What is the Difference Between Conjugated and Cumulated Dienes

The key difference between conjugated and cumulated dienes is that conjugated dienes have two double bonds separated by a single bond, whereas cumulated dienes have two double bonds connected to a similar atom.

Dienes are organic compounds that are very important in organic chemistry. These are also known as diolefins or alkadienes. These are covalent compounds consisting of two double bonds among carbon atoms. Therefore, these compounds consist of two alkene units along with the standard prefix “di” of systematic nomenclature.

CONTENTS

1. Overview and Key Difference
2. What are Conjugated Dienes 
3. What are Cumulated Dienes 
4. Conjugated vs Cumulated Dienes in Tabular Form
5. Summary – Conjugated vs Cumulated Dienes 

What are Conjugated Dienes?

Conjugated dienes are organic compounds with double bonds separated by single bonds. In other words, a conjugated diene has an alternating pattern of double bonds and single bonds. This makes a conjugated system of an electron cloud throughout the conjugated system. However, if the double bonds are separated by more than one single bond, then we call it an isolated diene. These conjugated dienes are mostly used as monomers in the polymer industry.

The preparation of conjugated dienes is similar to the way we use the elimination reaction of alkyl halides to make alkenes. We can use dihalides to convert them into conjugated dienes through two successive elimination reactions. In this process, it is important to use a strong, sterically hindered base. This is helpful in preventing the competing substitution reactions. There are possible byproducts of this process, including alkynes upon using strong bases such as sodium amide. Another method of preparing conjugated dienes is the elimination reaction of allylic halides.

Moreover, we can determine the stability of conjugated dienes to demonstrate the heat of hydrogenation. E.g., when comparing 1,3-butadiene and 1-butene, 1,3-butadiene has an extra double bond that needs an additional mol of hydrogen to reduce it into butane. Therefore, the hydrogenation of 1,3-butadiene generates more heat than that of 1-butene. However, the experimental values for the heat of hydrogen are less than expected because of the stability of this compound. The conjugated system provides extra stability to the molecule.

What are Cumulated Dienes?

Cumulated dienes are organic compounds having double bonds sharing a common atom. In other words, two double bonds are attached to the same atom. Typically, these compounds are less stable than conjugated dienes because the conjugated dienes form the conjugated system, which makes the molecule more stable. These compounds are also known as cumulated alkadienes.

The simplest cumulated diene molecule we can find is 1,2-propadiene. It is also known as an allene. There is a central carbon atom in these molecules which has sp hybridization. The geometry around this carbon atom is linear.

Generally, cumulated dienes are unstable compared to conjugated dienes. This is because these compounds are a probable state for the triple bond of an alkyne to move down the carbon chain towards the most stable position.

What is the Difference Between Conjugated and Cumulated Dienes?

There are three different ways the double bonds in dienes can be arranged in the molecule: conjugated, isolated, or cumulated. The key difference between conjugated and cumulative dienes is that conjugated dienes have two double bonds separated by a single bond, whereas cumulated dienes have two double bonds connected to a similar atom.

The following table summarizes the difference between conjugated and cumulated dienes.

Summary – Conjugated vs Cumulated Dienes

A diene is an organic compound consisting of two double bonds. In conjugated dienes, the two double bonds are separated by a single bond, whereas in cumulated dienes, two double bonds are connected to a similar atom. This is the key difference between conjugated and cumulated dienes.

Reference:

1. “Conjugated Dienes.” Chemistry LibreTexts, Libretexts, 13 Sept. 2020.
2. “Conjugated, Cumulated, and Isolated Dienes.” Chemistry Steps, 23 July 2021.

Image Courtesy:

1. “1,3-butadiene” By Lukáš Mižoch – Own work (Public Domain) via Commons Wikimedia
2. “Cumulated Diene EXAMPLE C V” By Jü – Own work (CC0) via Commons Wikimedia