Stereochemistry is
the study of the static and dynamic aspects of the three-dimensional
shapes
of molecules.
The study of stereochemical problems
spans the entire range of organic, inorganic, biological, physical and supramolecular
chemistries. Stereochemistry includes methods for determining and describing
these relationships; the effect on the physical or biological properties these
relationships impart upon the molecules in question, and the manner in which
these relationships influence the reactivity of the molecules in question (dynamic
stereochemistry).
You are already familiar with the concept
of isomers: different compounds which have the same molecular formula.
In this chapter we learn to make distinctions between various kinds of isomers,
especially the more subtle kind of isomers which we call stereoisomers.
Ø Constitutional Isomers: Isomers which
differ in "connectivity".
The latter term means that the difference is in the sequence in which atoms are
attached to one another. Examples of isomers pairs which are consitutional
isomers are (1)butane and methylpropane,i.e., isobutane, which are different in
that butane has a sequence of four carbon atoms in a row, but isobutane has a
three carbon chain with a branch (2)dimethyl ether and ethanol--the former has
a C-O-C chain, while the latter has a C-C-O chain (3) 1-pentene and
cyclopentane--the former has an acylic chain of 5 carbons, while the latter has
a 5-membered ring.
Ø Stereoisomers: Isomers which have the
same connectivity. Thus
all isomers are either constitutional or stereoisomers. Stereoisomerism is a
more subtle kind of isomerism in which the isomers differ only in their spatial
arrangement, not in their connectivity. Cis- and Trans-1,4-dimethylcyclohexane
are a good example of a pair of stereoisomers.
Stereoisomers
there are two sub-types of stereoisomers:
Enantiomers: Stereoisomers which are
mirror images
- mirror-image stereoisomers
- like left and right hands
To be isomers, molecules must not be
identical. The test for "identicality" is one of superimposability.
In a sample of butane, all of the molecules are identical because they can be
superimposed upon one another in some conformation. The same is true of ethanol
or propanol or 1-butanol, but in the case of 2-butanol there are two isomeric
forms which can not be superimposed. They do not differ in connectivity,
obviously, or they wouldn't both be called by the same name (2-butanol). They
also don't have a cis or trans prefix, to indicate that they are
diastereoisomers. They have a very specific, unique relationship to one
another, the same relationship which exists between an object and its mirror
image. A key aspect of this difference, as we all know, is that a mirror acts
to interchange left and right hands.
Enantiomer
Examples :
Properties of Enantiomers
- enantiomers have identical physical and chemical properties,
- interact with another chiral molecule differently (like trying on left- or right-handed gloves - left and right hands react differently)
- rotate the plane of plane-polarized light by equal amounts but in opposite directions
Optical Activity
- chiral compounds rotate the plane of plane-polarized light
- rotation
measured in degrees clockwise (dextrorotatory or +) or
counterclockwise (levorotatory or -) - polarimeter - instrument for measuring optical activity
Diastereoisomers:
Stereoisomers which are not mirror images
Diastereomers
(sometimes called diastereoisomers) are stereoisomers that are not enantiomers. Diastereomerism occurs when two or more stereoisomers of
a compound have different configurations at one or more (but not all) of the
equivalent (related) stereocenters and
are not mirror images of each other. When two diastereoisomers differ from each
other at only one stereocenter they are epimers. Each stereocenter gives rise
to two different configurations and thus increases the number of stereoisomers
by a factor of two.
Diastereomers
differ from enantiomers in that the latter are pairs of stereoisomers that
differ in all stereocenters and are therefore mirror images of one another.
Enantiomers of a compound with more than one stereocenter are also
diastereomers of the other stereoisomers of that compound that are not their
mirror image. Diastereomers have different physical properties (unlike enantiomers)
and different chemical reactivity.
Cis-trans
isomerism and conformational isomerism are also forms of diastereomerism.
A molecule or object which is not
identical to its mirror image molecule or object is said to be chiral.
This means it resembles a human hand in that the left and right hands are not
superimposabile but can be readily distinguished (at least by some of us). By
the same token, a molecule or any object is said to be achiral if it is
identical to its mirror image molecule or object. Many molecules are achiral,
but many are chiral, especially complex molecules such as are found in
biological systems.
Example of Chiral and achiral :
~~forgiveness if there are
mistakes~~
