Chapter 28, Problem 39P

Interpretation Introduction

Interpretation:

It should be explained that why two different products are formed from disrotatory ring closure of (2E, 4Z, 6Z)-octatriene, but only one product is formed from disrotatory ring closure of (2E, 4Z, 6E)-octatriene.

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

In an electrocyclic reaction “one new sigma- bond is formed or brocken.”

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

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

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

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

Enantiomers are a type of stereoisomers that have the same molecular formula and constitutions around the atom but differ in their spatial arrangement of groups around the atom.