Trans isomers can be explained in two views. One is inorganic and another is organic. On the basis of inorganic chemistry, it can be explained as, in geometrical isomerism the ligands occupy different positions round the central metal ion.
When the two identical ligands are opposite or diagonal to each other, the isomer is known as Trans isomer, (in Latin trans means across). If the two identical ligands are adjacent to each other, the isomer is known as cis isomer, (in Latin cis means same).
On the basis of organic chemistry, it can be explained as trans isomerism results from a restriction in rotation about double bonds, or about single bonds in cyclic compounds. Trans isomer occurs when two similar groups are on the opposite sides of the double bond.
All alkenes do not show cis-trans isomerism. It is possible only when each double bonded carbon atom is attached to two different atoms or groups.
- Each carbon atom shows tetra valency and four single bonds arranged in tetrahedral manner.
- When two three dimensional arrangements of a molecule are inter convertible to each other by the free rotation of bonds.
- These two forms are known as conformations of each other.
- However two forms of molecules which cannot be inter convertible into each other by free rotation of bond are called as configuration.
For example, in case of alkene, there is a pi bond between carbon atoms which form through the side way overlapping of unhybridised p orbitals.
There are two essential conditions for geometrical isomerism is
- In pi bond electron density distributed above and below the plane of single bond, hence it prevents the free rotation of the carbon atoms of the double bond.
- This restricted rotation creates different spatial arrangements of groups attached to the carbons of a double bond and form geometric isomers of molecule.
- Hence the substituted alkenes exist as isomers differing in the relative spatial arrangement of atoms or groups around the double bond.
- These isomers are called geometrical isomers.
- Geometrical isomerism is also known as cis-trans isomers.
- These isomers cannot rotate the plane polarized light and show different physical and chemical properties.
- The isomer in which similar atoms or groups lie on the same side of the double bond is called the cis isomer.
- However the isomer in which similar atoms or groups lie on opposite sides of the double bond is called the trans isomer.
- There must be restricted free rotation due to the presence of double bond or presence of cyclic structure.
- The molecule must contains WXC=CYZ form, hence double bonded carbon atom must be bonded with two different groups or atoms.
For example; 2-Butene shows cis and trans-forms. Both show different physical and chemical properties like trans-form has high boiling point compare to cis-form due to symmetrical structure of trans-form.
Another common example of geometrical isomerism is malice acid and fumaric acid in which carboxy groups (-COOH) bonded at terminal carbon atoms located at same side in cis-form and called as maleic acid. When carboxy group (-COOH) located at opposite sides in trans-form and called as fumaric acid.
Geometrical isomerism is also possible in cyclic structures due to restricted rotation bond in cyclic ring. For example, 1,2-Dichlorocyclohexane can show cis and trans form.
Another example of cis-trans isomerism is 1,2-Dicarboxycyclopropane, which show geometrical isomerism due to restricted rotation of carbon-carbon bond of cyclic ring.
If any one of the carbon atoms of the double bond has two identical groups attached to it, there cannot be any geometric isomers. For example; 1,1,2-trichloroethene cannot show cis-trans isomerism due to two similar chloro group on one carbon atom. So by flipping one molecule by 180Â° give same molecule.
Some other examples of cis-trans isomerism is as follows.
Properties of cis-trans isomers
Cis and trans-isomers are differing in their spatial arrangements of atom due to restricted rotation of bonded carbon atoms. They show different physical and chemical properties.
- Generally the dipole moment of trans-form is zero while cis-forms are polar in nature with certain value of dipole moment.
- The boiling point of cis-isomers is more than trans-isomers. For example; the boiling point of cis-2-pentene 310 k whiles for trans-form its 309 k. This is due to intermolecular dipoleâ€“dipole forces (or Keesom forces) due tom polar nature and London dispersion forces in cis-isomers which raise the boiling point. In the trans-isomer, these forces do not occur due to non-polar nature of molecule.
- The symmetrical nature of molecule is a key factor in determination of relative melting point of molecule. Because of symmetrical nature of trans-isomers, they show a better packing in the solid state and have high melting point compare to cis-isomers.
- For example, the cis-isomer of oleic acid has a melting point of 386.4 k and exists as a liquid at room temperature, while the trans-isomer, oleic acid, melts a 316 k temperature due to the straighter trans-isomer being able to pack more tightly, and exists as solid under normal conditions.
- The interconversion of cis- and trans-isomers is not possible as this interconversion requires a 180Â° internal rotation about the double bond in which hold the other carbon has to be stationary. This is not possible because of presence of pi bond.
- In highly substituted alkenes, especially when the substituents are not alkyl groups, the cis-trans nomenclature is ambiguous.
- The new nomenclature of such type of alkene is based on Cahn-Ingold-Prelog system, in which the two groups at each Carbon atom are ranked according to sequence rule.
- According to sequence rule substituents bonded to carbon atom are listed in order of decreasing atomic number i.e. atom with high atomic number take precedence over atom with low atomic number.
- If the high priority groups are on the same side then it named as zusammen (Z)-isomer, and if they are on opposite sides then it is an entgegen (E)-isomer. Just like cis and trans-form, E and Z notation also written as prefix.
For example, in 1-Bromo-2-chloro-1-fluoroethene, out of bromo and fluoro group, bromo group has high precedence. However in Chloro and hydrogen, Chloro group has high atomic number and take precedence over hydrogen.
If both higher group located at same side, termed as Z-isomer while both higher and lower at opposite side form E-isomer. E, Z notation can be written in some other double bonded compounds which contain C=N , N=N and C=S present in imines and oximes.
For example, in diazocompound if both groups located on same side, it will be (Z)-isomer. However in (E)-isomer both groups located at opposite side. In these compounds E and Z-notation better termed as syn and anti-forms.