[standarsh]

Hi!

I really don't understand this picture from my textbook https://imgur.com/a/x8PG8pX. Why aren't the rings in red conjugated? They have alternating double and single bonds?

And how to distinguish the electron donating from electron withdrawing group?

Thank you!

[Arkcon]

What is the definition of conjugated?  Are you leaving off a word, that tells you why the aliphatic rings aren't conjugated?

I will admit, the third does stump me.  I assume the nitrogens interfere, but exactly how, I would have to break out my old text books to know.

[standarsh]

What is the definition of conjugated?  Are you leaving off a word, that tells you why the aliphatic rings aren't conjugated?

I will admit, the third does stump me. I assume the nitrogens interfere, but exactly how, I would have to break out my old text books to know.

Thanks for the reply. I'm not leaving anything off in the picture. The picture is from Clayden's book, in the chapter about radicals. The definition of conjugated I think is alternating double and single bonds between the carbons.

[OrganicH2O]

I don't really know and maybe a more knowledgeable person should answer. But I think in this case "conjugated" must mean the pi bonds have the ability to effectively delocalize the radical in a series of overlapping P orbitals. In this context the allyl system would also be considered conjugated: http://www.ochempal.org/index.php/alphabetical/a-b/allylic-radical/

This delocalization should involve drawing at least half-way reasonable reasonable resonance structures. In the third molecule I think maybe there is a very slight stabilizing effect from the top nitrogen with two phenyl groups. But delocalization of the radical would involve some very ugly charge separation: one positive nitrogen adjacent to a negative nitrogen. 

So I'd guess that this molecule is basically two separate conjugated systems: one with the radical conjugated with the trinitro phenyl, and another with the top nitrogen conjugated with two phenyls.

The best way to know for sure would be to somehow determine the actual shape of the radical.

[standarsh]

Thank you for your answer. Some of what you wrote went a bit over my head, but I get what you're saying about resonance and that I got it wrong. Still don't understand the difference in the third molecule.

I was thinking... Since the upper nitrogen is marked as "electron donating", the unpaired electron will go to the lower phenyl group which enables resonance... but I'm just guessing...

Anyway, thanks again!

[OrganicH2O]

First issue: Your might need to redefine your definition of conjugated. "3 or more overlapping P orbitals" might be a better definition than "single bond/double bond/single bond." Read about allyl cation, anion and radical. These three species contain the smallest unit of conjugation. All three species are stabilized by this conjugation.

By my definition of conjugation, a molecule like butadiene is an example of conjugation, but not the prototype. Butadiene has 4 overlapping P orbitals, one more than it is necessary.

I have sometimes seen conjugated defined as "single/double/single etc." but I think it's probably better to define it in terms of overlapping P orbitals because this better describes the true reason for the stabilizing effect.

By any definition of the word, a phenyl group always contains a conjugated system. So you're not going crazy when you notice that!

As for my words about resonance structures, here is a link to a picture that might better explain what I was trying to say:

https://imgur.com/a/4536lQl

Drawing resonance structures is not the most rigorous explanation. The best thing to do would be to consider molecular orbital diagrams. But that's not my forte.

Edit: I should add also your are correct that the upper nitrogen is electron donating. So it pushes electrons down toward the radical nitrogen. Radicals in general are usually stabilized by both electron donating and withdrawing groups. But I think conjugation with benzene has a much stronger effect.