Two molecules are isomers if 1) they are neither superimposable, nor conformers 2) they have the same molecular formula
Two molecules are constitutional (structural) isomers if 1) they are isomers 2) they have a different semi-developed formula
Two molecules are configurational (stereo) isomers if 1) they are isomers 2) they have the same semi-developed formula
Different ramification of the hydrocarbon chain:
- Chain isomerism -
Different position of a functional group:
- Position isomerism -
- Function isomerism -
Rigidity of the C=C double bond:
- Cis-trans (E,Z or geometrical) isomerism -
Atom C with 4 different substituants (asymetrical C , C*):
- Enantiomerism -
The isomers can be separated at normal temperature.
Two isomer molecules do not belong to the same chemical substance.