Conformation Of Alkanes

Hydrocarbon chains are joined together by a carbon-carbon bond.

These bonds can be single or multiple in some compounds.

Alkanes are hydrocarbons which only contain single σ bonds between two carbon atoms.

Electrons are uniformly distributed in this molecular orbital.

Due to this uniform distribution, carbon atom at the ends of bond can rotate about bond.

Due to this rotation, the arrangements of atoms in the space can be changed into another arrangement

Such molecules in which spatial arrangements of atoms is different due to rotation of a sigma bond are called conformations

Conformations are also called conformers or rotamers.

Because σ bond can rotate 360⁰, an alkane can have infinite conformers

The properties of each conformer depend upon the arrangement between atoms in it

One of these properties is repulsion between atoms

In some conformers, the atoms will be just in front of each other. This will cause a repulsion between them

This repulsion causes a strain in the molecule called torsional strain.

The magnitude of torsional strain will depend upon the angle between the bond of atoms with the two carbons

This angle is known as torsional angle or dihedral angle.

Now, the conformers are hard to visualize on the plane sheet

So, for the ease of visuality of the conformers, we use Sawhorse and Newman projections.

In sawhorse projection, we represent the confirmation in a straight line.

In sawhorse, projection molecule is viewed at an angle rather than side on.

The first carbon is present at the lower end of the line and second carbon is at the upper end of the line.

Three lines attached to carbon represents three bonds. These may be attached to same or different groups.

If we rotate the bond between two carbons i.e & , then we will get a new conformation.

If we look at the sawhorse projection from front then a new projection will be obtained.

This new projection is called Newman's projection.

Newman's projection is obtained from sawhorse projection as shown :

In Newman's projection first carbon is shown by a dot and second by a circle.

The longer lines represent the bonds of first carbon. Shorter ones are bonds of back carbon.

If we rotate the bonds of back carbon, then we will get a new conformation.

REVISION

Conformational isomers of alkane are formed due to rotation around single bond.

Conformational isomers are also known as conformers or rotamers.

Alkanes form infinite conformational isomers from ethane onwards.

The conformers are difficult to understand on a plane sheet.

Hence, they are represented by two projection formula namely Newmann and sawhorse.

Sawhorse projection is a line representation of alkane and is obtained when the molecule is viewed at a certain angle.

The first carbon is represented on the lower line and the second carbon is represented on the upper line.

The three lines attached to carbon represents bonds with other atoms

Newmann projection is obtained from sawhorse projection on viewing the molecule head on.

In newman's projection first carbon is shown by a dot and second by a circle. The lines represent bonds with other atoms

The angle of rotation between two carbon is called a torsional or dihedral angle.

The End