Concept: Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom (like Fluorine, Oxygen, or Nitrogen) and is attracted to another highly electronegative atom in a different molecule or the same molecule. For hydrogen bonding to occur, two conditions must be met:

1. A hydrogen atom must be directly bonded to a highly electronegative atom (F, O, or N). This creates a highly polarized H-X bond.
2. There must be another highly electronegative atom (F, O, or N) with a lone pair of electrons available to accept the hydrogen bond.

Why (D) HF is correct:

In HF, hydrogen is directly bonded to fluorine, which is the most electronegative element. This creates a strong positive charge on the hydrogen atom and a strong negative charge on the fluorine atom. The positively charged hydrogen atom of one HF molecule can then form a hydrogen bond with the negatively charged fluorine atom of another HF molecule.

Option Analysis:

• A) ${\mathrm{CH}}_3\mathrm{F}$ (Methyl fluoride): Although fluorine is highly electronegative, the hydrogen atoms are bonded to carbon, not directly to fluorine. Therefore, ${\mathrm{CH}}_3\mathrm{F}$ cannot form hydrogen bonds with itself.
• B) ${\mathrm{CH}}_3\mathrm{Cl}$ (Methyl chloride): Chlorine is less electronegative than fluorine, oxygen, or nitrogen. More importantly, the hydrogen atoms are bonded to carbon, not directly to chlorine. Thus, ${\mathrm{CH}}_3\mathrm{Cl}$ does not form hydrogen bonds.
• C) ${\mathrm{CH}}_3\mathrm{Br}$ (Methyl bromide): Bromine is even less electronegative than chlorine, and the hydrogen atoms are bonded to carbon. Therefore, ${\mathrm{CH}}_3\mathrm{Br}$ does not form hydrogen bonds.
• D) HF (Hydrogen fluoride): Hydrogen is directly bonded to fluorine (a highly electronegative atom), fulfilling the primary condition for hydrogen bonding.

Correct Answer: (D)