Chapter 30.3, Problem 3P

Interpretation Introduction

Interpretation:

Only the 2E,4E isomer is obtained in practice by opening of trans-3,4-Dimethylcyclobutene by two conrotatory paths to give symmetry-allowed products either (2E,4E)-2,4-hexadiene or (2Z,4Z)-2,4-hexadiene.

Concept introduction:

An electrocyclic ring-opening reaction is a reaction in which a σ bond of a cyclic reactant is cleaved to form a conjugated product with one more π bond. It occurs only when like phases of orbitals can overlap to form a bond. Such a reaction is symmetry allowed. It cannot occur between orbitals of opposite phase. Such a reaction is symmetry forbidden. To form a bond, the p orbitals on the terminal carbons must rotate so that like phases can interact to form the new sigma bond. Two modes of rotation are possible - When like phases of the p orbitals are on the same side of the molecule, the two orbitals must rotate in opposite direction one clockwise and one counterclockwise. Rotation in opposite directions is said to be disrotatory.

When like phases of the p orbitals are on opposite sides of the molecule, the two orbitals must rotate in the same direction both clockwise or both counterclockwise. Rotation in the same direction is said to be conrotatory.