16.1 : Structure of Conjugated Dienes

Introduction

Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the double bonds.

Conjugated diene	Cumulated diene	Isolated diene

Synthesis of Conjugated Dienes

There are two common approaches for preparing conjugated dienes:

1. From allylic halides: Allylic halides undergo dehydrohalogenation in the presence of potassium tert-butoxide to form conjugated dienes.

2. From alcohols: Double dehydration of diols in the presence of aluminum oxide gives conjugated dienes.

Configurational Isomerism

The double bonds in substituted conjugated dienes can adopt an E or a Z configuration and exhibit configurational isomerism. For example, 1-chloro-2,4-heptadiene has four configurational isomers, as shown below.