How can lone pairs contribute to conjugated systems if they aren't in P orbitals?

[cwpatter]

When we look at conjugated systems, systems with delocalized electrons, we have to have two different P orbitals connecting across a sigma bond. This is easy enough to see in a conjugated diene.

But we also get a conjugated system when we have an allylic 1) Radical 2) Anion 3) Cation 4) Lone Pair

Here's where I'm confused

In the case of the lone pair/anion, aren't these lone pair of electrons in an SP3, NOT a P orbital?

How can they contribute to a conjugated system if they themselves aren't in P orbitals?

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[brood910]

There's an exception rule for this.
When an SP3 atom with one or more lone pairs is connected to SP2 atom, it is considered SP2, not SP3.
When it's SP2, there's one free P orbital to allow electrons to participate in conjugation.
Remember that all atoms must be SP2 to be a conjugated system

 

[cwpatter]

I had no idea the rule even existed. That's the problem with science, all the the little loopholes. Thanks though, I appreciate it.

 

[Blueprint MCAT Tutor]

Just as an aside, it's often easier to remember specific examples, or at least easier to remember the rule if you pair it with a specific example.

So for this one, my go-to example is pyrrole. It's the classic example of a heterocyclic aromatic compound: http://en.wikipedia.org/wiki/Pyrrole

Notice that the lone pair on the nitrogen has to count as part of the conjugated p orbital system to get to the 4n+2 number of electrons required by Huckel's rule.

So, you remember that pyrrole is aromatic and that can help you remember the thing about lone pairs.

Hope this helps!

 

[shklorg]

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[aldol16]

I had no idea the rule even existed. That's the problem with science, all the the little loopholes. Thanks though, I appreciate it.

It's not that science has loopholes but rather that people try to apply general "rules" universally when those rules really aren't universal rules. For example, you have to think about where the rule comes from. It comes from the fact that molecules tend towards stability. That's a good rule. When you apply that, everything makes sense. Conjugation would give an allylic anion a lot of stability so it will want to conjugate. To conjugate, the lone pair needs to be in a p-orbital. Therefore, it will be in a p-orbital. Conceptually, you can also imagine the opposite case. Imagine you can design a system where you have 6 electrons tied up in pi-bonds and then on a carbon that is immediately adjacent to one of the pi-bonding carbons, you have a lone pair. Based on the observation that molecules tend towards stability, the lone pair will not want to delocalize because that will actually break aromaticity and thus make the molecule less stable.

In science, there are few things that are known as truths - that's why we have the problem of applying purported "rules" so broadly. The known truths are generally called "laws" (though not all laws are necessarily absolutely correct - gravity is an example).