Why conjugated alkenes are more stable than isolated alkenes

In today’s post, we will mainly focus on conjugated dienes and some of their unique properties. So, first, what are dienes?

These are compounds that possess two C=C double bonds. The classification of denes is based on the proximity of the π bonds.

When they are adjacent (connected), we have cumulated dienes which are called cumulenes. Allenes is another name used to describe these compounds.

 When separated by one single bond, it is a conjugated diene. And when the double bonds are separated by more than one single bond, we have an isolated diene.

Preparation of Conjugated Dienes

Just like we use the elimination reaction reactions of alkyl halides to prepare alkenes, dihalides can be converted into conjugated dienes by two successive elimination reactions:

It is important to use a strong, sterically hindered base, to prevent the competing substitution reactions.

Possible side products here are alkynes especially if stronger bases such as sodium amide is used:

Another approach to prepare conjugated dienes would be performing elimination reactions on allylic halides:

Stability of Conjugated Dienes

The relative stability of isolated and conjugated dienes can be demonstrated by their heats of hydrogenation.

For example, let’s compare the heats of hydrogenation of 1-butene and 1,3-butadiene:

1,3-butadiene has an extra double bond which requires an additional mol of hydrogen to be reduced to butane. In this prospective, 1-butene can be looked at as an intermediate to the final product butane and we can expect that hydrogenation 1,3-butadiene will produce twice the amount of heat as 1-butene would.

However, the experimental data revealed that the heat of hydrogenation for the conjugated diene is less than expected.

The process is exothermic and smaller value of the hydrogenation indicate that the conjugated diene is more stable than if it was an isolated one. Therefore, we can conclude that conjugated double bonds bring additional stability. In this case, the stabilization associated with the conjugated nature of double bonds is determined to be ~15 kJ/mol.

This is a general trend for all the conjugated compounds whether they are dienes, trienes or aromatic compounds.

A similar observation is seen when we compare the heats of hydrogenation of 1,4-pentadiene (an isolated diene) and (3E)-1,3-pentadiene (a conjugated diene) to pentane. This time, both molecules have two double bonds, and the experiment might be seen as more relevant.

No surprise here and as expected, 1,3-pentadiene has a smaller heat of hydrogenation and therefore, it is lower in energy meaning it is more stable than the isolated 1,4-pentadiene:

So, why are conjugated dienes more stable than isolated dienes?

Although, the root of this question, and many others in organic chemistry, lie in the molecular orbital theory, the simple answer can be that conjugated dienes are capable of more resonance structures. The overlapping p orbitals on adjacent atoms allow the electrons to be delocalized over the four or more atoms. Keep in mind that to achieve this delocalization, all the p orbitals must be aligned parallelly:

The delocalization breaks when p orbitals are separated by and sp3-hybridized carbon because it does not have an unhybridized p orbital capable of participating in the electron flow. This is the case with isolated dienes and that is why they are less stable. Going back to our example, we cannot draw resonance structures for 1,4-pentadiene involving the electrons on carbon atoms on both double bonds i.e., they are not delocalized. This delocalization, however, is possible for the conjugated 1,3-pentadiene:

Reactions of Conjugated Dienes

The chemical properties of isolated dienes resemble those of alkenes since the isolated double bonds behave just like two separate alkenes. Therefore, all the reactions of alkenes are characteristic of isolated dienes with the difference that they can react twice.

Conjugated dienes are still subject to the principles of addition reactions such as the Markovnikov’s rule, however, these reactions are more complicated since the resonance forms contribute to the formation of different products. They are classified as 1,2 and 1,4 additions and we will go over these reactions in the next articles:

Conjugated dienes can also adopt what is called “s-cis” and “s-trans” conformations which occur through the rotation about the sigma bond connecting the two π bonds.

The difference between these and the traditional cis and trans isomerism is that here we have conformers rather than geometrical isomers. Conformers can be interconverted through a rotation about single bonds while cis and trans isomers are locked due to the structural nature of the double bond.

In the s-cis conformation, the dihedral angle between the two double bonds is 0°, while in the s-trans conformer this angle is 180°. In both cases, the two double bonds are in the same plane – in the given representation below, it is the plane of the paper/display. This implicates that all the p orbitals overlap giving a continuous, conjugated π system.

Any combination of the conformational and configurational isomerism is possible in a given diene, triene or a polyene.

The most common and important reaction of conjugated dienes is the Diels-Alder reaction in which the diene reacts with a dienophile and a six-membered ring is formed.

Interestingly, the reaction only occurs when the diene is in the cis conformation:

We will talk about the Diels-Alder reaction in quite details later and the good starting point for that would be reading about the reactivity of dienes and dienophiles as well as the general trends of the Diels-Alder reaction. 

1.

Classify each diene as isolated or conjugated and draw a resonance structure involving the maximum number of pi bonds:

2.

Carotenes are highly conjugated compounds responsible for the color of autumn leaves, many fruits and vegetables such as apricots, melons, carrots, and pumpkins.

Draw a resonance structure using all the pi bonds of the ß-carotene to demonstrate its highly-conjugated character.