# Organic Chemistry/Stereochemistry

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Organic Chemistry table of contents \>

# Stereoisomerism

**Stereoisomers** are compounds that have the same connectivity
(constitution) and the same chemical formula, but are isomers because
they differ in the spacial arrangement of the atoms attached to the
stereocenters (chirality centers) throughout the molecule. All
stereoisomers are unique and possess their own physical, chemical and
biological properties (with the exception of meso compounds).

!Two enantiomers of the common psychoactive drug
methamphetamine{width="450"}

To illustrate the importance of stereoisomerism in organic chemistry,
let\'s consider the well-known substance methamphetamine, which has two
enantiomers: (R)-N-methyl-1-phenylpropan-2-amine and
(S)-N-methyl-1-phenylpropan-2-amine. The S enantiomer, commonly called
D-methamphetamine is a controlled substance in most countries worldwide.
It is highly psychoactive, giving users an intense rush of euphoria when
ingested. Use of D-methamphetamine often leads to severe drug dependence
and even death in chronic users. By contrast, the R enantiomer of
methamphetamine, commonly called L-methamphetamine is a non-psychoactive
substance that is legal to possess in most jurisdictions. While not very
psychoactive, L-methamphetamine is an excellent sympathomimetic
vasoconstrictor and can be found in some OTC medications used to relieve
nasal congestion caused by the common cold or allergic sinusitis, often
under the name \"levmetamfetamine\".

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## Types of Stereoisomers

**Enantiomers** are chiral molecules with non-superimposable mirror
images. When comparing stereoisomers, the enantiomer is always the
isomer without any internal planes of symmetry and that has had each of
its stereogenic centers flipped. In other words, each stereocenter has
gone from an R-configuration to an S-configuration.

!Two enantiomers of
3-chloro-1-ethylcyclohexane{width="225"}\
In this example, the two molecules are enantiomers of one another
because (1) this molecule lacks any internal plane of symmetry and (2)
all of its stereocenters have been flipped. The molecule on the right is
(1S,3S)-3-chloro-1-ethylcyclohexane, while the molecule on the left is
(1R,3R)-3-chloro-1-ethylcyclohexane.

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**Diastereomers** are also chiral molecules, but differ from enantiomers
in that they are not mirror images of each other due to some **but not
all** of the chirality centers being flipped.

!Two diastereomers of
3-chloro-1-ethylcyclohexane{width="225"}\
The following two molecules are diastereomers of one another because (1)
they are not mirror images, and (2) only one of the stereocenters has
been flipped. The molecule on the left is
(1R,3S)-3-chloro-1-ethylcyclohaxane while the molecule on the right is
(1S,3R)-3-chloro-1-ethylcyclohexane.

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**Meso compunds** are achiral molecules that contain stereogenic
centers. They are achiral molecules because their mirror images are
superimposable onto each other as they possess an internal plane of
symmetry, meaning that meso compounds are identical molecules with
identical properties.

!The meso compound
(1R\*,2S\*)-1,3-dichlorohexane.-1,3-dichlorohexane."){width="300"}\
A common mistake is thinking that these two molecules are enantiomeric
due to both stereogenic centers having been flipped. However, upon
closer inspection these two molecules are not enantiomeric because by
manipulating this molecule in space it could be superimposed on top of
its mirror image due to it possessing an internal plane of symmetry.

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------------------------------------------------------------------------

-   Configurations
    
-   Enantiomers
    

:   Optical
    activity
    
:   R-S notational
    system
    
:   Meso
    compounds
    

-   Diastereomers
    

------------------------------------------------------------------------

\<\< Alkanes and cycloalkanes \|
Stereochemistry \|  Haloalkanes

