Chapter 28, Problem 25P

Interpretation Introduction

Interpretation:

The product formed in the given reactions should be differentiated.

Concept introduction:

Pericyclic reactions are “ any concerted reaction in which bonds are formed or brocken in a cyclic transition state”.  There is a single transition state from start to finish, in contrast to a stepwise reaction.

There are mainly three types of pericyclic reactions,

1. 1) Electrocyclic reactions
2. 2) Cycloaddition reactions
3. 3) Sigmatropic reactions

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

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

A photochemical reaction takes place when a reactant absorbs light and a thermal reaction takes place without the absorption of light.

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

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