Question

How many benzene rings are present in Naphthalene?
(A) $2$
(B) $3$
(C) $4$
(D) $5$

Answer 1

Hint:We all know about Naphthalene. It is obtained from crude oil or coal tar. It is also used as a fumigant in households. It is the simplest polycyclic aromatic hydrocarbon. Also, it has the molecular formula ${C_{10}}{H_8}$. Like benzene, it is a planar molecule but the bond lengths between atoms in naphthalene are different.

Complete answer:We all know about Naphthalene. It is generally used as a fumigant in households. Fumigants are those substances that are used to kill insects, nematodes, etc by turning themselves from solid into toxic vapors. These substances are poisonous and toxic. Naphthalene is an organic compound, with a molecular formula ${C_{10}}{H_8}$ . It is a polycyclic aromatic hydrocarbon with two fused benzene rings. It is a white crystalline solid. It has the following structure:

We know that bond length is the average distance between the nuclei of two bonded atoms. Bond length is a function of hybridization. The bond lengths between carbon-carbon atoms are not the same in Naphthalene. As naphthalene is a polycyclic aromatic hydrocarbon, it consists of two fused benzene rings, therefore there are alternate double bonds and single bonds. Therefore the hybridization changes and thus the bond lengths changes. Resonance also occurs in the naphthalene structure.

Resonance is a property in which an aromatic compound can be represented by two or more resonating structures. These structures have the same number of electrons but differ in the arrangement of electrons. Resonance occurs due to delocalized pi-electrons. The double-headed arrows used in resonating structures represent that the actual structure is the average of the resonance structures.

Thus, the correct option is $A$.

Note:It is important to note that Naphthalene is the simplest polycyclic aromatic hydrocarbon. According to Huckel’s rule for a molecule to be aromatic, it should be planar, cyclic, and must have $(4n + 2)$ pi electrons. Naphthalene is a planar, cyclic molecule with $10\left[ {4 \times 2 + 2} \right]$ pi electrons. Thus Naphthalene is an aromatic molecule.