Isomers

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In organic chemistry, isomers are compounds with the same molecular formula but they do not necessary share the similar properties. There are two main groups of isomerization: constitutional isomer and stereoisomer.


Constitutional isomers have the same chemical formula, but may have different structures and connectivity.

e.g. C5H12 can be dimethylpropane which looks like a caltrop, or n-pentane which is a straight chain.

Note: Branching decreases boiling point


Stereoisomers not only have the same chemical formula, but also the same connectivity and only differ by spatial orientation.

These isomers are further subdivided into Conformational isomers and Configurational isomers. Conformational isomers are stereoisomers that have a different arrangement of atoms in space, but can be interconverted by rotations about sigma bonds. Configurational isomers have different arrangement of atoms in space that can't be interconverted by rotation about sigma bonds.Configurational isomers are subdivided into enantiomers and diastereomers.


Enantiomers are stereoisomers which are mirror images of each other while diastereomers are stereoisomers which are not mirror images of each other. Most diastereomers contain at least two stereocenters.


Some useful terminology

Stereocenter: An atom in a molecule bonded to four different substituents.

racemic mixture: getting equal amount of two enantiomers.

optical activity: rotation of plane polarized light

meso: A stereoisomer with an chiral centre which has an internal plane of symmetry. Even though meso compounds have chiral centres, they are achiral. achiral: property of a molecule that does not exhibit optical activity

Stereocenter: An atom in a molecule bonded to four different substituents.