Conformations of Alkanes and Sigma Bonds

Conformations of Alkanes

• Alkanes contain carbon-carbon sigma (σ) bonds, which permit free rotation about C-C single bonds due to symmetrical electron distribution around the internuclear axis.
• This rotation results in different spatial arrangements of atoms in space, known as conformations or conformers or rotamers.
• Alkanes can have infinite number of conformations by rotation around C-C single bonds.

Conformations of Ethane

• Ethane molecule (C2H6) contains a carbon-carbon single bond with each carbon atom attached to three hydrogen atoms.
• Rotation around the C-C axis results in infinite number of spatial arrangements of hydrogen atoms attached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom.
• There are two extreme cases: eclipsed conformation (hydrogen atoms attached to two carbons are as close together as possible) and staggered conformation (hydrogen atoms are as far apart as possible).
• Any other intermediate conformation is called a skew conformation.
• Bond angles and bond lengths remain the same in all conformations.

Representations of Conformations

• Sawhorse projections: molecule viewed along the molecular axis, projected on paper by drawing the central C-C bond as a straight line.
• Newman projections: molecule viewed at the C-C bond head-on, with the carbon atom nearer to the eye represented by a point and the rear carbon atom represented by a circle.

Relative Stability of Conformations

• Staggered form of ethane has minimum repulsive forces, minimum energy, and maximum stability due to electron clouds of carbon-hydrogen bonds being as far apart as possible.
• Eclipsed form of ethane has increased electron cloud repulsions, resulting in higher energy and lesser stability.
• Torsional strain, the repulsive interaction between electron clouds, affects stability of a conformation and depends on the angle of rotation about the C-C bond.