Question about Conjugated Systems

[Deucedano]

Does a nitrogen doubled bonded to carbon or oxygen double bonded to carbon contribute to conjugation like a regular carbon to carbon double bond? Thanks

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[12thandSouth]

Yes. I think what's important is having the correct proximity between the two different pi bonds, not whether the pi bonds exist between two carbon atoms, or a carbon atom and an oxygen atom (carbonyl), or a carbon atom and a nitrogen atom.

So, for instance, say you have an aldehyde: carbon 1 is double bound to an oxygen (terminal carbonyl group -- definition of an aldehyde). If a double bond exists between carbon 2 and carbon 3, then you have a conjugated system that exists between the carbonyl on carbon 1 and the alkene between carbons 2 and 3. If you were looking at an IR spectrum, the peak for the carbonyl would show up somewhere around 1690 1/cm instead of the usual 1710 1/cm because it is a 'conjugated carbonyl'.

Hope this helps.

 

[xlr8]

yeah, and its evident in aldol condensations. when you dehydrate, the double bond forms in the more stable position, where it is conjugated with the carbonyl double bond.

 

[Deucedano]

Thats what I thought but wasnt sure. Thanks guys

 

[jeesapeesa]

yes. an example off the top of my head would be pyridine. the lone pair on the nitrogen would be out of the ring and would not take part in Huckel's Rule (4n + 2), so you have a conjugated system where n = 1, so pi e- would equal 6 (there fore it's conjugated, 4n + 2 suffices, and it's cyclic).