Hydrogen bonding is found in compounds in which the hydrogen atom is directly bonded to nitrogen, oxygen or fluorine This bond mode is comparably strong (10-40 kJ/mol) Many biological systems use this bond mode to stabilize a specific structure (i.e., DNA base pairing) The presence of hydrogen bonds in water explains its high melting and boiling point compared to its weight (H 2 S: m.p.= -82 o C, b.p.= -60 o C; H 2 Se: m.p.= -66 o C, b.p.= -41 o C; H 2 Te: m.p.= -49 o C, b.p.= -2 o C) Hydrogen fluoride also displays a high boiling point (b.p.= 20 o C) compared to hydrogen chloride (b.p.= -85 o C) and hydrogen bromide (b.p.= -67 o C) due to the same reason Hydrogen bonding is also observed in ammonia (b.p.= -33 o C) and in amines, but to a lesser degree because nitrogen is less electronegative than oxygen and fluorine (PH 3 : b.p.= -88 o C) The relatively high boiling points of alcohols and carboxylic acids can also be attributed to this bond mode as well i.e., dimers for benzoic acid
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