3.3 Configuration and conformation
3.3 Configuration and conformation
We have used the term conformer to explain isomers related to the rotation about C-C single bond of ethane and butane derivatives, and the term configuration to define some substituted methanes and ethylenes in the previous chapters. At first glance it seems straightforward to distinguish conformation and configuration. The stereoisomerism which is due to the rotation about a single bond is referred to as conformation. Conformers are easily interconvertible and it is difficult to isolate the isomer. To the contrary, when two compounds are different in their configuration, e.g., a pair of enantiomers of bromofluoromethane, or a pair of geometrical isomers, maleic acid and fumaric acid, these are distinguishable compounds, and their isolation is possible. However, if maleic acid can be converted into fumaric acid by heat, there remains some ambiguity to classify conformational isomers and configurational isomers by their possibility of interconversion. It would be more practical to classify them by their facility of interconversion. A new nomenclature was proposed where stereoisomers with lower energy barrier of conversion are conformers (conformational isomers), while those with higher energy barrier are configurational isomers. If the barrier of interconversion is above 100 kJ mol-1, these are configurational isomer while if it is lower than 100 kJ mol-1, these are conformers.
▶go to S3.3 Conformers and configurational isomers
It was previously explained that the rotation about a C=C double bond is prevented by the overlap of p-orbitals, while the rotation about C-C single bond is relatively free. The rotation about a C=C double bond, however, can occur during the reaction; e.g., fumaric acid is converted into mareic acid by heating. Thus, the difference between the rotation about a C-C bond and that about a C=C bond might better be regarded as the difference of the required energy to achieve the transition state involved in the rotation. Then, let us examine the twist angle-energy diagram of the rotation about the double bond. We shall follow the process of the rotation starting from E isomer 25, via Z isomer 23, to 25, the starting structure. The energy of the molecule tends to maximal when the two p planes of each carbon atoms become orthogonal. In this state, the C=C bond is completely cleaved into a single bond, and by a rotation of the single bond, the E isomer 25 (θ=180o) was obtained which is in the state of second energy minimum. This minimum is usually lower than that of Z isomer. The diagram from θ=180o to θ= 360o Situation involved in θ=180o to θ= 360o is similar to the first half of the diagram.
