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Single Double and Triple Bonds

December 10, 2012

Atoms in a molecule are bound by bonds.

Each bond is made up of two electrons, each coming from an atom.

Introduction to single, double and triple bonds:

If the bonds are formed by donating and accepting of electrons it is called as ionic bond.

If it is formed by sharing of electrons, it is called as covalent bond.

When two electrons are involved in the bond formation, it is called single bond.

However certain atoms are bound by more than one bond, may be two or three.

These are called double and triple bonds respectively. The compounds with these bonds are also called as unsaturated compounds.

Formation of Single Bond:

Whenever only two electrons are involved in a bond, it forms a single bond. The single bond is always of sigma type of bond. It could be ionic or covalent as discussed above.

E.g. Ionic bonds in Na-Cl, K-Cl etc.

Covalent bonds in CH4, C2H6

Single covalent bonds are formed by direct overlapping. The hybridization is sp3 hybridization. It means that a 2s and three 2p orbitals get hybridized to form the 4 sp3 hybridized orbitals.

Single bond

single bond methane

Formation of Double Bond:

Let us consider molecule of propene.

It has molecular formula C3H6

The central carbon has sp2 hybridization. It means one of the 2s electron is promoted to the 2p orbital and the remaining electron from the 2s and two of the 2p orbitals get mixed up, recast, rearrange and 3 new hybridized orbitals are formed for double bond.

All these orbitals form sigma bonds. However the lone unutilized p orbital forms parallel overlap and a pi bond is formed.

This pi bond is only called double bond.

CH3-CH=CH2

Existence of pi bond in the form of a doule bond attributes coplanar shape to the molecule.

Double bond

Double bond in ethene

Formation of Triple Bond:

Consider the molecule of acetylene.C2H2

The central carbon has sp hybridization. It means one of the 2s electron is promoted to the 2p orbital and the remaining electron from the 2s and one of the 2p orbitals get mixed up, recast, rearrange and 2 new hybridized orbitals are formed.

Both these orbitals form sigma bonds, one with carbon and another with adjacent hydrogen. However the two unutilized p orbitals form parallel overlaps and two  pi bonds are formed.

These two  pi bonds along with the sigma bond are only called triple bond.

  CH=CH

Existence of two  pi bonds in the form of triple bond attributes ‘linear’ shape to the molecule.

Triple bond

triple bond acetylene

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