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