Isomerism In Alkanes

This page teaches isomerism in alkanes. Make sure you have read isomerism before reading this section.

Alkanes show both structural isomerism and conformational isomerism.

Structural Isomerism in Alkanes

First three members of alkane family — methane, ethane and propane have only one structure. The higher alkanes can have more than one structure. In fact, in higher alkanes the number of isomers increases with increase in number of carbon atoms.

Let us discuss structural isomerism in butane : The four carbon atoms in butane can be linked together either in a continuous chain or with a branched chain in the following two ways :

Notice that the above isomers are chain isomers. Hence, butane has two chain isomers.

Conformations

Alkanes contain carbon-carbon sigma bonds. A sigma bond between two carbon atoms is formed when two sp³ hybrid orbitals of each carbon overlap along their internuclear axis. The electron distribution of the sigma molecular orbital is symmetrical around the internuclear axis of the C−C bond which is not disturbed due to rotation about its axis. As a result, the molecule of an alkane can have different spatial arrangements of atoms in space. Such arrangements are known as conformations or rotamers.

Torsional strain

The rotation around C−C single bond is not completely free. In fact, the rotation requires energy about 1-20 KJ mol⁻¹ due to weak repulsive interaction between the adjacent bonds. This repulsion is called torsional strain.

Conformations of ethane

Ethane molecule (CH₃−CH₃) contains a C−C single bond. If we keep one of the CH₃ groups stationary and rotate the other group, we will get infinite number of spatial arrangements of hydrogen atoms with respect to the hydrogen atoms attached to the other carbon.

These infinite arrangements are the conformations of ethane. Out of all those arrangements there are two extreme cases :

1. Eclipsed conformation
2. Staggered conformation

Eclipsed Conformation : In eclipsed conformation, the hydrogen atoms on two carbon atoms are as close as possible.
Staggered Conformation : In staggered conformation, the hydrogen atoms on two carbon atoms are as far apart as possible.

Any other intermediate conformation is called a skew conformation. It may be noted that the bond angles and the bond lengths remain the same in all arrangements. Eclipsed and staggered conformation can be represented by Sawhorse projections and Newman projections

Sawhorse projection

Sawhorse projection is a simple method of representing three dimensional formulae on paper. The following rules are used in this method :

• The C−C bond is represented by a straight line drawn diagonally. The line is somewhat longer for better clarity.
• The lower end of the line represents the front carbon, and the upper end of the line represents the rear carbon.
• Each carbon has three lines attached to it to represent hydrogen atoms. The lines are inclined at an angle of 120° to one another.

Newman projection

In Newman projection, the molecule is viewed at the C−C bond head on. The carbon atom nearer to the eye is represented by a point, whereas the rear carbon is represented by a circle. The hydrogen atoms on both the carbons are shown by lines inclined at an angle of 120° to one another.

Relative stability of conformations

In staggered form, any two hydrogen atoms on adjacent carbons are as far apart as possible. As a result, the repulsion between the electron clouds (torsional strain) of σ-bonds of two non-bonded hydrogen atoms is minimum. On the other hand, the carbon-hydrogen bonds are closer to each other in eclipsed form resulting in increase in electron cloud repulsion thereby reducing its stability. Hence, the staggered form of ethane is more stable than its eclipsed form.The energy difference between the staggered and eclipsed conformation is 12.55 KJ mol⁻¹. This energy difference is not large enough to prevent rotation. As a result, it is not possible to separate different conformational isomers of ethane.

Conformations of n-Butane

n-Butane has the following structure :

If we rotate any of the carbon numbered 2 or 3 keeping other one fixed, we will get infinite number of spatial arrangements. Of all those conformations, six are very important :

Clearly, there are actually four distinct conformations in the above six conformations : Fully eclipsed, eclipsed, gauche and anti.

Relative stability of conformations

The stability of four distinct conformations of n-butane follows the order :

Anti  >  Gauche  >  Eclipsed  >  Fully eclipsed

The reasons are given below :

• Anti : Since the two methyl groups and four hydrogen atoms are as far apart as possible. it is the most stable conformation.
• Gauche : The two methyl groups in gauche form are only 60° apart that causes some repulsion,known as steric strain, between the methyl groups. Due to this reason gauche conformations are slightly less stable than anti conformations. The two gauche conformation, however, are equal in energy.
• Eclipsed : In eclipsed conformations, there are two methyl-hydrogen eclipsed interactions and one hydrogen-hydrogen eclipsed interaction. As a result, the stability of eclipsed conformations is less than those of anti and gauche conformations.
• Fully Eclipsed : Since there is a strong methyl-methyl eclipsed interaction in addition to two weak hydrogen-hydrogen eclipsed interactions, fully eclipsed conformation is the least stable.

These conformations of n-butane cannot be isolated due to small energy difference between them.