[bedlam17]

What are the resonance structures of caffeine?  Please post the answer on this website and also send it to bedlam17@hotmail.com . Thanks.
(I know that one of the structures is the one at http://www.cs.virginia.edu/~an4m/T-shirt/caffeine-molecule.gif).

[Grafter]

Do you know how to draw resonance structures? The aim of the exersize is probably to get you practicing them.

Do you want to know how to draw resonance structures?

[bedlam17]

Yes, I know how to draw resonance structures for most molecules.  However, caffeine is a very complex molecule and doesn't have a regular pattern of double bonds....

The aim of the exercise isn't really to draw resonance structures---it's to see which nitrogen on caffeine can act as a base---but it's necessary to draw resonance structures to see this.

[Grafter]

Ok, start by drawing on the hybridisation state of each atom. If you are interested in the base strength, see how each nitrogen lone pair may be delocalised into the molecule. If you can get the charge density to end up on an oxygen, the lone pair will be very delocalised. The more delocalised the lp, the less basic the nitrogen.

[Winga]

I think the most basic nitrogen atom is in the 5 member ring which double bonded with the carbon atom.

No matter there is a resonance through this nitrogen atom, its non-bonding sp2 orbital is still non-bonding (doesn't take part in resonance).

[Grafter]

i agree