Question

The observed dipole moment is greater than its calculated value in:
(A) P- fluoro phenol
(B) 1- bromo-2- chlorobenzene
(C) O- fluoro phenol
(D) O- xylene

Answer 1

Hint: To answer this answer, you must recall the formula for the calculation of dipole moment in a chemical bond. Dipole moment is a vector quantity, that is, it has magnitude and direction. The direction of dipole moment in a bond is from the electropositive atom towards the electronegative atom. The difference in dipole moment results when the compounds form bonds like hydrogen bonding.

Formula used: $ \mu = q \times d $
Where, $ \mu $ represents the dipole moment in the bond
 $ q $ represents the magnitude of the charges induced on both the atoms
And, $ d $ represents the inter- nuclear distance between the two atoms forming the polar bond.

Complete step by step solution:
When there is a difference in the electronegativities of the two atoms that form a bond, the more electronegative atom attracts the shared pair of electrons towards it more than the electropositive atom. This delocalization of the charge density is known as dipole moment. The formation of a polar bond in the compound increases dipole moment of the molecule.
In para- fluorophenol, the two electronegative atoms are present exactly opposite to each other. Their dipole moments cancel out each other.
In 1- bromo-2- chlorobenzene, both the electronegative atoms give a resultant dipole moment, but due to less electronegativity of bromine atom, the magnitude will be lesser.
In ortho fluorophenol, the dipole moments of both the electronegative atoms combine to give a large resultant dipole moment. Also a hydrogen bond is formed between the fluorine atom and the hydrogen in the hydroxyl group. So it will have the highest dipole moment.
In o- xylene, the dipole moments cancel each other out.
The correct answer is C.

Note:
A hydrogen bond is also a type of a dipole- dipole interaction, which is caused due to large difference in the electronegativities between the atoms which induces charges, thus increasing the total dipole moment in the compound. The dipole moment increases with increase in the charge separation.