# Dienes

## Overview

### Description

Compounds containing two double bonds are known as dienes. The arrangement of these double bonds within the molecule affects how they will react with each other and with other molecules. This reactivity may be explained by an understanding of frontier molecular orbital theory and an examination of the $\pi$ orbitals of the alkenes that are used for bonding. If these $\pi$orbitals are near enough to each other to overlap, they may either share electrons (providing resonance and stability in a molecule) or transfer electrons, allowing the creation of a new bond.

### At A Glance

• Dienes are classified as cumulated, isolated, or conjugated; conjugated dienes are more stable than isolated dienes because of electron delocalization over the $\pi$ system.
• Conjugated dienes undergo electrophilic addition reactions in which the electrophile reacts with either one double bond (1,2-addition) or both double bonds (1,4-addition).
• The Diels-Alder reaction is a [$4+2$] cycloaddition in which a diene ( 4 $\pi$ electrons) and a dienophile (2 $\pi$ electrons) combine to form a cyclohexene molecule (or a cyclohexadiene if an alkyne is used as the dienophile).
• Frontier molecular orbital (FMO) theory dictates whether a reaction will occur naturally or will need to have the electrons excited to form a product. The Woodward-Hoffmann rules predict the stereochemistry and activation energy of pericyclic reactions.