Question

The observed dipole moment is greater then its 8 calculated value in
(1) p-fluorophenol
(2) 1-bromo-2-chlorobenzene
(3) o-fluorophenol
(4) o-xylene

Answer 1

Hydrogen bond is a type of dipole –dipole interaction, larger electronegativity difference between the atoms induces charge on both the atoms so total charge of the molecule increases.
$Dipole moment={Charge(q) \times interatomic difference}$
Dipole moment of a molecule increases with increasing the charge. Dipole moment is a vector quantity that has both magnitude and direction. Direction of dipole moment is from an electropositive atom to toward the electronegative atom. Polarization of bond creates a dipole in the molecule, an induced dipole comes from polarization which is a completely different permanent dipole.

Complete answer:
Hydrogen bond is a type of covalent bond and polarity of this bond is measured in terms of dipole moment. Electronegativity difference is the measure of bond polarity while dipole moment is the measure of molecular polarity. Hydrogen bond is a type of polar covalent bond because of the large electronegativity difference in between the bonding atoms. So, formation of a polar bond within a molecule increases the dipole moment of the molecule.
(1) In para-fluorophenol both the electronegative atoms are completely opposite to each other. So these atoms cancel out the dipole moment of each other.
(2) In 1-bromo-2-chlorobenzene both the electronegative atoms give a resultant dipole moment. Since bromine is less electronegative atom so the magnitude of dipole moment will be less.
(3)In ortho-fluorophenol both the electronegative atoms give a vector sum of dipole moment, and an intramolecular hydrogen bond formed in between the fluorine atom at ortho position and hydrogen atom of (-OH). Fluorine is a highly electronegative atom, which induces positive charge on the nearest hydrogen atom and forms a hydrogen bond. Since hydrogen bond is a polar bond which will increase the dipole moment of the molecule.
So, option (3) is the correct answer.
(4) In o-xylene the net dipole moment will cancel out to each other, so this molecule has very less dipole moment.

Note
Do not confuse dipole moment and polarization. Dipole moment is the product of total charge and distance between the charges, so it is basically separation of charge in a molecule. However polarisation is a relative tendency of charge dispersion in between the molecule or ability of an atom to induce charge on the other atom.