Are these molecules conjugated?

by James

in Alkenes, Chemical Bonds, Organic Chemistry 2, Where Electrons Are

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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{ 18 comments… read them below or add one }

A. Easton

I love your posts, they’re always informative! Thanks!

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James

Thanks!

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laxmi

your posts are very helpful to me sir,thank u somuch

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ap_13

I would have loved it if you explained this phenomenon in terms of vbt n mot

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Binh

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!

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james

Thanks Binh.

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danielle

Thank you so much! I am in Orgo II right now and am struggling but these really help me a lot!

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james

Glad it was helpful Danielle!

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John Henry

Thank you for helping me remember my Chemistry!

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Kendall

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!

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stick

Whats the answers to the structure above.

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James

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 ? : – )

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Alyaa

So all of it can form resonance except the first one?

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James

For the purposes of determining aromaticity, the first one is not conjugated all the way around the ring. The others are.

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Angela

Thank you so much for this great resource!

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Asmat Elahi

Thanks for helping to understand conjugated and not conjugated molecules.

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suleman

sir please explain isolated,congugate and cumulated type differentiate among them

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James

That’s something you can google.

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