Chapter 27, Problem 27.9P

Interpretation Introduction

Interpretation: The structure of provitamin ${\text{D}}_{\text{3}}$ is to be drawn.

Concept introduction: Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds. These reactions can take place in photochemical and as well as thermal conditions. The photochemical and thermal reactions take place in the presence of light and heat respectively.

Curved arrows aid in determining the movement and flow of electrons in the reaction. The electrons that take part in the chemical reactions are shown by the curved arrows.