Aldehydes and Ketones (e.g., Propanal and Propanone).

Carboxylic Acids and Esters (e.g., Propanoic acid and Methyl acetate).

Metamerism: Different alkyl groups attached to the same polyvalent functional group.

Example: Diethyl ether ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{OCH}_2\mathrm{CH}_3$) and Methyl propyl ether ($\mathrm{CH}_3\mathrm{OCH}_2\mathrm{CH}_2\mathrm{CH}_3$).

Tautomerism: Special type of functional isomerism where isomers exist in dynamic equilibrium, usually involving migration of a proton and a double bond.

Keto-enol Tautomerism: Most common. Requires an $\alpha$-hydrogen.
```
R-CO-CH2-R'  <--H+ shift-->  R-C(OH)=CH-R'
   (Keto form)                (Enol form)
```
Example: Acetone ($\mathrm{CH}_3\mathrm{COCH}_3$) and Propen-2-ol ($\mathrm{CH}_2=\mathrm{C}(\mathrm{OH})\mathrm{CH}_3$).

2. Stereoisomerism

Same molecular formula, same connectivity, different spatial arrangement of atoms.

Conformational Isomerism: Different spatial arrangements arising from rotation about single bonds. Interconvertible at room temperature.
- Example: Ethane (staggered, eclipsed), Cyclohexane (chair, boat).
Configurational Isomerism: Interconversion requires bond breaking.
- Geometrical Isomerism (cis-trans or E/Z): Restricted rotation around a double bond or in a cyclic structure. Requires each carbon of the double bond to be attached to two different groups.
  - Cis/Trans: For simple cases. Cis if identical groups are on the same side, trans if on opposite sides.
  - E/Z (Entgegen/Zusammen): For more complex cases. E (opposite) if higher priority groups are on opposite sides, Z (same) if on the same side (Cahn-Ingold-Prelog rules for priority).
  - Example: But-2-ene. Cis-but-2-ene has two methyl groups on the same side of the double bond. Trans-but-2-ene has two methyl groups on opposite sides.
- Optical Isomerism: Due to chirality (non-superimposable mirror images).
  - Chiral Center: An atom (usually carbon) bonded to four different groups.
  - Enantiomers: Stereoisomers that are non-superimposable mirror images of each other. They have identical physical and chemical properties (except rotation of plane-polarized light and reaction with other chiral molecules).
    - Rotate plane-polarized light in equal but opposite directions (dextrorotatory 'd' or '+', levorotatory 'l' or '-').
    - R/S Configuration: Absolute configuration based on Cahn-Ingold-Prelog priority rules.
  - Diastereomers: Stereoisomers that are not mirror images of each other. They have different physical and chemical properties.
  - Meso Compounds: Compounds that possess chiral centers but are optically inactive due to the presence of an internal plane of symmetry.
  - Racemic Mixture: An equimolar mixture of a pair of enantiomers. Optically inactive due to external compensation. Resolution is the process of separating enantiomers.

Nomenclature (IUPAC)

Systematic naming of organic compounds.

Identify the longest continuous carbon chain: This is the parent chain. If a functional group is present, the chain containing the functional group is chosen, even if it's not the longest.
Number the parent chain: