Question

Please explain why pentadienyl has five pi orbitals not 4 (causing it to have five molecular orbitals)? Also please explain step by step how to determine the number of p orbitals in a molecule?

Answer 1

With alkenes, dienes and trienes all the nodes always occur between atoms, but with the three p orbital allyl system and the five p orbital pentadienyl systems, the nodes sometimes occur at an atom. As a node corresponds to a region of zero electron density these MOs are designated as non-bonding orbitals.

When looking at a molecule and deciding how many lone pairs are in p orbitals, it is first important to decide which atoms have p orbitals. The first thing to look for is to find atoms that are involved in double or triple bonds. These atoms have to have p orbitals because the p orbital overlap is what is involved in the pi bond.

Next you have to look at other atoms that are not involved in a pi bond and ask yourself whether that molecule would benefit if this atom had a p orbital. If the atom can become involved in conjugation the answer is usually yes.

After you find all of the p orbitals, you need to decide which electrons are in the p orbitals. Automatically you know that the electrons involved in double bonds are in the p orbitals. Next look at lone pairs. We will have to decide which lone pairs are in sp2 orbitals and which are in p orbitals. Now add up all the electrons in the p orbitals.