Question

Dipole moment of phenol is smaller than that of methanol. Why?

Answer 1

A dipole moment occurs when there is a charge separation in a system. As a result, they can occur in both ionic and covalent connections. The difference in electronegativity between two chemically linked atoms causes dipole moments.

Complete answer:
The polarity of a chemical bond between two atoms in a molecule is measured by the bond dipole moment. It uses the notion of electric dipole moment, which is a measure of how far negative and positive charges are separated in a system.
The electron-withdrawing effect of the benzene ring makes the CO bond in phenol less polar, whereas the electron-releasing impact of the $-C{{H}_{3}}$ group makes the CO bond more polar in methanol.
The C-O bond in phenol is less polar due to the electron-withdrawing (-I effect) of the benzene ring, whereas the C-O bond in methanol is more polar due to the electron-donating (+I effect) action of the $-C{{H}_{3}}$ group.
Because of the electron withdrawing action of the phenyl ring, the dipole moment of phenol is lower than that of methanol. The polarity of the C-O bond in phenol lowers as a result of the resonance.
The electron density on the oxygen atom rises in methanol due to the presence of an electron-releasing methyl group, making the C-O bond more polar.

Note:
Alkyl halide has electron withdrawing halogen atoms, whereas the alkyl groups have electron giving tendencies. The positive charge is communicated to the other atoms in the chain if the electronegative atom (without an electron, therefore having a positive charge) is connected to a chain of atoms, generally carbon. This is the -I effect, or the electron-withdrawing inductive effect. In a nutshell, alkyl groups give electrons, resulting in the +I effect. The ionisation constant serves as its experimental foundation. It differs from, and frequently opposes, the mesomeric effect.