Question

Which of the set of isomers of ${{C}_{6}}{{H}_{4}}C{{l}_{2}}$ is having equal dipole moment with ${{C}_{6}}{{H}_{5}}Cl$and ${{C}_{6}}{{H}_{6}}$ respectively?
A. Ortho and Meta
B. Meta and Para
C. Ortho and Para
D. Para and Ortho

Answer 1

When there is separation between the charges then dipole moment arises or we can say dipole moment occurs due to difference in electronegativity between two chemically bonded atoms. It measures the polarity of chemical bonds.

Complete step by step solution:
Isomers are those molecules which have the same molecular formula but different arrangement of atoms. But it is not necessary that they have the same physical and chemical properties. Dipole moment is calculated by the formula
${{\mu }_{R}}=\sqrt{{{\mu }_{1}}^{2}+{{\mu }_{2}}^{2}+2{{\mu }_{1}}{{\mu }_{2}}\cos \theta }$
In case of ${{C}_{6}}{{H}_{4}}C{{l}_{2}}$the dipole moment ${{\mu }_{R}}$= ${{\mu }_{1}}$= ${{\mu }_{2}}$and in case of benzene ${{\mu }_{R}}$= 0.
In meta dichlorobenzene bond angle between two carbon chlorine bonds is ${{120}^{o}}$ so the value of $\operatorname{Cos}\theta =\operatorname{Cos}{{120}^{o}}$= -0.5 and in this case ${{\mu }_{R}}$= ${{\mu }_{1}}$= ${{\mu }_{2}}$ which is equal to the chlorobenzene and in case of para dichlorobenzene bond angle between two carbon chlorine bonds is ${{180}^{o}}$ and the value of $\operatorname{Cos}\theta =\operatorname{Cos}{{180}^{o}}=-1$ which is equal to benzene molecule.

Thus we can say that option B is the correct answer.

Note: The terms ortho, meta and para are prefixes used in organic chemistry which tells us about the position of non-hydrogen substituents on a hydrocarbon ring. Ortho defines a molecule with substituents present at the 1 and 2 positions, in case of meta substituents are present at 1 and 3 position while para molecule tells us that substituents are present at 1 and 4 positions on an aromatic compound.