Aromaticity

For a compound to be considered aromatic, it must satisfy the following four rules:

1.       The molecule must be cyclic.

2.       The molecule must be planar.

3.       The molecule must be completely conjugated.

4.       The molecule must satisfy Huckel’s rule.

Determining if a compound is cyclic is rather simple. If the compound is a ring and all bonds are connected to form this ring, then the compound satisfies the first rule.

 A planar molecule has all atoms in one plane; basically the molecule is “flat”.

Conjugation means the compound must have alternating double bonds. Benzene is considered completely conjugated because each pi orbital (double bond) overlaps and is separated by a sigma bond (single bond). However, 1,3-cyclohexadiene is not conjugated because there are three sigma bonds (single bonds) with no pi orbital (double bond).

Benzene                                                                                   

1,3-cyclohexadiene

Huckel’s rule applies to the number of pi electrons in the compound.  The equation used to find the acceptable number of pi electrons is 4n+2, (n=1, 2, 3, etc.). The acceptable numbers of pi electrons are 2, 6, 10, 14, 18, etc. The trend is adding 4 to each number. A pi electron is usually shown in the compound by a dot around the molecule with the electrons, or by a double or triple bond. Each double bond has two pi electrons. For example, the benzene molecule above has 3 double bonds, which means it has 6 pi electrons.

Let’s look at a few examples of compounds and find out if they are considered aromatic.

1.

Let’s begin with rule number one; the molecule must be cyclic. This molecule is considered cyclic, it is a ring and all bonds are connected to form that ring.
Rule 2: The molecule must be planar. This compound is not flat due to the carbons that are sp³ hybridized. The two hydrogen atoms connected to the carbon are in two different planes.
Rule 3: The molecule is not completely conjugated, if there were one more double bond, the molecule would satisfy this rule.
Rule 4: The molecule has 3 double bonds which means it has 6 pi electrons.
This molecule is NOT considered aromatic because it does not satisfy rule 2 or 3.

2.  

This molecule is commonly known as Pyrrole. Let’s begin with rule 1: the molecule must be cyclic. This molecule is considered cyclic; all bonds are connected to form a ring.
Rule 2: The molecule must be planar. All carbon atoms are sp2 hybridized and the hydrogen on the nitrogen is in the same plane as all the other atoms. This molecule is considered planar.
Rule 3: The molecule must be conjugated. This molecule has alternating double bonds, each pi orbital overlaps, so this molecule is considered to be conjugated.
Rule 4: Huckel’s Rule; there are 2 double bonds which equals 4 pi electrons, but there are also two pi electrons on nitrogen. Therefore there are six pi electrons, satisfying Huckel’s rule.
This molecule IS considered to be aromatic.