Hydrocarbons: Physical Properties of Alkanes and Conformation of Ethane (For CBSE, ICSE, IAS, NET, NRA 2022)

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Physical Properties of Alkanes

Boiling Point

  • The boiling points of alkanes also increase with the increase in the molecular mass.
  • In straight chain alkanes, the increase in boiling points due to the increase in surface area of the molecules.
  • Branching in a chain reduces the surface area and therefore, decreases the boiling point of alkanes.

Melting Point

  • The melting points of alkanes increase with the increase in their molecular mass but there is no regular variation in their melting point.
  • The melting points of alkanes depend not only upon the size and shape of the molecules, but also on the arrangement of the molecules in the crystal lattices.
  • The alkanes with odd number of carbon atoms have lower melting point than those having even number of carbon atoms because alkanes containing odd number of carbon atoms are symmetrical and can be more closely packed.

Conformation of Ethane

In ethane molecule, , the two carbon atoms are linked together by a single bond called sigma bond.

  • The groups bonded through a sigma bond can easily rotate with respect to each other i.e.. the two — groups in ethane can rotate with respect to each other.
  • The different arrangement of atoms resulting from such a rotation are called conformations and each such specific conformation is called a conformer (from conformational isomers) .

The conformational isomers can be represented in the following two ways:

  • Sawhorse representations
  • Newman projections

The Sawhorse representations show the carbon – carbon bond from an oblique angle and indicate the spatial arrangement of all C — H bonds.

Conformation of Ethane

In Newman projections the two carbon atoms are viewed along the C — C bond axis. The front carbon and its bonds are represented as and back carbon and its bonds are represented as .

Conformation of Ethane
  • Several conformations of ethane are possible. But there are two extreme possibility.
  • In this conformation all the six C — H bonds are as far away as possible This conformation is called staggered conformation and shown below.
Conformation of Ethane

conformation in which all the six C — H bonds are as close as possible and shown below:

Conformation of Ethane

The staggered conformation is the most sable conformation whereas the eclipsed conformation is the least stable conformation of ethane.

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