[orgo]

[ippie]

PICTURE IS ATTACHED.
Select only heterocyclic aromatic compounds where the unshared electron pair of nitrogen occupies an sp2 orbital in the plane of the ring.
all the three are aromatic, but the reason III is not the answer is because N in III is SP3, not SP2???

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[glk2101]

correct, the first two are sp2 since three things are attached to the nitrogen counting the double bond as one thing and a lone pair as one thing. the third is sp3 since it is connected to four things

 

[ippie]

but III has resonance, if we consider the resonance form, N in III is also SP2. right?

 

[vvvv]

PICTURE IS ATTACHED.
Select only heterocyclic aromatic compounds where the unshared electron pair of nitrogen occupies an sp2 orbital in the plane of the ring.
all the three are aromatic, but the reason III is not the answer is because N in III is SP3, not SP2???

I and II is sp2 because the have 2 bonding plus 1 pair e-. so total is 3 and it's sp2. also the pair e is on the plane of the ring they not contribute to the pi bond of the ring.
III is sp3 because it has 3 bonding and 1 pair e- so total is 4 = sp3. also the pair e- of this N is NOT on the plane of the ring. it contribute to the pi bond of the ring.

 

[doc3232]

Resonance in the 3rd isn't 100%, I think.
The electrons will mostly be donated to the ring because it is 1000x more stable if it donates the electrons.
So it will somewhat be sp2 and also somewhat sp3.
Mostly sp2 though as it will donate the electrons.