If two different 3D arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about the bonds, then they are called conformations. Conﬁgurations represent isomers that can be separated whereas Conformations represent conformers, which are rapidly interconvertible and thus nonseparable. The terms ‘‘conformational isomer’’ and ‘‘rotamer’’ are sometimes used instead of ‘‘conformer.’’
Barriers to rotation:
The rate of a chemical process is associated with an energy barrier (this holds both for reactions and simple bond rotations): the lower the rate, the higher the barrier. Rotation about single bonds is fast at room temperature.
The energy barrier for rotation about the single bond in butadiene (B) is slightly larger because of the weak conjugation between the double bonds, but the barrier to rotation about the genuine double bond in but-2-ene (D) is enormous and no rotation is seen. The energy barrier to the rotation about the C–N bond in an amide such as DMF (C) is usually about 80 kJ mol−1, translating into a rate of about 0.1 s−1 at 20 °C. The conjugation in amides is well developed, and the C–N bond has signiﬁcant double-bond character.
Conformations of ethane:
The staggered and eclipsed conformations of ethane are not identical in energy: the staggered conformation is lower in energy than the eclipsed by 12 kJ mol−1, the value of the rotational barrier. The dihedral angle, θ (sometimes called the torsion angle), is defined as the angle between a C–H bond at the nearer carbon and a C–H bond at the far carbon. In the staggered conformation, θ = 60° whilst in the eclipsed conformation, θ = 0°. The energy level diagram shows the staggered conformation as a potential energy minimum whilst the eclipsed conformation represents an energy maximum. Thus the eclipsed conformation is not a stable conformation as any slight rotation will lead to a conformation with lower energy. The molecule will actually spend the vast majority of its time in a staggered or nearly staggered conformation and only brieﬂy pass through the eclipsed conformation en route to another staggered conformation.
But why is the eclipsed conformation higher in energy than the staggered conformation? There are two reasons. The ﬁrst is that the electrons in the bonds repel each other and this repulsion is at a maximum when the bonds are aligned in the eclipsed conformation. The second is the stabilizing interaction between the C–H σ bonding orbital on one C-atom and the C–H σ* antibonding orbital on the other C-atom, and is highest when the two orbitals are exactly parallel and happen only in the staggered conformation. The same effects—repulsion between ﬁlled orbitals (a form of steric effect, see p. 129) and stabilization by donation into antibonding orbitals—govern the favoured conformations about all rotating bonds.
Conformations of butane:
A slightly more complicated case than ethane is that of a 1,2 disubstituted ethane (YCH2-CH2Y or YCH2-CH2X), such as n-butane. An energy diagram is given for this system. The gauche conformation of butane or any other similar molecule is chiral. The lack of optical activity in such compounds arises from the fact that and its mirror image are always present in equal amounts and interconvert too rapidly for separation.
The conformations with dihedral angle 60° and 300° are actually mirror images of each other, as are the conformations with angles 120° and 240°. This means that we really only have four different maxima or minima in energy as we rotate about the central C–C bond: two types of eclipsed conformation, which will represent maxima in the energy–rotation graph, and two types of staggered conformation, which will represent minima. In the syn-periplanar and anti-periplanar conformations the two C–Me bonds lie in the same plane; in the synclinal (or gauche) and anticlinal conformations they slope towards (syn) or away from (anti) one another.
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 furthest from each other in the anti-periplanar conformation so this will be a slightly lower minimum than the two synclinal (gauche) conformations.
As in ethane, the eclipsed conformations are not stable since any rotation leads to a more stable conformation. The staggered conformations are stable since they each lie in a potential energy well. The anti-periplanar conformation, with the two methyl groups opposite each other, is the most stable of all. We can, therefore, think of a butane molecule as rapidly interconverting between synclinal and anti-periplanar conformations, passing quickly through the eclipsed conformations on the way. The eclipsed conformations are energy maxima, and therefore represent the transition states for interconversion between conformations For butane and for most other molecules of the forms YCH2-CH2Y and YCH2 -CH2X, the anti conformer is the most stable, but exceptions are known.
One group of exceptions consists of molecules containing small electronegative atoms, especially fluorine and oxygen. Thus 2-fluoroethanol, 1,2-difluoroethane, and 2-fluoroethyl trichloroacetate (FCH2CH2OCOCCl3) exist predominantly in the gauche form and compounds, such as 2-chloroethanol and 2-bromoethanol, also prefer the gauche form. It has been proposed that the preference for the gauche conformation in these molecules is an example of a more general phenomenon, known as the gauche effect, that is, a tendency to adopt that structure that has the maximum number of gauche interactions between adjacent electron pairs or polar bonds. It was believed that the favorable gauche conformation of 2-fluoroethanol was the result of intramolecular hydrogen bonding, but this explanation does not do for molecules like 2-fluoroethyl trichloroacetate and has in fact been ruled out for 2-fluoroethanol as well.