Question

The number of \[\sigma \] bonds in o-xylene is
(A) $6$
(B) $9$
(C) $12$
(D) $18$

Answer 1

Hint: There are two types of covalent bonds. One is the sigma(σ) bond and the other is the pi ($\pi $) bond. Sigma bond is formed by head to head overlap of atomic orbital. Whereas pi bond is formed due to lateral or sideways overlap of atomic orbitals.

Complete Step by Step Solution:
Xylene is an organic compound with two methyl groups on the benzene ring. Different isomers are formed due to placement of two methyl groups over the benzene ring. Fix one methyl group over benzene. Mark it as position $1$ and the position of another benzene ring will define the naming of the compound. If the second methyl group is adjacent to the other methyl then it is called ortho xylene or o-xylene. Or in other words, the two methyls are at the \[1,2\] position of benzene then o-xylene.

If the second group is an alternative to methyl group, then it is meta xylene or m-xylene. The two methyls will be at the \[1,3\] position of the benzene ring. If the second methyl group is exactly the opposite position or next to the meta position then it is called para xylene or p-xylene. The two methyl groups will be at \[1,4\]positions.
The structure of ortho xylene is as follow

To find the number of sigma bonds, let us divide the structure into various parts.
The first is the benzene ring. Benzene had formula\[{C_6}{H_6}\]. It has \[6\] sigma bonds between $C$and $C$. There are $4$ sigma bonds between $C$ and $H$.So this makes a total of \[10\] sigma bonds.

Next are the two methyl groups. Each methyl group has formula\[C{H_3}\]. Thus, the number of sigma bonds between $C$ and $H$ is \[3\] in each methyl so a total of \[6\] sigma bonds. There are two sigma bonds through which each methyl group is attached to the benzene ring.
Thus, total number of sigma bond $ = 10 + 6 + 2$$ = 18$
The correct answer is D.

Note: A sigma bond exists independently but a pie bond cannot exist independently. A pi bond is always found with a sigma bond.