[edwinksl]

Dear all,



I would like to ask there is any conjugation between the pi bond of the =NR₂ group and the lone pair of N in -NR₃R₄. If yes, doesn't it mean these 2 nitrogen atoms are sp² hybridised? An amide would be a suitable analogue, in which the nitrogen atom is sp² hydridised so as to allow conjugation between its lone pair (in the p orbital) with the pi bond of the carbonyl group.

Thanks.

[Mitch]

Just because it can hypothetically have conjugation doesn't mean it will. The number of pi electrons in that system would be 4, which is antiaromatic and most unlikely to occur.

[edwinksl]

Just because it can hypothetically have conjugation doesn't mean it will. The number of pi electrons in that system would be 4, which is antiaromatic and most unlikely to occur.

Mitch,

How do you explain the conjugation in an amide then? The pi system there also has 4 electrons as well. The 4n antiaromatic rules only applies to monocyclic systems, and amides and amidines clearly are not cyclic, therefore you cannot say amides and amidines are antiaromatic.

[Yggdrasil]

I don't see why you wouldn't have conjugation.  It seems reasonable to me to think that the tertiary amine would be sp² hybridized.

[Edit: fixed tags]

[edwinksl]

I don't see why you wouldn't have conjugation.  It seems reasonable to me to think that the tertiary amine would be sp^{2[/sub] hybridized.}

I think so too. The catch is that my book (Clayden's; turn to page 202 if you have the book ) says that the N at the tertiary amine is sp³ hybridized.

I guess I should reproduce the exact phrasing:

An amidine has 2 nitrogen atoms that could be protonated - one that is sp³ hybridized, the other sp² hybridized. We might expect the sp³ nitrogen to be more basic but protonation occurs at the sp² nitrogen atom. This happens because we have the situation as with an amide: only if we protonate on the sp² nitrogen can the positive charge be delocalized over both nitrogens.

I agree the N at the =NR2 group has to be protonated in order to delocalize the positive charge. I only have a problem with the author claiming that the N at the tertiary amine is sp³ hybridized, when it should be sp² hybridized if we agree there is conjugation in the amidine molecule.

Hmm, so who is right? Us (I would think so) or the book? Haha. Thanks Yggdrasil.

[Yggdrasil]

I think that the fact that the =NR₂ group is the more basic of the two is more evidence that there is conjugation.  It doesn't make sense (to me at least) how he can say that "the positive charge be delocalized over both nitrogens" but state that the tertiary amine is sp³ hybridized.

[edwinksl]

I think that the fact that the =NR₂ group is the more basic of the two is more evidence that there is conjugation.  It doesn't make sense (to me at least) how he can say that "the positive charge be delocalized over both nitrogens" but state that the tertiary amine is sp³ hybridized.

Indeed. Thanks anyway. I just needed to confirm my conjecture.