Isomerism is the phenomenon whereby compounds have the same molecular formula but different structural arrangements or spatial orientations of atoms, leading to different properties.
Isomerism is broadly classified into:
Occurs when compounds have the same molecular formula but differ in the way atoms are connected or bonded.
Types of Structural Isomerism include:
| Type | Description |
|---|---|
| Chain Isomerism | Different carbon skeleton arrangements |
| Positional Isomerism | Functional group at different positions |
| Functional Group Isomerism | Different functional groups entirely |
| Metamerism | Different alkyl groups on both sides of functional group |
| Tautomerism | Dynamic equilibrium between two structures (e.g. keto–enol) |
In this type of isomerism, compounds differ in the arrangement of the carbon chain. For example, 2-methyl propane is an isomer of butane.
n–butane (C4H10)
CH3 — CH2 — CH2 — CH3
2–Methylpropane (C4H10)
CH3
|
CH3 — CH — CH3
Similarly, Pentane has 3 isomers.
n–pentane (C5H12)
CH3 — CH2 — CH2 — CH2 — CH3
2–Methylbutane (C5H12)
CH3
|
CH3 — CH — CH2 — CH3
2,2 — dimethylpropane (C5H12)
CH3
|
CH3 — C — CH3
|
CH3
Hexane has 5, Heptane has 9, and Octane has 18 isomers
In this isomerism, functional group or substituent occupies different positions on the same carbon skeleton.
Propan-1-ol (C3H6O)
CH3 — CH2 — CH2 — OH
Propan-2-ol (C3H6O)
OH
|
CH3 — CH — CH3
Chlorobutane (C4H9Cl)
Cl
|
CH2 — CH2 — CH2 — CH3
2-Chlorobutane (C4H9Cl)
Cl
|
CH3 — CH — CH2 — CH3
In this isomerism, compounds have the same molecular formula but different functional groups.
Ethanol (alkanol) (C₂H₆O) CH3 — CH2 — OH Dimethyl ether (ether) (C₂H₆O) CH3 — O — CH3
Propanal (alkanal)(C3H6O)
CH3CH2CHO
Propanone (alkanone) (C3H6O)
CH3COCH3
In this isomerism, functional group remains the same but the alkyl groups on both sides differ.
Ethoxyethane (Diethyl ether) CH3 — CH2 — O — CH2 — CH3 Methoxypropane (Methyl Propyl ether) CH3 — O — CH2 — CH2 — CH3
Tautomerism is a type of isomerism where two or more molecules (tautomers) are in equilibrium, often involving the movement of a hydrogen atom and a double bond. This process is also known as keto-enol tautomerism, where a keto form (containing a carbonyl group) is in equilibrium with an enol form (containing an alkene and an alcohol group). The conversion between these forms involves the transfer of a proton (H+ ion) and a rearrangement of electrons.
For example, Propanone (CH3COCH3)
⇌
Prop-1-en-2-ol (CH3C(OH)=CH2)
Keto form: CH3—CO—CH3
OH
|
Enol form: CH3 — C = CH2
Stereoisomerism refers to a type of isomerism in which compounds have the same molecular formula and the same structural (bonding) arrangement, but differ in the spatial arrangement of their atoms or groups in three-dimensional space. The atoms are connected in the same order, but positioned differently around a chiral center or a double bond.
There are two main types of stereoisomerism:
1. Geometric (cis–trans) isomerism: This occurs due to restricted rotation around a C=C double bond or within ring structures. Groups may be on the same side (cis) or opposite sides (trans) of the double bond.
2. Optical isomerism: This occurs in molecules that contain a chiral carbon (a carbon atom attached to four different groups). Such molecules exist as non-superimposable mirror images called enantiomers. They rotate plane-polarised light in opposite directions.
Thus, stereoisomerism focuses on how atoms are oriented in space rather than how they are connected, making it different from structural isomerism.
| Type | Description |
|---|---|
| Geometric (cis–trans) | Restricted rotation around double bond or ring leads to spatial difference |
| Optical Isomerism | Non-superimposable mirror images (chiral molecules) |
Occurs due to restricted rotation around a C=C double bond or ring.
Properties of Cis and Trans Isomers
Cis–trans (geometric) isomers arise due to restricted rotation around a double bond or within ring structures. Although they have the same molecular formula and connectivity, their spatial arrangement leads to noticeable differences in physical and sometimes chemical properties.
1. Cis Isomers
- Have similar groups on the same side of the double bond.
- Usually have higher boiling points because the molecule is often polar, allowing stronger intermolecular forces (dipole–dipole attractions).
- Often have lower melting points because their asymmetrical shape makes them pack less efficiently in solids.
- Generally have a net dipole moment due to uneven distribution of atoms/groups.
2. Trans Isomers
- Have similar groups on opposite sides of the double bond.
- Usually have lower boiling points because the molecule is often non-polar, leading to weaker intermolecular forces.
- Often have higher melting points because their symmetrical shape packs more efficiently in solids.
- Usually have zero or very small dipole moment due to symmetry.
Do Cis and Trans Isomers Have Similar Chemical Properties?
Generally, yes — cis and trans isomers often have similar chemical properties because they contain the same functional groups and the same type of chemical bonds.
However, their rates of reaction or reactivity can differ due to the spatial arrangement of groups. For example:
- Cis isomers may undergo reactions more readily if groups are close together.
- Trans isomers may be more stable and less reactive in some cases.
Therefore, they have similar chemical properties but often show different physical properties and different stabilities due to their geometry.
Cis but-2-ene CH3 CH3 \ / C = C / \ H H Trans but-2-ene CH3 H \ / C = C / \ H CH3
Cis 1,2-dichloroethene Cl Cl \ / C = C / \ H H Trans 1,2-dichloroethene Cl H \ / C = C / \ H Cl
Occurs in molecules with a chiral carbon —a carbon attached to four different groups.
Example: Lactic acid (C₃H₆O₃)
1. Left-handed (L) form
OH
|
H — C — COOH
|
CH3
2. Right-handed (D) form
HO
|
COOH — C — H
|
CH3