organic chemistry - Dipole moments of pyrrole and furan

Why do pyrrole and furan have dipoles oriented in different directions?

Answer

Both pyrrole and furan have a lone pair of electrons in a p-orbital, this lone pair is extensively delocalized into the conjugated pi framework to create an aromatic 6 pi electron system.

Where pyrrole and furan significantly differ is that,

• in pyrrole there is an $\ce{N-H}$ bond lying in the plane of the ring and directed away from the ring


• whereas in furan, there is a full lone pair of electrons in roughly the same position.

The localized lone pair of electrons pointing away from the ring has a very significant effect on the dipole vector and is enough to cause the observed reversal in dipole moment direction between furan and pyrrole.