Overview

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.

Procedure

Dienes are organic compounds containing two carbon–carbon double bonds. They are classified as cumulated, conjugated, or isolated.

Cumulated dienes have two adjacent double bonds. In comparison, the double bonds in conjugated dienes are separated by one single bond and in isolated dienes by two or more single bonds.

Each carbon atom in conjugated dienes is sp²-hybridized with an unhybridized p orbital perpendicular to the plane of the molecule.

The adjacent p orbitals overlap to form a continuous π system, where the π electrons are delocalized across the entire molecule.

This electron delocalization imparts a partial double bond character to the carbon–carbon single bond, making it shorter than a regular carbon–carbon single bond.

Additionally, the carbons in conjugated systems being sp² hybridized, have a higher s character than sp³-hybridized carbons. Since electrons in an sp² orbital are closer to the nucleus, the overlap between two such orbitals leads to shorter bonds.