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Hydrogen Bond : Sigma bond and Pi bond with example

Hydrogen Bond

The bond which is formed between an atom of hydrogen and an atom of a negative electronegative element due to the mutually stable electrostatic attraction force of their partial positive and negative charges, is called hydrogen bond.

A covalent bond between a hydrogen and an atom of a strongly electronegative element is a polar covalent bond. For this reason, there is a partial positive charge on the hydrogen atom and a partial negative charge on the atom of the negative electronegative element.

Example: In Hydrogen Fluoride (HF), the bond formed between H and F is a polar covalent bond and due to this there is a partial positive charge on H and a partial negative charge on F.

H+ – F

That atom of hydrogen which is connected to an atom of a negative electronegative element by covalent bond, can also be attached to any other atom of that negatively electronegative element by means of a stable electrostatic attraction force. This type of bond is called hydrogen bond and it is represented by dotted lines ( – – -).

In this case, the atomic negative electronegativity of Hydrogen forms a bridge between the two atoms of the element. The strength of hydrogen bond depends on the negative electronegativity of the negative electronegativity element.

When the negative power is high, the strength is high. The following is the sequence of occurrence of negative energies of the major elements.

F > O > Cl ~ N > Br > C ~ S ~ I > H

If the atoms of hydrogen and negative electronegativity between which hydrogen bond is formed are part of different molecules of a compound, then this type of bond is called intermolecular hydrogen bonding.

intermolecular-hydrogen-bonding

Example: Intermolecular hydrogen bonding occurs in HF and H2O.

Compounds which have intermolecular hydrogen bonding, their molecules are attracted to each other due to hydrogen bonding. Therefore, more energy is required to separate the molecules of these compounds. Therefore, the melting and boiling points of such compounds are often high.

Example: HF and H2O are liquids at room temperature and NH3, HCl and H2S are gases.

In other words, the boiling points of HF and H2O are higher than those of NH3, HCl and H2S. The reason for this is that F and O are strongly negatively electronegative elements, so hydrogen bonding is present in HF and H2O.

In HF and H2O there are hydrogen bonds between atoms of different molecules. Therefore, intermolecular hydrogen bonding takes place in HF and H2O. N , Cl and S are not so strongly negatively electronegative elements and hydrogen bonding in NH3, HCl and H2S is not very or very weak.

Therefore, the boiling points of HF and H2O are higher than those of HCl, H2S and NH3. Compounds which have intermolecular hydrogen bonding, when water is added to them, the molecules of the compound form hydrogen bonds with water molecules. Therefore, these types of compounds are usually soluble in water.

If the atoms of hydrogen and the negative electronegative element, between which hydrogen bond is formed, are part of a molecule of a compound, then this type of bonding is called intramolecular hydrogen bonding.

Intramolecular-hydrogen-bonding

Example: Intramolecular hydrogen bonding occurs in ortho nitrophenol. Similarly, intramolecular hydrogen bonding occurs in selycylaldehyde and celycilic acid.

Compounds that have intramolecular hydrogen bonding usually have low melting and boiling points and are volatile.

Example: Intermolecular in aortho nitrophenol and intramolecular hydrogen bonding in per nitrophenol. The melting point of aortho nitrophenol is 45°C while the melting point of pera nitrophenol is 114°C. Aortho nitrophenol steam is volatile. Whereas pera nitrophenol is not water volatile.

Compounds that have entermolecular hydrogen bonding are often insoluble in water or have low solubility in water.

Example: The solubility of aurtho nitrophenol in water is less than that of pera nitrophenol in water.

Concentration of the Ore

According to the modern concept of atomic structure, electrons in an atom reside in different orbitals. There can be a maximum of two electrons in any one orbital and their spin is in the opposite direction.

If the second electron is acquired by transfer or sharing by the orbitals which have only one electron, then the direction of spin of this second electron is opposite to the direction of spin of the first electron.

The opposite direction of the magnetic field produced by the spins of both electrons results in neutralization of the magnetic field and loss of energy, resulting in stability.

Based on the knowledge of atomic structure, it is necessary to explain how the orbitals participate in the sharing of electrons when covalent bonds are formed by the sharing of electrons. Two theories have been proposed to explain the sharing of electrons with respect to orbitals.

  • valency bond theory
  • molecular orbital theory

According to both these theories, those orbitals of the atom which are half-filled i.e. which have only one electron, participate in forming covalent bonds. Covalent bonds are formed by overlapping of half filled orbitals. This theory of formation of covalent bond by overlapping of orbitals is called orbital theory of covalency.

Sigma and Pi Bonds

The bond formed by the linear overlapping of two half-filled orbitals is called a Sigma bond(σ – Bond).

When a sigma bond is formed by the overlapping of two orbitals, the axes of these orbitals are in the same straight line while overlapping.