Hydrogen Bonding Definition.
A hydrogen bond is not a true bond but a strong form of dipole-dipole interaction. A hydrogen atom can participate in hydrogen bonding if it is bonded to oxygen, nitrogen, or fluorine. Thus H – F, N – H, and O – H hydrogens (which are partially positive), are hydrogen bondable hydrogens.
Hδ+ in these formulas show hydrogen bondable hydrogens. Hydrogen bonding in methanol and dimethylamine is shown by dotted lines as follows.
The O – H and N – H bonds are strongly polarized, leaving the hydrogen atom with a partial positive charge. This partially positive hydrogen atom forms inter or intramolecular attachments with the non-bonding electrons on oxygen, nitrogen, or fluorine atom. Although hydrogen bonding is a strong form of intermolecular attraction, it is much weaker than a normal C – H, N – H, or O – H covalent bond. Breaking a hydrogen bond requires about 20-40 kJ mol-1 compared with about 400 kJ mol-1 energy required to break a C – H, N – H, or O – H bond.
Hydrogen bonding has a large effect on the physical properties of organic compounds. The structures and boiling points of ethanol and dimethyl ether, (two isomers of molecular formula C2H6O), are as follows:
These two isomers have the same size and the same molecular weight; however, ethanol has an O – H hydrogen, and it is extensively hydrogen-bonded. Dimethyl ether has no O – H hydrogen, and it cannot form hydrogen bonds. As a result of its hydrogen bonding, ethanol has a boiling point over 100°C higher than that of dimethyl ether. The effect of N – H hydrogen bonding on boiling points can be seen in the isomers of formula C3H9N shown below.
Trimethylamine has no N – H hydrogens, and it is not hydrogen bonded. Ethylmethylamine has one N – H hydrogen atom and the resulting hydrogen bonding raises its boiling point about 34°C above that of triethylamine. Propylamine, with two N – H hydrogens, is more extensively hydrogen-bonded and has the highest boiling point of these three isomers.
In addition to b. p. the acidity of carboxylic acids can also be affected by hydrogen bonding. p-Hydroxy benzoic acid is 40 times weaker acid than salicylic acid because the anion produced after ionization of salicylic acid is stabilized through intramolecular hydrogen bonding, whereas the anion of p-hydroxybenzoic acid has no intramolecular hydrogen bonding. This is shown below: