Diene

A diene is an organic compound, more specifically a hydrocarbon, that contains two double bonds. The way these double bonds are arranged is what give dienes their specific properties.

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What is a Diene?

When we hear the word “diene” we can break it apart into 2 words: “di” & “ene”. With that in mind, we know that a diene is basically 2 double bonds.

The “ene” part comes from an alkene which is comprised of a pi bond. While the “di” part indicates that specifically 2 pi bonds are present.

One common example of this is 1,3-butadiene or buta-1,3-diene. It is the simplest of the conjugated dienes. Conjugated is simply one way to classify dienes, along with isolated or non-conjugated, and cumulated.

General Diene Structure

We will discuss the difference between all 3 types of dienes below, however notice that in the image of 1,3-butadiene above, there are 2 pi bonds separated by 1 sigma bond.

To name this diene, we will use the positions of the carbons where the double bonds begin, which in this case is carbons 1 and 3 (more on naming later).

Much like alkenes, a diene is reactive due to having 2 pi bonds that can undergo reactions. We will go over some of the ways to prepare dienes, as well as the specific reactions conjugated dienes in particular can undergo.

Types of Dienes:

We have already learned about the 3 categories of dienes above, however now we will dive a bit deeper into how we can identify them.

Here is a brief introduction into the conjugated, cumulated and isolated (non-conjugated) dienes which are highlighted in purple. See if you can figure out which is the most stable, and which is the least stable. What do you think?

The 3 Types of Dienes

1. Conjugated Dienes 

We’ve heard the word conjugated thus far, however it’s important to understand what exactly it means. It basically is an adjective to describe the concept of resonance.

Therefore, a conjugated molecule has 3 or more atoms that are capable of resonating which make a conjugated diene the most stable. The overlap of multiple p orbitals, due to the alternating single and double bond nature, allow for this increased level of stability.

Structurally, we can identify a conjugated diene by this alternating single and double bond appearance. This means that a conjugated diene has 2 pi bonds separated by 1 sigma bond.

The C=C−C=C unit is very unique in that it can undergo reactions such as conjugated hydrohalogenation, as well as be the key diene for the Diels-Alder reaction (discussed below.)

Conjugated Diene

2. Isolated Dienes

*Also called non-conjugated dienes

This type of diene acts like 2 regular double bonds. It is not as stable as conjugated dienes, however it is more stable than cumulated dienes.

While conjugated dienes contained primarily sp2 hybridized carbons, isolated dienes are 2 pi bonds separated by an sp3 hybridized carbon. This means that there are not 3 atoms in a row capable of resonance.

As the word isolated alludes to, this diene has 2 pi bonds separated by at least 2 sigma bonds, which can be seen in the image below.

Isolated Diene

Functionally, isolated dienes are seen in Organic Chemistry. One common reaction where they are involved is as the product of a reaction termed Birch Reduction.

Cyclic Isolated Diene

The specifics of this reaction are not necessary to memorize as the isolated diene above is merely a product. However, below we will get into certain reactions of conjugated dienes that are crucial to understand as the diene is the major reactant involved.  

3. Cumulated Dienes 

The last type of dienes we will discuss are called cumulated dienes. We can remember this term by thinking of this molecule as having consecutive double bonds.

Cumulated Diene

The functional group you see below is characterized by having a central sp hybridized carbon that is connected to 2 pi bonds creating sp2 hybridized carbons. If you see the term “allene”, this is the term given to the molecule we just described.

Cumulated Diene Hybridization

Allenes, are commonly seen when discussing atropisomers. These compounds are known for their inability to freely rotate making them a special type of chiral molecule because they have no chiral centers.

When it comes to allenes, we have specific rules that we can apply to say if they are chiral or not. The molecule below is one example of this.

Allene Structure

As you can see, there are different types of dienes with different characteristics and importance in Organic Chemistry. The last category of cumulated dienes are the least stable of the 3 and exhibit properties similar to alkynes due to the sp character of the central atom.

Preparation of Dienes:

Later we will learn 2 very important reactions dienes can undergo (there are more), but for now let’s focus on how to prepare a diene.

Since we are trying to form a compound that has two double bonds, we will be focusing primarily on elimination reactions to form conjugated dienes. However, we need to isolate types of reactions that do their work at the allylic position.

A. Starting with a double bond 

What is the allylic position? Allylic refers to the position next to a double bond as seen in the image below by the “R group”.

Allylic Position

Now, why is this important? Because we can use reactions such as allylic bromination to give us a halogen (Br) at that position if starting with a double bond, which we can then eliminate to form a conjugated diene.

Allylic Bromination Product

Once we form the compound above, we are just one step away from forming a diene. Can you guess what the next step would be?

If you said E2 elimination that is correct! Remember, an E2 elimination is when we use a base and sometimes heat to form a double bond and get rid of a leaving group. In this case, since our leaving group is a halogen, we can refer to this specifically as a dehydrohalogenation reaction.

Another way to form a diene is if instead of a Br in the allylic position, we had an alcohol (OH) group. We can still prepare dienes this way, however the mechanism to do so is slightly different because alcohols aren’t great leaving groups like halogens are.

This type of reaction is called a dehydration and looks something like this:

Now here, we do not have an allylic alcohol, however the reaction would work the exact same. For 1˚ (primary) alcohols, we will follow the E2 mechanism after forming water (a good leaving group) in the first step.

However, for 2˚ (secondary) and 3˚ (tertiary) alcohols we would follow the E1 mechanism after we protonate to form water. In both cases, our product will be a double bond.

*The molecule above is a special case because there are no B-hydrogens. Here, we would have to go through the E1 mechanism and a carbocation rearrangement to form our product.

For example, here is what the mechanism would look like. Keep in mind that we will primarily follow the E2 mechanism when forming conjugated dienes from alcohols, because they will be substituted (2˚ or 3˚).

Dehydration Mechanism

B. Starting without a double bond 

Lastly, we discussed what to do if given an allylic compound with a leaving group, but what if we were simply given this compound (an alkane):

Simple Alkane

Normally, simple alkanes don’t undergo reactions. However, through a radical halogenation we can form a functional unit.

Alkyl Halide

This compound now contains a leaving group, which means it could then undergo elimination to form our alkene.

Elimination of Alkyl Halide

At this point we could then use the allylic bromination we mentioned above, or simply any kind of allylic halogenation to form an allylic halide and continue with the steps mentioned above.

Reactions of Dienes:

Now that we have learned ways to prepare dienes, let’s discuss certain reactions that dienes, conjugated in particular, can undergo. They are:

1. Conjugated hydrohalogenation 

We learned about hydrohalogenation of alkenes before, however now we will add a new dimension. Just in case you forgot, the reaction proceeds with a carbocation intermediate and produces an alkyl halide as our product in the Markovnikov position.

Hydrohalogenation 

Now, instead of an alkene as our starting material, we will have a conjugated diene. What do you think are the key differences?

Conjugated Hydrohalogenation

Well, just know that the first step still forms a carbocation intermediate. However, once this occurs we have an intermediate that can resonate.

Conjugated Hydrohalogenation Intermediates

The blue arrow above shows the 1 arrow mechanism that is typical for the movement of cations we learned about with resonance structures.

Movement of Cations

This creates a new resonance structure in which we have a less stable carbocation, but a more stable double bond (seen in blue below). At this point we have 2 possible products that can be formed once our nucleophile (X-) attacks.

Nucleophilic Attack

We call these the 1,2 product (kinetic product) and the 1,4 product (thermodynamic product).

Conjugated HydrohalogenationProducts of 1,2 occurs at lower temperatures and comes from the positions where the H (hydrogen) and X (halogen) add to the original conjugated diene. Lower temperature is considered to be at or below 0˚ C.

For the 1,4 product (seen above on the right), we see the halogen now adding to the #4 carbon, while the hydrogen in red is added to the same position as before. This will occur at higher temperatures or above 40˚ C and is therefore called the thermodynamic product. 

Thermodynamic vs. Kinetic

*Hint: Higher numbers (1,4 product) = higher temperature (above 40˚ C)

2. Diels-Alder 

The next reaction conjugated dienes can undergo is called Diels-Alder, which is a cycloaddition reaction. This can be remembered by the fact that it produces a cyclic product and goes through a cyclic transition state that looks like this:

Diels-Alder General Mechanism

To start the reaction, we first need a conjugated diene, but not just any one. It specifically requires s-cis 1,3 dienes. What does that mean? Well, an s-cis diene is one that exists in the cis position, relative to where the pi bonds are in relation to the sigma bond.

S-Cis Conformation

Once this is established, we can finally begin our reaction and speak about the other requirement to this reaction. It is a dienophile. We’ve heard the term “phile” before when talking about nucleophiles and electrophiles.

Here however, the diene is what is being sought after. Therefore, dienophile simply describes a molecule that is “attracted to” or “has an affinity” for dienes. Common examples are:

• Alkenes and alkynes (seen below)

Common Dienophiles

The diene and dienophile therefore react via a 3-arrow mechanism to produce a six-membered ring. Some variations do exist including the possibility to form bridged products which occurs when the diene is already a part of a ring:

Diels-Alder Bridged Product

Also, you may sometimes see EWG or Electron-Withdrawing groups on the dienophile, and EDG or Electron-Donating groups on the diene. Common examples of this are seen below, however just realize this has to do with the nature of the reaction.

• EDG’s: lone pairs (on N or O in particular), negative charges, R groups

Electron Donating Groups

• EWG’s: positive charges, carbonyls, CX3, CN [halogens are weak EWG’s]

Electron Withdrawing Groups

Therefore, just imagine that we want the diene to be electron rich or “have electrons” and the dienophile to be electron poor to produce the best yields.

Pop Quiz: Which of the following dienes will react via the Diels-Alder reaction if any?

Alder Multiple Choice QuestionDiels-

(Answer below)

Special Topics Related to Dienes:

1. Naming

To name a diene, we will use the same rules we learned for naming alkenes and with IUPAC nomenclature

• This includes finding the longest chain with both double bonds while giving them the lowest priority possible (including other substituents).

The only difference would be that instead of the suffix “-ene”, we would now say “-diene” and after the prefix “hex-” we add the letter “a”. For example: 2,4-hexadiene

Also, you may need to use the terms, “Cis/Trans and E/Z” to accompany your complete name as well. Therefore, a complete name for a molecule may be (2E,4E)-hexa-2,4-diene. 

*Quiz: Can you draw out what this molecule will look like? 

Now, one trick with naming that can help you distinguish between the different types of dienes we mentioned above is their numbering.

• If we see a diene that has double bonds at positions 2 and 5, we know this is an isolated diene because the double bonds are separated by multiple carbons. 
• If we see a diene that has double bonds at positions 2 and 3, we know this is a cumulated diene because the 2 doubles bonds are consecutive
• If we see a diene that doubles bonds at positions 2 and 4, we know is a conjugated diene because the double bonds are separated by a sigma bond only. 

2. Aromaticity

One unique place we see dienes is in cyclic, conjugated dienes. That is because it allows certain molecules to become aromatic in one easy step: the donation of a hydrogen.

Now, this may not come up a lot, however it is good to be aware of where dienes can be seen. For example, if we look at the molecule below we see a diene.

Cyclopentadiene Structure

Well, the hydrogens located at the top carbon of the structure are uniquely acidic because if we react with a base, we can create an extremely stable molecule. This molecule which is known to display these properties is called cyclopentadiene.

The cyclopentadienyl anion, is the molecule that results from deprotonation and an aromatic molecule to be on the lookout for later on in Organic Chemistry II.

Cyclopentadiene Reaction

3. Stability 

When we talked about conjugation above, we explained how it overlaps with resonance. That is because when discussing conjugation, as in conjugated dienes, we look to stability.

However, what we didn’t mention much about before is this concept of overlapping orbitals.

Now, I know what you’re thinking. Any time we hear the word “orbital” we immediately get off track, so let’s break it down into simple terms we can understand.

Think of conjugation as a path or rather highway for electrons. Conjugation, allows for these electrons to cover more distance and “delocalize” across multiple atoms that would otherwise not be possible.

Therefore, delocalization of charge allows for conjugated dienes to be stable, and have less energy associated with it than normal.

Pi Bond Orbital Overlap

This shows the overlap of p orbitals for 1 pi bond. However, when looking at dienes we see this concept at a greater scale since 2 adjacent pi bonds are present.

Think about the additional resonance structures that result from allylic radials, cations, and anions. The image below can be used as a guide to show how conjugation allows for the charge to spread out across multiple atoms which contributes to the stability of each.

Resonance Structures

This concept can be seen quite a lot in organic chemistry. For example, when talking about allylic radicals, we know that they show an increased amount of stability compared to primary, secondary, and tertiary radicals.

Allylic and Benzylic Radicals

Before we move on, here is another image showing the overlap of multiple p orbitals in a pi bond. Remember, pi bonds are what make up the double bonds we see in conjugated dienes.

P Orbital Model

Commonly Asked Questions:

How is wavelength affected by conjugated molecules?

• The higher the conjugation, the higher the UV wavelength. Another way to say this is the molecule has overall lower energy. 

What is a polyene?

• Polyene is the overarching term to describe a molecule with multiple double bonds or a molecule that is polyunsaturated. A diene is a subset of this, but trienes, amongst others are possible. 

Can we also perform a halogenation on conjugated dienes?

• Yes, we can add directly (1,2) or indirectly via the conjugate 1,4 addition just as we saw for conjugated hydrohalogenation. As before we can favor the 1,4 product by doing the reaction under high temperature (40˚ C <).

Conjugated Halogenation

Above is the reaction of a conjugated halogenation, while below is the two different mechanisms shown. The blue is the 1,4 addition which requires a carbocation rearrangement, while the green is a simple halogenation of an alkene.

Conjugated Halogenation Mechanism

What to Remember: 

1. Conjugated dienes are composed of a pi bond, a sigma bond, and a pi bond. Therefore, lookout for structures like 1,3-butadiene.

2. Cumulated dienes have consecutive pi bonds with an sp-hybridized carbon separating the two and therefore appear as a straight line due to its linear geometry

3. Isolated dienes are simply 2 isolated alkenes on a molecule and are more spread out than what we see for conjugated dienes. These pi bonds have multiple sigma bonds in between them.

Answers to above questions:

     1. Diels-Alder

     - At first glance, you may have thought that none of these dienes would participate. However, in answer choice C. we have a sigma bond that is able to rotate to become an s-cis diene.

S-Cis Rotation

     - In all the other answer options, there are either isolated dienes or s-trans dienes that cannot be rotated to s-cis because they are in a ring.

     2. Naming

Diene Nomenclature

Pop Quiz:

There are 3 types of dienes: Can you name them all? Which is the most stable?

Types of Dienes

(Scroll down for answer)

I. Cumulated diene

II. Isolated diene

III. Conjugated diene – most stable