Question

The increase in stability and decrease in energy of aromatic compounds is due to:
A. Localization of $\pi $ electrons.
B. Delocalization of $\sigma $ electrons.
C. Localization of $\sigma $ electrons.
D. Delocalization of $\pi $ electrons.

Answer 1

Hint:The term aromatic is derived from Greek word ‘aroma’ meaning pleasant odour. Aromatic compounds have cyclic, planar structures that have $(4n + 2)\pi $ electrons. They have sigma and pi bonds in them. All aromatic compounds obey Huckel rule.

Complete answer:
-Aromatic compounds are a class of organic compounds. Generally these aromatic compounds contain carbon and hydrogen in their ring structure. But there are some aromatic compounds that contain hetero atoms like sulphur, nitrogen and oxygen.
-According to the Huckel rule, aromatic compounds have cyclic, planar structure and have $(4n + 2)\pi $ electrons where $n$ is an integer $(n = 1,2,3,....)$ .
-Aromatic compounds have sigma and pi bonds. Sigma electrons make up the sigma bond and pi electrons make up the pi bond. There are alternate sigma and pi bonds in these compounds.
-Aromatic compounds have extra stability due to delocalization of the pi electrons. This can be explained as follows-
-Benzene is the simplest aromatic hydrocarbon. 

Its structure can be represented as follows,

-All the six carbon atoms in benzene are $s{p^2}$ hybridized. The $s{p^2}$ hybridized orbitals overlap with each other and with $s$ orbitals of the six hydrogen atoms forming $C - C$ and $C - H$ $\sigma $ bonds. Since these bonds are formed from the overlap of planar $s{p^2}$ orbitals, all carbon and hydrogen atoms in benzene lie in the same plane.
-After the formation of six $C - C$ and six $C - H$ sigma bonds, one $2p$ unhybridized orbital is left on each carbon atom. They are parallel to each other and perpendicular to the plane of the $s{p^2}$ orbitals. The lateral overlap of these orbitals produces $3\pi $ molecular orbitals containing six electrons. As the overlapping on the both sides is equal, all the six $p - $ orbitals unite to form a continuous $\pi - $ molecular orbital containing six electrons. These molecular orbitals are low energy orbitals available for the electrons.
-The formation of continuous molecular orbital suggests that there is delocalization of pi electrons and all the six pi electrons are free to move over six carbon atoms. Since the delocalized electrons have less energy than the localized electrons, benzene becomes more stable.
-Such delocalization of pi electrons is observed in all aromatic compounds imparting them extra stability and decreasing their energy. 

Hence, the correct option is D.

Note:
-When an electron is shared by more than two atoms it is said to be delocalized.
-Coal and petroleum are the major sources of aromatic hydrocarbons and aromatic compounds.
-Aromatic compounds can be monocyclic or polycyclic. These polycyclic can be further of two types-isolated and fused ones. Some of the examples are-