[Arrowx7]

Hello everyone,
I just have two quick questions.
Is caffeine aromatic or not?  I know the 5-membered ring is aromatic, but what about the 6-membered ring?  It has 6 electrons in the pi orbitals, but carbon would have 5 bonds - but if we move the carbonyl electrons to the oxygen to make the oxyanion, carbon can now form double bonds with adjacent nitrogens and you have aromaticity in the ring (with positive nitrogen groups)  Is it aromatic then? 

http://en.wikipedia.org/wiki/Caffeine

2.  Would you call the nitrogens sp2?  Just by looking at it, it seems like they have 4 groups: a methyl, two carbons, and a lone pair, making it sp3, but are the bonds planar?  aren't they sp2?

Thanks!

[Yggdrasil]

1.  In judging whether a ring is aromatic or not, I like to look at whether all the atoms around the ring are sp² hybridized or not, not whether you can form a system of alternating double bonds around the ring.

2.  You are correct that the nitrogens would normally be sp³ hybridized, but atoms with lone pairs can change hybridization from sp³ to sp² by moving the lone pair from an sp³ orbital to a p-orbital.  Although this costs energy, the energy gained from aromatic stabilization outweighs the cost of moving the lone pair to the p-orbital.

So, yes, caffeine is aromatic.

[Arrowx7]

Thanks a lot for your reply.  So if those nitrogens undergo sp2 hybridization, to gain aromaticity, then why isn't caffeine drawn in its lowest energy state like this:  (attached)

Thanks!

[Yggdrasil]

The way caffeine is normally drawn is its lowest energy state.  When you protonate the carbonyls (as shown in your diagram), you lose some of the stabilization from delocalization of the ring electrons onto the oxygens.

Remember that the way in which we draw chemical structures is never 100% accurate, especially when you have conjugated pi bonds.  Our standard drawing of caffeine does not accurately depict the electronic structure and chemical bonding in the caffeine molecule.

[Arrowx7]

oh I see, so even though the conjugation includes the carbonyl oxygens, it doesn't mean the 6C ring isn't aromatic.  Both are most likely occurring at the same time when you look at the crystal structure and bond lengths.  Am I correct?

Thanks!  This helped a lot!

[Yggdrasil]

Yup.

[Rico]

Just to finish up your little discussion, the caffeine molecule can be drawn in two different resonance structures (see jpeg attached) which to some extent both contribute to the electrondistribution in the molecule i.e. the real structure of caffeine has an electrondistribution somewhere in between these two structures.
The structure you have drawn in your attachment, Arrowx7, is a di-protonated version and this is therefore not the caffeine-molecule anymore, but say a kind of derivative of it.
A good way to verify experimentally that a molecule is aromatic, is to analyse the 13C-NMR spectrum. I have also attached a 13C-NMR spectrum of caffeine and here it can be seen that almost all of the carbons on the ring is definitely in the aromatic region.

Rico