Organic chemistry deals with a large number of compounds, many of which have the same molecular formula but differ in their structures and properties. This phenomenon is known as isomerism. It arises due to differences in the arrangement of atoms within the molecule or in the spatial orientation of atoms. Isomerism is an important concept that helps in understanding the diversity and behaviour of organic compounds.
Types of Isomerism
Isomerism is classified into structural isomerism and stereoisomerism depending on how atoms differ in a molecule.
1. Structural (Constitutional) Isomerism
In this type, compounds have the same molecular formula but differ in the connectivity of atoms. This difference in structure leads to changes in physical properties like boiling point and sometimes chemical properties as well.
Further classified into various types:
a) Chain Isomerism
Chain isomerism arises due to different arrangements of the carbon skeleton. One isomer may have a straight chain, while another has a branched chain.
- This happens because carbon atoms can form chains of different shapes.
- Branching decreases surface area, leading to weaker van der Waals forces and hence a lower boiling point.
Example: Butane and 2-methylpropane
Same formula (C4H10), but different chain arrangement.
b) Position Isomerism
In position isomerism, the position of a functional group, substituent, or multiple bond differs, while the carbon skeleton remains unchanged.
- The change in position can affect reactivity and physical properties.
- Common in alcohols, alkenes, and halides.
Example: 1-butanol and 2-butanol
Only the position of –OH group is different.
c) Functional Group Isomerism
This occurs when compounds have the same molecular formula but different functional groups.
- Since functional groups determine chemical behavior, these isomers show completely different chemical properties.
- They may belong to entirely different classes of compounds.
Example: Ethanol (alcohol) and dimethyl ether (ether), C2H6O
d) Metamerism
Metamerism is shown by compounds having a divalent functional group such as –O–, –NH–, etc.
- It arises due to different distribution of alkyl groups on either side of the functional group.
- These isomers often have similar chemical properties but different physical properties.
Example: CH3–O–C3H7 and C2H5–O–C2H5
e) Ring-Chain Isomerism
In this type, one compound has an open-chain (acyclic) structure, while the other has a closed-ring (cyclic) structure.
- This leads to significant differences in properties due to ring strain and structure.
- Common in compounds with formula CnH2n.
Example: Propene and cyclopropane
2. Stereoisomerism
In stereoisomerism, compounds have the same molecular formula and same connectivity, but differ in the three-dimensional arrangement of atoms in space. These differences affect properties like polarity and optical activity.
Further classified as:
a) Geometrical Isomerism
This type arises due to restricted rotation around a double bond or in cyclic structures.
- Double bonds prevent free rotation, fixing the position of groups.
- This leads to different spatial arrangements.
- Each carbon of the double bond must have two different substituents.
They are further classified into two types:
- Cis-isomer: Similar groups on the same side. Cis isomers are generally more polar and may have higher boiling points due to dipole–dipole interactions
- Trans-isomer: Similar groups on opposite side. Usually more stable due to less steric hindrance.
Example: But-2-ene
Nomenclature of Geometrical Isomers
Geometrical isomers are named using two systems: the cis–trans system and the E–Z system. These systems help to distinguish isomers based on the relative positions of groups around a double bond.
1. Cis–Trans Nomenclature
This is the simpler and commonly used system for compounds having similar groups attached to the double-bonded carbons. This system works only when we can clearly identify similar groups on both carbons.
Example: But-2-ene
- In cis-but-2-ene, both methyl (–CH₃) groups are on the same side
- In trans-but-2-ene, the methyl groups are on opposite sides
2. E–Z Nomenclature System
The E–Z system is a more accurate and universal method based on priority rules.
Step 1: Assign Priority
- Look at the two groups attached to each carbon of the double bond
- Assign priority based on atomic number
- Higher atomic number = higher priority
Step 2: Compare Positions
- Z (Zusammen = together): Higher priority groups are on the same side
- E (Entgegen = opposite): Higher priority groups are on opposite sides
Example: Consider a compound where
- One carbon has –Cl and –H
- Other carbon has –CH3 and –H
Priority order:
- Cl > CH₃ > H
- If higher priority groups (Cl and CH₃) are on the same side, then it is Z-isomer
- If they are on opposite sides, then it is E-isomer
- Priority is assigned based on the atomic number of the atom directly attached
b) Optical Isomerism
Optical isomerism occurs in compounds having a chiral carbon atom (a carbon attached to four different groups). These compounds form mirror-image isomers called enantiomers.
- The images are non-superimposable, like left and right hands.
- They rotate plane-polarized light in opposite directions. One is dextrorotatory (+), other is levorotatory (–).
- Chemically similar but differ in interaction with plane-polarized light.
Example: Lactic acid