Why do pyrrole and furan have dipoles oriented in different directions?enter image description here


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.

enter image description here


enter image description hereIt is due to more than just the presence of the lone pair on the oxygen. The oxygen is more electronegative than nitrogen therefore although both furan and pyrrole have the inductive effect of electron withdrawing, this effect is greater in furan because of the greater electronegativity. So the dipole moment of furan faces towards the oxygen, a result of greater inductive effects than resonance effects. Whilst the dipole moment of pyrrole faces away from the nitrogen as a result of greater resonance effects


This may be the same as other answers, but viewed a different way.

Imagine a strong base were to remove the (most acidic) proton attached to nitrogen in pyrrole, giving a structure isoelectronic and isostructural with furan. With the lone pair on the most electronegative atom in both cases we would then expect similar dipole moments (the charge on our deprotonated pyrrole would exist independently of the dipole moment). But the actual pyrrole has that proton attached, at what would be the negative end of the furan-like dipole.

So it is the extra hydrogen atom in pyrrole, which would effectively be a proton in a comparison with furan, that reverses the dipole in the former.


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