Question

How many atomic p orbitals overlap to form the de localized system of pi molecular orbitals for each structure of Thymine, Pyrimidine, and cytosine?

Answer 1

are the conditions that must be satisfied for a molecule to be considered aromatic:

1. It must have a cyclic structure.
2. The ring must be planar.
3. Each atom in the ring must be sp2-hybridized, so that π electrons can be delocalized around the ring.
4. The number of π electrons in the ring must be such that, in the ground state of the molecule, all bonding MOs are completely filled, and all nonbonding and antibonding MOs are completely empty.

It turns out that, in order to satisfy condition #4, the ring must contain a specific number of π electrons. The set of possible numbers is quite easy to remember - the rule is simply 4n+2, where n is any positive integer (this is known as the Hückel rule, named after Erich Hückel, a German scientist who studied aromatic compounds in the 1930’s). Thus, if n = 0, the first Hückel number is (4 x 0) + 2, or 2. If n = 1, the Hückel number is (4 x 1) + 2, or 6 (the Hückel number for benzene). The series continues with 10, 14, 18, 22, and so on. Cyclooctatetraene has eight π electrons, which is not a Hückel number. Because 6 is such a common Hückel number, chemists often use the term 'aromatic sextet'.

Benzene is best visualized as a planar ring made up of carbon-carbon sbonds, with two ‘donut-like’ rings of fully delocalized π electron density above and below the plane of the ring (the fact that there is a ring of π electron density on both sides of the molecule stems from the fact that the overlapping p orbitals have two lobes, and the electron density is located in both). This general picture is valid not just for benzene but for all other aromatic structures as well.

Let’s look at some different aromatic compounds other than benzene. Pyridine and pyrimidine both fulfill all of the criteria for aromaticity.

In both of these molecules, the nitrogen atoms are sp2 hybridized, with the lone pair occupying an sp2 orbital and therefore not counted among the aromatic sextet. The Hückel number for both pyridine and pyrimidine is six.

Rings do not necessarily need to be 6-membered in order to have six π electrons. Pyrrole and imidizole, for example, are both aromatic 5-membered rings with six π electrons.

so as it is in thymine and cytocine because of it has also pyrimidine type structure.