Chemical molecules are composed of different types of atoms which are bonded through chemical bonds. The chemical bond is formed by sharing or transfer of valence electrons of the bonded atoms. The formation of chemical bonds between atoms provides stability to them due to completion of octet configuration. A chemical molecule can be represented in two manners; as molecular formula and as structural formula. The molecular formula represents the number of atoms and also the constituent atoms which are written in the form of their atomic symbols.
The structural formula of a molecule represents constituent atoms with the bonds between them in the form of lines. A single covalent bond represents as ‘-‘ and double covalent bond as ‘=’ and so on. So we can say that the structural formula represents complete bonding between atoms in a molecule whereas molecular formulae of a molecule do not provide information about the bonding. The molecules which have same molecular formulae but different structural formulae due to different bonding between atoms are called as isomers. This phenomenon is known as isomerism. There are mainly two types of isomerism, structural and stereoisomerism. Structural isomers have same molecular formulae but different structural formulae due to different bonding or position of atoms in molecules.
There are six types of structural isomers. Chain isomers have different number of carbon atoms in the parent chain of the molecule such as n-butane and 2-methylpropane have same molecular formula but have different number of carbon atoms in the parent chain. Another type of structural isomer is positional isomerism in which the position of functional group is different in the molecule. For example; 1-butene and 2-butene have same molecular formula but have different position of double bond in the molecule. Functional isomers have same molecular formulae with different functional group in the molecule. Due to presence of different functional group in the molecule, they have different physical and chemical properties. For example; alcohol and ether are functional isomers of each other. Metamers are usually found in ethers and secondary amines in which alkyl groups are different at both side of the functional group.
Tautomerism is due to shifting of H-atom which results the formation of keto and enol forms. Ring-chain isomers are also examples of structural isomers which have ring and open chain structure with same molecular formule. For example propene and cyclopropane are ring-chain isomers. Stereoisomers have same molecular and structural formulae but have different arrangement of atoms in 3-D space. There are two types of stereoisomers; cis-trans or e-z isomers or geometrical isomers and optical isomers. Optical isomers are non-super imposable mirror images of each other. Geometrical isomers have different arrangement of atoms due to restricted rotation of carbon=carbon bonds.
Geometrical isomers have different arrangement of atoms in a molecule which have restricted rotation of carbon atoms in the molecule. Geometrical isomers are also called as ez isomers which can be defined as isomers which have same molecular and structural formulae but have different arrangement of atoms due to restricted rotation of atoms. For example; alkenes have double bonded carbon atoms which can not rotate and form e and z-isomers.
Alkenes are typical examples of geometric isomerism in which the same groups are arranged different manner. They are mainly named as cis and trans or e and Z.
There are two main conditions for ez isomerism listed below.
- Isomers must show restricted rotation between atoms like alkenes
- Double bonded carbon atoms must contain different groups.
The double bonded carbon atoms are sp2
hybridized. Therefore each carbon atom contains three hybridized orbitals with one un-hybrid p-orbital. The hybrid orbitals form sigma bonds with another carbon atom and two other atoms. The un-hybrid p-orbital is placed perpendicular to the sigma bond between carbon atoms. These p-orbitals involve in the formation of pi-bond which also remain perpendicular to the sigma bond.
The perpendicular placement of pi-orbitals with respect to sigma bond restrict the rotation between carbon atoms and fix the position of atoms or groups which are bonded with double bonded carbon atoms. It results the formation of two geometrical isomers; cis and trans. The cis-isomers have same groups at the same side whereas trans-isomers have same groups at opposite sides.
The geometrical isomers are formed due to restricted rotation of carbon atoms which usually occurs due to presence of double bond. The e-z system is applied for such isomers where cis and trans naming does not work. The E-Z system is based on the same priority rules which are often called the Cahn-Ingold-Prelog (CIP) rules. Here ‘Z’ represents zusammen that means together whereas if these groups are placed on opposite sides, they will be said to be E-isomers which comes from entgegen means opposite at the double bond. For example; in cis-2-bueten, both methyl groups are placed at same side therefore it is also called as z-2-butene whereas trans-2-butene is also named as E-2-butene.
The E-Z naming is beneficial to name such compounds in which double bonded carbon atoms do not contain same groups. As given below; cis-2-chlorobut-2-ene is E-2-chlorobut-2-ene because Cl-group is more prior group compare to H-atom.
In the geometrical isomerism
, the cis-trans naming is based on the longest chain whereas the E-Z naming is based on a set of priority rules. The E-Z naming follows Cahn-Ingold-Prelog priority rules which based on certain rules such as in alkene, both double bonded carbon atoms are sp2 hybridized and for their naming, first we have to assign priority as high and low to each bonded atom on double bonded carbon atoms based on atomic number. Now determine the relative position of the higher priority groups. If high priority groups are placed on the same side then it results the formation of (Z)-alkene otherwise it will be E-alkene. Let’s take one example of 1-bromo-2-chloro-2-fluoro-1-iodoethene.
In this molecule the double bonded carbon atoms are bonded with four different groups. According to CIP rule, out of Br and I, I is high priority group compare to Br due to high atomic number. Similarly out of Cl and F, Cl is high priority group. Hence high priority group at same side forms Z-isomer and at opposite side it forms E-isomer.