Ch. 3 Textbook

3.1 Cis-trans isomerism

Of various isomerism, geometrical isomerism with C=C double bond would be easiest to understand. As is often used as an example, there are three isomers 1~3 with dichloroethylene. Though all of these are isomers, 1 and 2, and 3 are the structural isomers with different rational formula. On the other hand, 2 and 3 are stereoisomers because these have an equal rational formula (ClCH=CHCl), while have a different in their configuration of atoms in space. For the isomer 2, the same ligands are placed on the same side of the plan made by the double bond, while these ligands are placed on the opposite side for the isomer 3. Thus, these are the isomers with different configuration. The former is named a cis isomer, and the latter is named a trans isomer. This kind of stereoisomerism is called cis-trans isomerism or geometrical isomerism.

An isomer which is a stereoisomer but not an enantiomer is named a diastereomer (see Chapter 6). The cis and trans isomers are not enantiomers, but these are diastereomers. A pair of diastereomers is completely distinguishable substance with different chemical and physical properties. Since (unlikely to a single bond) a rotation about a double bond is prohibited by the overlap of p orbitals, geometrical isomers are usually isolable. For example, the melting and boiling points of trans-1,2-dichloroethylene 3 is -50 and 48.4oC respectively, while those of cis-1,2-dichloroethylene 2 is -80.5oC and 60.3oC, respectively. Their density is 1.259 for 3, 1.265 for 2, thus these possess completely different properties.

3.2 E, Z nomenclature

The cis-trans nomenclature leaves some vagueness as is shown in the examples as Q3.1 and Q3.4. Cis-trans nomenclature is originally a trivial system, and useful when simple compounds are treated. It is not competent, however, as a general nomenclature. There was proposed a general nomenclature of geometrical isomers based on the sequence rule. The procedure is as follows:

  • 1) To determine the priority of two ligands (including lone pair) bonded to the each atom forming the double bond based on the sequence rule.
  • 2) To name the isomer as below when the ligands in upper rank are located on:
    • the same side of the double bond: Z (abbreviation of zusammen) or seqcis
    • the opposite side of the double bond: E (abbreviation of entgegen) or seqtrans

Prefixes such as seqcis and seqtrans are equal to cis and trans, respectively, in the sequence rule (SEQuence rule). Symbols Z and E are, however, exclusively used today.

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.

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.

3.4 Conformational analysis of butadiene

Butadiene 27 is a molecule consisted in four successively bonded sp2carbon atoms. When the structure is presented, in a usual manner, as 27, it seems as if there are two independent double bonds.

Now we draw the structure of butadiene in which the overlap of p orbitals are emphasized. Since the maximum overlap, hence, the maximum stabilization, is obtained when adjacent p orbitals are parallel, there should be two stereoisomers of butadiene, 28 and 29. If p orbitals are omitted, 28 and 29 will be represented simply as 30 and 31, respectively. The two double bonds are placed on trans position for the central singles bond in 30, and cis in 31. Hence these are named s-trans and s-cis isomer, respectively. Prefixes s means the single bond. The s-cis isomer is rare except for a fixed ring structure like cyclohexadiene 32, since s-trans isomer has much less steric hindrance.