why is the lone pair electron of N in pyrrole put in the p orbital but that of pyridine is put into the sp2 orbital? how to determine if lone pair electrons are pi electrons?
Hi there! If we will further analyze the structure of pyrrole, the N-H bond of it uses the sp2 orbital while the lone pair is in the p orbital. On the other hand, the lone pair of pyridine uses the sp2 orbital which is perpendicular to the p orbitals and pi bonds in the ring.
In general, we can say that if a double bond is connected to a nitrogen like in pyridine, the pi bonds uses the p-orbital while the lone pair uses the sp2 orbital and does not participate to the aromaticity of the ring. However, if there is no double bond drawn to the N, where an N-H is present like in pyridine, this N-H bond uses the sp2 orbital and the lone pair uses the p orbital that can contribute to the aromaticity of the ring.
Below is a diagram of the orbitals of pyridine and pyrrole, you can learn more about these here from UC Davis Chemwiki: http://bit.ly/2mZW7Mt
Hope that helps!