Compounds that have the same molecular formula but different structures and, therefore, different chemical properties.
For example, hexose sugars - glucose, galactose and fructose - share the same molecular formula C6H12O6. But their molecular structures are different.
The bonding pattern differs and functional group arrangement at the 4th carbon is inverted.

Also called Structural Isomers. These isomers have the same molecular formula; differ in the covalent arrangement of their atoms. e.g. ethanol and dimethyl ether (C2H6O), and hexose sugar.

Molecules have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space.

Diasteoreomers are defined as non-mirror image non-identical stereoisomers

Cis-trans Isomers have the same covalent partnerships but differ in the spatial arrangement of atoms around a carbon-carbon double bond.

Interconverted just by rotations about formally single bonds.

Conformational isomers that differ by rotation about a single σ bond

Enantiomers are substances that are mirror images of each other due to the presence of an asymmetric carbon. Enantiomers are possible when four different atoms or groups of atoms are bonded to an asymmetric carbon.

  • The four groups can be arranged in space in two different ways that are mirror images of each other.
  • They are like left-handed (L) and right-handed (D) versions of the molecule.
  • Biologically important enantiomers
    • amino acids are most L-type.
    • sugars are most D- type.
  • Usually one is biologically active, while the other is inactive.