Aromatic compounds have been discovered in the 19th century. They were unusually stable and had pleasant odors, hence the term aromatic. Benzene is the most famous aromatic compound that has been studied extensively.
AROMATIC, NONAROMATIC AND ANTIAROMATIC
In order for a molecule to be aromatic, it has to meet four criteria.
The structure must be cyclic (a ring).
The ring must be planar. This is to ensure effective overlap of the p orbitals.
The ring must be fully conjugated. Each atom on the ring has to have an unhybridized p orbital.
Huckel's Rule of 4N+2 pi electrons must be followed, where N can be equal to any integer such as 0,1,2,3...
If the molecule has a ring and is planar but has 4N pi electrons, it is said to be ANTIAROMATIC.
If the molecule fails either one of the first 3 criteria, it is said to be NONAROMATIC.
Let's now go through the criteria. The toughest ones are number 3 and 4. First, how do we know if an atom on the ring has an unhybridized p orbital? I am listing the most common possibilities below:
The following have unhybridized p orbitals as shown above: carbon with a double bond, carbon with three bonds and a positive charge, boron with three bonds, nitrogen with a double bond, nitrogen with a lone pair of electrons, oxygen with two bonds and two lone pairs, sulfur with two bonds and two pairs of electrons. The most common case of an element that DOES NOT have an unhybridized p orbital is a carbon with four single bonds since it is sp3 hybridized. Any atom that is sp3 hybridized will not have an unhybridized p orbital since all of the p orbitals have to been used to make sp3 hybrid orbitals.
Next, in order to follow Huckel's rule, we need to know how to count pi electrons. Again, I am listing the most common case scenarios:
A double bond has 2 pi electrons. Carbon with a positive charge has 0 pi electrons. Boron with three bonds has 0 pi electrons. Nitrogen with a lone pair and three bonds has 2 pi electrons, Oxygen with two lone pairs and two bonds has 2 pi electrons (one lone pair is not in the same plane as the other and doesn't overlap with the rest of the p orbitals). Same goes for sulfur with two bonds and two lone pairs. Carbon with a lone pair and three bonds has 2 pi electrons. Nitrogen with a double bond and one lone pair has 2 pi electrons.
Let's take a look at some examples now to determine whether they are aromatic, nonaromatic or antiaromatic.
The molecule on the left is a planar ring. It is conjugated because it has 4 carbons with the double bond and one carbon with a positive charge (see my tricks for conjugation above). It has 4pi electrons since it has two double bonds and each double bond is two pi electrons. 4n+2=4 gives us n=1/2 which is not an integer. Therefore molecule is not aromatic. It follows 4n rule and is antiaromatic.
The molecule in the middle is a planar ring. It is conjugated because it has two carbons with a double bond and one carbon with a positive charge. It has 2 pi electrons and therefore follows Huckel'l rule giving us n=0. It is aromatic.
The molecule on the right is a planar ring that is conjugated ( 5 carbons on a double bond and one nitrogen on a double bond). It has 6 pi electrons (three double bonds, lone pair on the nitrogen doesn't count since it is in a different plane). This follows Huckel's rules giving us n=1 which is an integer. The molecule is aromatic.
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