When a hydrogen atom forms a covalent bond with a highly electronegative atom (such as oxygen or nitrogen) in a molecule, a polar bond is formed between them.
The highly electronegative atom draws the shared electron pair to itself most of the time and thus attains a partially negative charge.
Consequently, the hydrogen atom attains a partially positive charge, being virtually electron deficient most of the time.
As shown from the figure above, the electron cloud is asymetrically located between the two bonding atoms. The arrow simply indicates the net dipole moment of the molecule and the direction of increasing electronegativity.
This frequent uneven sharing of electrons between the two atoms imparts a polarity to the molecule containing these bonded atoms.
The resulting polarity creates a dipole-dipole attraction between molecules called hydrogen bonding.
Hydrogen bonding accounts for the following phenomena:
- associated compounds have higher boiling point than non-associated compounds with nearly the same molecular weights;
- greater solubility of associated compounds in water;
- variations in the boiling points and solubility in water of the isomers of certain phenolic compounds.
Hydrogen bonding can occur between:
- the same molecules;
- different molecules (intermolecular hydrogen bonding);
- within the molecule (intramolecular hydrogen bonding).
Intermolecular Hydrogen Bonding
In the following figures below, the conceptualized intermolecular hydrogen bonding of several organic molecules with themselves and with water molecules is shown.
The hydrogen bonding of the ethyl alcohol molecules above explains its higher boiling point than those of propane's and dimethyl ether's.
The wide difference in the boiling points of the primary, secondary and tertiary amines can be attibuted to hydrogen bonding. Primary amines, like methyl amine molecules shown in hydrogen bonding above, and secondary amines form hydrogen bonds while tertiary amines can not do so due to lack of hydrogen atom bonded to nitrogen atom.
Phenol has higher boiling point than toluene because of hydrogen bonding of phenol molecules as illustrated above.
The association of ethyl alcohol molecules (above) and phenol molecules (below) with water molecules through hydrogen bonding explains their relatively greater solubility in water as compared to those compounds with nearly the same molecular weights.
Intramolecular Hydrogen Bonding
The isomers of some phenolic compounds exhibit great difference in their boiling points and water solubility because of intramolecular hydrogen bonding (that is, association within the molecules) that occurs in the ortho isomers. This intramolecular hydrogen bonding reduces the chances of intermolecular hydrogen bonding of the said isomers resulting to their relatively lower boiling points and lower water solubility.
