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Dienes and MO Theory

By James Ashenhurst

Are these molecules conjugated?

Last updated: March 26th, 2019 |

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.

1-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’.

2-conjugated

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….

3-rings

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29 thoughts on “Are these molecules conjugated?

  1. 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!

  2. 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!

  3. 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!

    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 ? : – )

  4. 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.

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