Can nitrogen ever have 2 bonds and 2 lone pairs, or even 1 bond and three lone pairs?

Many organic molecules I've seen either had 3 bonds and 1 lone pair or 4 bonds to nitrogen. I've not seen nitrogen with 2 bonds or 1 bond. Why is this the case?

1 Answer
Truong-Son N.
Feb 17, 2016

Because it's very unlikely that a nitrogen of that sort wouldn't just steal a proton from its solvent.

In the first case, a nitrogen with two bonds and two lone pairs would be #-"N"^((-))-#. That is, it would have a charge of #-1#. It would be very strongly basic (kind of like #"O"^((2-))#), and thus would want to donate its lone pairs to get a proton and stabilize itself.

The pKa of an amine is about #\mathbf(36)#, and deprotonating it raises the pKa even higher than #\mathbf(36)#.

#-"NH"-# is a secondary amine, and that is more stable than #-"N"^((-))-#. The anion has two lone pairs, instead of one, and it can donate one of those pairs to grab a proton and stabilize the charge.

Can you reason this out for #-"N"^((2-))#, an amine with two protons removed? It's similar.

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