Page 17 - Reference Guide For Foreign Pharmacy Licensing Exam Theory

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Enantiomers: Stereoisomers that are mirror images of each other are called enantiomers.

Diastereoisomers: Stereoisomers that are NOT mirror images of each other are called diastereomers.

Chiral: Molecules that are NOT superimposable on their mirror images are defined as Chiral. Molecules that are
superimposable on their mirror images are called Achiral.

Configuration: The fixed three-dimensional relationship of the atoms in a molecule, defined by the bonds
between them is commonly known as configuration.

Geometric isomer: The particular kinds of diastereomers that owe their existence to hindered rotation about
double bonds are called geometric isomers. They are normally prefixed by “cis” (on the same side) or “trans”
(across).

Cis 2-Butene Trans 2-Butene
Cis = Same Side Trans = Across

There is an interesting characteristic about the above two geometric isomers of 2-Butene. The Cis isomer of 2-
Butene is less stable than the Trans isomer of 2-Butene. The reason behind this is the orientation of methyl
groups. Methyl groups are very well separated in trans-isomer of 2-Butene, whereas they are close enough in
cis-isomer of 2-Butene to cause crowding and instability.

Configurational isomers: They are interconverted by inversion at the chiral center.

Conformational isomers: They are interconverted by rotation about single bonds.

Geometric isomers: They are interconverted by rotation about double bonds.

IUPAC Rules:

1. Select the parent structure as the longest continuous chain. In numbering the parent carbon chain, start at
whichever end results in the use of the lowest numbers; thus the following compound is called 2-
methylpentane rather than 4-methylpentane.

Correct name: 2-Methylpentane
Incorrect name: 4-Methylpentane

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