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Isomerism In Alkanes

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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 :

Isomers of C4H10

isomers of butane : butane
isomers of butane : 2-methylpropane

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

How many chain isomers does alkane with formula C5H12 have ?

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.

Conformation of ethane

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 :

sawhorse projection : eclipsed
sawhorse projection : staggered

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.

Newman projection, eclipsed form
Newman projection, staggered form

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 :

Structure of n-butane

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 :

Conformation of n-butane, fully eclipsed form Conformation of n-butane, Gauche form Conformation of n-butane, Eclipsed form Conformation of n-butane, Anti form Conformation of n-butane, Eclipsed form Conformation of n-butane, Gauche form

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 :

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

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