \text{Disrotatory}& \text{trans}\\ & \text{Conrotatory}& \text{cis}\end{array}$