Dienes and MO Theory

By James Ashenhurst

Are these molecules conjugated?

Last updated: May 23rd, 2021 |

If Org 2 has an overall theme, it’s “resonance”. Today’s post has a very simple message. Atoms with lone pairs, π bonds, radicals, and carbocations can participate in resonance: atoms that lack any of these features cannot.

One way in which this comes up is in determining whether two alkenes are “conjugated” or not.  A full discussion of conjugation is for another time, but “conjugation” is the name we give for the phenomenon where π  electrons (that is, “electrons in p orbitals”)  can be shared over more than 3 or more atoms.

Here’s an example of a conjugated diene and a non-conjugated dienes.

are these molecules conjugated adjacent double bonds conjugated not conjugated if separated by sp3 carbon ie carbon with 4 sigma bonds carbocations radicals anions are conjugated

The thing is, we can also have alkenes which are conjugated with each other even through they are not directly attached. The only requirement is that we require an atom in between which can participate in resonance. This can take at least four forms:

  • A carbon (or other atom) with an empty p orbital (e.g. a carbocation)
  • A carbon with a half-filled p orbital (e.g. a radical)
  • A carbon with a lone pair (carbanion)
  • Any other atom with a lone pair (e.g. N, O, S, etc.)

In each of these we can draw resonance forms where the middle atom participates in a π bond. This implies that the electrons are being shared (“delocalized”) between this central atom and the alkenes on the side, and thus the whole system is ‘conjugated’.

examples of conjugated and unconjugated dienes with carbocations radicals carbanions atoms with lone pairs

Why does this matter, you may ask? Because some day, I have a hunch that you might be asked whether the following rings are conjugated or not, and you might need to come up with a good answer….

small quiz on whether or not molecules are conjugated or not


Comment section

40 thoughts on “Are these molecules conjugated?

  1. Hey,
    This is so helpful content. It is so informative content. It is a higher level of readable content. Students can easily realize the topic by reading this content. As a textile chemistry specialist I can say is the most info content ever seen.
    THis is
    Md. Kamrul Islam
    B.Sc in textile engineering (Specialization in textile chemistry)
    Blogger of https://practicaltextile.com/

  2. I cannot thank you enough for these posts! I can only imagine how much time and energy it takes to create them. Know that you are helping students so much and that we so appreciate it!

  3. Can the electrons in a triple bond participate in resonance/conjugation? I expect so because they’re just part of another p orbital, but nothing I’ve read has explicitly sanctioned that.

    1. Yes, of course! However as you touched on, the two pi bonds on the alkyne are at 90 degree angles to each other, so only one pi bond will be able to interact with any given adjacent pi system.

  4. For the carbanion resonance structure, the lone pair is on the last carbon. That carbon has 1 C-C bond, 2 C-H bonds, and a lone pair. So wouldn’t that technically be sp3? or is it because its a resonance structure, so the the true structure is something in between the two resonance structures and therefore sp2?

  5. I have a doubt regarding the stability of conjugated systems. In general, will a cross conjugated system be more or less stable when compared to one that is continuously conjugated?
    For eg, consider 1-methylene-2,4-cyclohexadiene and 1-methylene-2,5-cyclohexadiene. Which would be the more stable alkene?

    1. The key point comes up in the context of “aromaticity”. In order for a molecule to be aromatic, ALL of the carbons of the ring have to be able to participate in resonance. In the question at the bottom of the post, can you see any rings where there are carbons that CAN’T participate in resonance ? : – )

  6. I’m so glad you put the “exceptions” to the rule on your posts. My teacher always forgets to tell us when the rule doesn’t apply, and I always come to your blog to gain a better understanding of the material. Thanks!

  7. This is really helpful, I’m going over this to help refresh my chemistry knowledge whilst studying for the MCAT! I hope you add more lessons!

  8. Thank you for your posts!!! AND your website! I love that I’m able to access all the information for free. I’ve been struggling with chemistry for a very long time. Your website is very helpful!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.