Conformational Isomerism in aliphatic systems

Session Objectives

By the end of this session, students will be able to:

• Explain the conformation of ethane

• Explain the conformation of butane

Why staggered conformation of ethane is lower in energy than the eclipsed conformation?

• Steric interaction between hydrogen atoms in the eclipsed conformation

• The hydrogen atoms are too small to get in each other’s way

• It has been estimated that steric factors make up less than 10% of the rotational barrier in ethane

• Can be justified with two important reasons

1) The first is that the electrons in the bonds repel each other and this repulsion is at a maximum in the eclipsed conformation

2) The second is that there may be some stabilizing interaction between the C–H σ bonding orbital on one carbon and the C–H σ* anti-bonding orbital on the other carbon, which is greatest when the two orbitals are exactly parallel: this only happens in the staggered conformation.

Ethane Conformations

• The torsional energy of ethane is lowest in the staggered conformation.

• The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in energy.

• At room temperature, this barrier is easily overcome, and the molecule rotate constantly.

Conformational Isomerism Definitions

Eclipsed conformation: A conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as close as possible to the atoms or groups on an adjacent carbon

Staggered conformation: A conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far as possible to the atoms or groups on an adjacent carbon

Dihedral angle: An angle created by two intersecting planes

Anti-conformation: A conformation about a single bond in which two groups on adjacent carbons lie at a dihedral angle of 180⁰

Gauche conformation: A conformation about a single bond of an alkane in which two groups of adjacent carbons lie at a dihedral angle of 60⁰

Conformations of butane

• Coming to butane, structure is slightly complicated

• We have replaced two hydrogen atoms in ethane by larger methyl groups

• Steric factors become a significant contribution to the rotational energy barriers.

• Also when we rotate C-C bond, not all the staggered conformations are same, neither are all the eclipsed conformations

• The six conformations butane can adopt as the central C-C bond is rotated in 60⁰ is explained in the next slide

Visualization of Conformers of butane

Energy relationship of conformers

• If we look those conformers, can observe the conformations with dihedral angles 60⁰ and 300⁰ and with angles 120⁰ and 240⁰ are mirror images of each other

• Means we have only four different maxima or minima in energy on rotation of central C-C bond

• In the four, two types of eclipsed conformations which represents the maxima in energy rotation graph and two types of staggered conformations which represents the minima

• All the conformations have been explained with respective names

Relationship between energy and dihedral angle

Observations in the graph

• Each of the eclipsed conformations will be energy maxima but the syn-periplanar conformation (θ = 0°) will be higher in energy than the two anticlinal conformations (θ = 120° and 240°)

• In the syn-periplanar conformation two methyl groups are eclipsing each other whereas in the anticlinal conformations each methyl group is eclipsing only a hydrogen atom

• The staggered conformations will be energy minima but the two methyl groups are farthest from each other in the anti-periplanar conformation

• Anti-periplanar will be a slightly lower minimum than the two synclinal (gauche) conformations

• Staggered conformations are stable since they each lie in a potential energy well

• Anti-periplanar conformation, with the two methyl groups opposite each other, is the most stable of all

• Eclipsed conformations are energy maxima, and therefore represent the transition states for interconversion between conformers

Summary

• Steric interaction between hydrogen atoms in the eclipsed conformation

• Six conformations butane can adopt as the central C-C bond is rotated in 600

• Eclipsed conformations are energy maxima