Isomerism of Class 11
(a) Hindered rotation about carbon-carbon bond
A double bond consists of a s-bond and a p-bond. The p-bond is formed by the sideways overlapping of unhybridized p-orbitals of two carbon atoms above and below the plane of carbon atoms. If one of the carbon atoms of the double bond is rotated with respect to the other, the p-orbitals will no longer overlap and the p-bond should break, but the breakage of this bond requires 251 kJ mol-1 of energy which is not provided by the collisions of the molecules at room temperature. Consequently, the rotation about a carbon-carbon double bond is not free but is strongly hindered or restricted. In other words, a p-bond prevents free rotation of the carbon atoms of the double bond with respect to each other. Due to this hindered rotation, the relative positions of atoms or groups attached to the carbon atoms of the double bond get fixed. For example, Ha and Hb cannot exchange their positions by rotations of C1 with respect to C2 without breaking the p-bond.
(i) Cis-trans isomerism
Alkenes can exist in two distinct isomers, which differ from each other only in the relative positions of atoms or groups in space around the double bond. For example, but-2-ene can exist in the following two forms (I and II).
Both these isomers have the same structural formulae but differ in the relative spatial arrangement of hydrogen atoms and methyl groups around the double bond. The isomer I,
in which the similar atoms or groups lie on the same side of the double bond is called the cis-isomer whereas the isomer II, in which the similar atoms or groups lie on the opposite sides of the double bond is called the trans-isomer. It is because of this reason that geometrical isomerism is also called cis-trans isomerism.
(ii) E & Z nomenclature
In the case of cis-trans nomenclature, the atoms or groups attached to the C-atoms should be similar but if all the four groups are different, then E, Z nomenclature comes into picture. According to this nomenclature, if the atoms or groups of higher priority are on the same side of the double bond, the isomer is designated as Z (zusammen, in German means together) and if the two atoms or groups of higher priority are on the opposite sides, the isomer is designated as E (Entegegan, in German means opposite). The priority of a group or atom is based on the given below rules.
Rule 1: The atom of higher atomic number gets higher priority. If the two atoms attached to the double bond are isotopes, the isotope of higher mass number gets the higher priority.
For example, in 1-bromo-2-chloro-2-fluro-1-iodo ethene, C1 has two atoms viz. Br and I. Since I (Z = 53) has higher atomic number than Br (Z = 35), therefore I is assigned priority 1 while Br is assigned priority 2. Similarly, Cl is assigned priority 1 while F is assigned priority 2 on C2.
Rule 2: If two atoms directly attached to the doubly bonded carbon have the same atomic number, then the relative priority of the group is determined by a similar comparison of the atomic numbers of the next element in the group. (and so on, if necessary, work outwards till the first point of difference is reached). For example, in the following compound,
One of the carbon atoms of the double bond carries CH3 and CH3CH2 groups, since the first atom (i.e. C) attached to the carbon atom of the double bond is the same in CH3 and CH3CH2 groups, compare the atomic number of the atoms attached to each of these first atoms.
Rule 3: Double bond and triple bonds are treated as if they have duplicate or triplicate single bonds.
For example, consider the following compound,
One of the atoms of the double bond carries CH3 and C6H5 group. Since, in C6H5 group, the first carbon is attached to two other carbons by a double bond and the other by a single bond. Therefore, phenyl gets higher priority than CH3 group. Similarly, CHO gets higher priority than CH2OH. Thus, the given isomer is ‘E’.
(c) Necessary and sufficient conditions for geometrical isomerism
All compounds containing double bond necessarily do not show geometrical isomerism.
The necessary conditions for a molecule to exhibit geometrical isomerism are
(i) The molecule must having restricted rotation due to the presence of a C=C, C=N, N=N and cyclic structure.
(ii) Each of the two atoms having restricted rotation must have different substituents attached to it, which may be same or different. Thus, alkenes of the type abC=Cab and abC=Cde show geometrical isomerism.
However, geometrical isomerism is not possible, if one or both the doubly bonded carbon atoms carry two similar substituents. This is because in such cases, the two possible configurations are, identical as shown below.
It is because of this reason that terminal alkenes such as propene, but-1-ene, 2-methyl prop-1-ene and alkenes carrying identical substituents on one of the doubly bonded carbon atoms such as 2-methyl but-2-ene and 2, 3-dimethyl but-2-ene etc. do not show geometrical isomerism.
The nomenclature for aldoximes is syn (when H and OH are present on same side of the double bond) and anti (when H and OH are present on opposite sides of the double bond).
Cyclic compounds too have restricted rotation because of the impossibility of rotation around C-C single bond as the conformation of cyclic compound would twist on rotation. Appropriately placed substituents on cycloalkanes would be capable of showing geometrical isomerism.
From a study of the physical properties. The difference in the structure of cis, and trans isomers is reflected in their physical properties. Some such properties are illustrated below.
a) Dipole Moments: The trans isomers have normally less dipole moments than their corresponding cis isomers. The reason for this is clearly understood if we consider the cis and trans isomers of 1,2-dichloroethylene. The trans isomer has a dipole moment of zero. This is due to the fact that the two bond moments of C–Cl bonds are opposed because of the symmetry of the molecule. On the other hand, the cis isomer being non-symmetrical has a finite dipole moment because here the bond moments are not opposed.
In such alkenes which have one polar substituent different from the other, the dipole moment will not normally be zero but would be smaller than the corresponding cis isomer. If, however, one substituent is electron-donating and the other electron-withdrawing, the bond moments are fully additive in trans isomer. Thus the trans isomer in this case has a higher dipole moment than the corresponding cis isomer.
Therefore, it is possible to assign configuration to a pair of isomers on the basis of dipole measurements, provided the nature of substituents is known.
b) Melting Points and Related Phenomena: In general, a trans isomer has greater symmetry than the corresponding cis isomer. Thus it packs more easily in the crystal lattice and hence has higher melting points. Cis compounds, on the other hand, have low melting points since they being less symmetrical do not pack well in the crystal lattice. Moreover, the poor packing leads to weaker forces of attraction between the molecules in the crystal lattice. The weaker forces of attraction can be easily broken by the dielectric constant of the solvents and hence the cis isomers have greater solubilities than their trans isomers.
Cis compounds have also been found to have higher heats of formation and ionization constants as acids. Due to these differences in properties, it is sometimes possible to assign configurations to a pair of geometrical isomers.
c) By Chemical Methods: The formation of a cyclic molecule from an open chain molecule takes place easily only when the reacting groups are close to each other. This fact has been most useful in assigning configuration to cis-trans isomers in which the doubly bond carbon atoms carry groups that are capable of reacting with each other. The configuration of maleic and fumaric acids the two groups are nearer to each other than they are in fumaric acid. That is,
Obviously maleic acid is the cis form and fumaric acid the trans form. In addition to the methods given above, other physical measurements such as the measurement of the distances between certain atoms by means of X-rays, measurement of absorption spectra etc., may be of help for deciding upon the configuration in some cases.