Question

Intermolecular hydrogen bonding is strongest in:
A.Methylamine
B.Phenol
C.Formaldehyde
D.Methanol

Answer 1

Hint: To answer this question, recall the concept of hydrogen bonding. The phenomenon of hydrogen bonding refers to the formation of Hydrogen bonds. These hydrogen bonds are a special class of attractive intermolecular forces that arise due to the dipole-dipole interaction between a hydrogen atom that is bonded to a highly electronegative atom and another highly electronegative atom while lies in the vicinity of the hydrogen atom.

Complete step by step answer:
We know that hydrogen bonding is more extensive for oxygen atoms as compared to the nitrogen atom. This is because of the higher electronegativity of oxygen atom as compared to the nitrogen atom. So, option A can be eliminated.
 There is a presence of polar bond in aldehydes in which the carbon atom has a partial positive charge and oxygen atom has a partial negative charge. Hence, there is the presence of dipole in aldehydes leading to dipole-dipole interactions instead of hydrogen bonding. Hence, option C can be eliminated.
Amongst all options, phenol and methanol both exhibit H-bonding. But phenol exhibits the strongest hydrogen bonding because it has an electrophilic benzene ring attracting the lone pair of oxygen, thus increasing its tendency to attract an electron pair of hydrogen.

Hence, the correct option is B.

Note:
In a molecule, when a hydrogen atom is linked to a highly electronegative atom, it attracts the shared pair of electrons more causing development of partially charged poles. This generation of charged poles leads to the development of attractive forces between opposite poles of different molecules. This results in a weak bond between them. As a result of hydrogen bonding, a hydrogen atom links the two electronegative atoms simultaneously, one by a covalent bond and the other by a hydrogen bond. The conditions for hydrogen bonding are:
  1. The molecule must contain a highly electronegative atom linked to the hydrogen atom. More the electronegativity higher will be the development of charged poles, hence, the polarization of the molecule is more.
  2. Electronegative atoms should be small in size because smaller dimensions mean higher charge density which leads to greater electrostatic attraction.