Hydrogen Bond Theory

Hydrogen Bond


Many hydrogenated compounds have properties some abnormal that can be interpreted by assuming that amongst its molecules is formed a special type of link, characteristic of hydrogen atoms, called as Hydrogen Bond, called hydrogen bridge in a principle, being the hydrogen atom which acts as a kind of bridge between two very electronegative atoms.

Hydrogen bonding occurs in all compounds where hydrogen is linked to a very electronegative atom X such as, for example, water, alcohols, phenols, organic acids, with OH groups, and in amines, amides, etc., with NH groups. These compounds may form molecular associations with themselves or with other molecules having electron donor atom such as, for example, ketones (group C = 0), ethers (-O-), ammonia, etc.
Due to the hydrogen bonding, all these compounds have something anomalous properties; in particular, have melting points and boiling abnormally high in relation to other similar compounds.

Moreover, the small formation energy of hydrogen bonding between 2 and 7 kcal / mol, allows that the link may break and forms, in quite mild conditions, so it plays an important role in many biological processes.
There are two types of hydrogen bonds:

  • Intermolecular à hydrogen bonding that occurs between two molecules.
  • Intramolecular à hydrogen bonding that occurs within the same molecule.


The hydrogen bond in water.

Ordinary water is undoubtedly the most common substance in which hydrogen bonding occurs. On ice, hydrogen bonds keep water molecules in a rigid but rather open structure. When the ice is melted, only a fraction of hydrogen bonds breaks. One indication of this is the relatively low heat of melting ice, much less than it could be expected if all the hydrogen bonds broke during melting.
When the ice melts, some hydrogen bonds are broken. This allows water molecules to be arranged more compactly, which justify the increase in density when the ice melts. That is, the number of water molecules per unit volume is higher in the liquid than in solid.
When liquid water is heated above the melting point, the hydrogen bonds are continuing breaking. Molecules are packed more and the liquid water density continues increasing. Liquid water reaches its maximum density at 3.98ºC. Above this temperature, water has a “normal” behavior: its density decreases with the temperature increase. This unusual behavior freezing explains why a freshwater lake freezes from up to down. When the water temperature drops below 4 ° C, the denser water sinks to the bottom of the lake and the colder surface water freezes. Then the ice of the lake surface tends to isolate the water below the ice, heat loss.
Without the hydrogen bonding, all lakes freeze from the bottom to up; and fish, small animals that live on the bottom and aquatic plants would not survive in the winter.


Other properties which are affected by hydrogen bonding.
There are many substances whose properties are affected by hydrogen bonding. In acetic acid (CH3COOH), molecules tend to join forming pairs of dimers (double molecules) as in liquid state as in vapor. When this substance is evaporated, not all the hydrogen bonds are broken and as a result, the vaporization heat is abnormally low.
Hydrogen bonding can also help us to understand some viscosity aspects. In alcohols, the H atom in an -OH group in a molecule may form a hydrogen bond with an atom or molecule of neighbor alcohol. An alcohol molecule with two OH groups (diol), is more likely to form hydrogen bonds than one with a single group -OH.
By having stronger intermolecular forces can be expected that diol has higher viscosity and therefore flow more slowly. The more -OH groups present, the greater amount of viscosity is expected it has.





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