The pyrrole molecule, shown below, is a planar (flat) molecule. The nitrogen ato

Question

The pyrrole molecule, shown below, is a planar (flat) molecule. The nitrogen atom in pyrrole has a non-bonding electron pair (not shown) and thus according to our simple rules from last semester should be sp^3 hybridized. a) Explain how you can know the N atom is not sp^3 hybridized and tell what its hybridization is. b) Describe in terms of the hybridization of nitrogen and aromaticity in pyrrole how and why the nitrogen atom breaks the rule. c) Why is the molecule have a flat, planar structure?

Explanation / Answer

2-

a) The nitrogen in pyrrole is a part of the aromatic system. Here the lone pair of nitrogen is placed such that it is in the same plane as the other carbon atoms of the ring. Thus the lone pairs can now be used for completing the aromaticity by being in resonance with the other p-orbitala. This gives the nitrogen not an sp3 hybridization which would be a regular tetrahedron, but a sp2 hybridized trigonal planar geometry. The energy for such transformation comes from the extra stability the molecule gains once it aromatizes.

b) As discussed above, nitrogen lone pars are adjusted such that they are in a p-orbital planar to the p-orbitals of the neighbouring carbon orbitals. Thus the overlapping is possible between these orbitals to form larger molecular orbitals and give enhanced stability due to the aromaticity in the structure by contributing the lone pairs. This thus breaks the rule of nitrogen being a tetrahedral.

c) In order to become a more energetically favoured aromatic structural moiety, the pyrrole structure adopts a planar conformation wherein, all the atoms of the ring are in the same plane. This allows for the nitrogen lone pair to be in the same plane as the other carbon orbitals and hence the electrons can be used for comleting the 4n+2 pi electron rule needed for the aromatic structure.

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