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Alkanes: Conformation

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Alkanes: Conformation - Lesson Summary

Alkanes with two or more carbon atoms, like ethane and propane, consist of carbon-to-carbon single covalent sigma bonds in which the electrons are shared symmetrically around the inter-nuclear axis of the carbon-carbon single bonds.

The shape of an alkane molecule can be twisted into different three-dimensional arrangements which are interchangeable and can exist for short spans of time are called conformations.



The rotation between the inter nuclear axis of carbon to carbon bond causes infinite different spatial arrangements of the hydrogen atoms of one carbon atom with respect to the hydrogen atoms of another carbon atom. These arrangements are called conformational isomers.

The arrangement in which two hydrogens of two adjacent carbon atoms are close to each other is known as eclipsed conformation.

The arrangement in which two hydrogens of two adjacent carbon atoms are far to each other is known as staggered conformation. All the arrangements between the eclipsed and staggered positions are known as skew conformations.
 

 
In the eclipsed conformation, the molecule is the least stable as the repulsive force between the electrons is the maximum. In the staggered conformation, the molecule is the most stable as there is minimum repulsive force between the electrons.
 
The repulsive interaction between the electron clouds in a molecule, which affects the stability of a conformation is called torsional strain and its angle is known as torsional or dihedral angle. Due to this, it is not possible to separate different conformation isomers of ethane.

These are known as Sawhorse projection and Newman projection. Using Sawhorse projection, the three-dimensional structure of the eclipsed and staggered conformations can be easily visualized.
 

 
In Newman projection, the carbon-to-carbon bond conformation is viewed head on front to back.  The hydrogen atoms connected to the carbon atom in the front are represented by longer lines, and those connected to the carbon atom at the back are represented by shorter lines.
 

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