Tuesday, June 11, 2013

Hydrogen Bond

                      a hydrogen bond is a type of attractive ( dipole-dipole ) interaction between an electronegative atom and a hydrogen atom bonded to another electronegative atom. Hydrogen is bound to a highly electronegative  atom such as nitrogen ( N ), oxygen ( O ), or fluorine ( F ). Hydrogen bond can occur between molecules or within parts of a single molecule. The hydrogen bond is stronger than Vander Walls interaction but weaker than covalent or ionic bonds. This type of bond can occur in inorganic molecules such as and in organic molecules like DNA and proteins. Highly electronegativity of the atom bonded to hydrogen atom so that bond sufficiently polar.
Types Of Hydrogen Bonds :

  1. Intra molecular hydrogen bond
  2. Inter molecular hydrogen bond
Inter molecular hydrogen bond :
                This bond occur between separate molecule in a substance . They can occur between number of like or unlike molecules as long as hydrogen donors and acceptors are present in position, which they can interact.
  • An example of hydrogen bond donor is ethanol. which has a hydrogen bonded to oxygen.
  • An example of hydrogen bond acceptor , which does not have a hydrogen bonded to it is the oxygen atom on diethyl ether.  
For Example :
Inter molecular hydrogen bonds can occur between  NH3  molecules alone between  H2O  molecules alone or between NH3 and H2O molecules.

Intra molecular hydrogen bond :
          Intra molecular hydrogen bonds occurs within one single molecule. Both hydrogen donor and acceptor must be present within one molecule. This type of hydrogen bond is commonly called chelation and is more frequently found in organic compounds.
Example :

Strength of hydrogen bonds :
  • Its strength depends upon the electronegativity of atom to which H atom is covalently bonded.
  • Electronegativity of  F > O > N . The strength of hydrogen bond is in the order of H-F - - - H > H-O --- H > H-N --- H .
  • Hydrogen bonds are much weaker than covalent bonds.
  • The bond strength of different bonds is in the order. Ionic bond > Covalent bond > Hydrogen bond > dipole-dipole interaction, vander wall's ( London forces ).

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