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Summary Stereochemistry
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This summary is just about the stereochemistry chair conformations
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What is Stereochemistry?Stereochemistry deals with relative arrangements in space (i.e. three-dimensional arrangements
in space) of atoms in molecules.
It is sometimes referred to as 3-D chemistry since the prefix "stereo-" refers to the "three-
dimensionality of a system.
What does Stereochemistry cover and Why Study it?
The study of stereochemistry:
I. covers the three-dimensional relationships between stereoisomers
II. the methods for determining and describing the three-dimensional relationships between
molecules,
III. rationalizes the influences these arrangements impart on the physical, chemical and
biological properties of the molecules. Structure and reactivity
Molecular Structure Relationships
Isomerism
Isomers are compounds which have the same molecular formula (i.e, the same composition of
elements) but have different sequences of arrangements of their atoms within the molecule.
Categories of isomeric relationships
Structural or constitutional isomers have the same molecular formula, but different in their
bond connections or order of bonded atoms/groups.
1
, Chemical Formula:C3H6O3
H H
O O H
H3C C CO2H HOH2C C CO2H
HOH2C CCH2OH HC CH2OH
OH H
H
Chemical formular C3H6O3
O
O
H3C
OH HO
OCH3
OH
Stereoisomers have the same molecular formula and sequence of bonded atoms, but differ only
in the three-dimensional orientations of their atoms in space.
Stereoisomers have the same order of attachment of atoms, but differ in arrangement of the
atoms in space.
2
,Starch and cellulose are two more examples of molecules that have the same connectivity, but
different 3-D arrangement of atoms. They are both composed of the same repeating unit,
glucose, joined by an oxygen atom. The only difference is one ring carbon. In cellulose, it is
equatorial, and in starch, it is axial
Thus Cellulose: Composed of long chains held together by intermolecular hydrogen bonds, thus
forming sheets that stack in an extensive 3-D network.
Starch: The axial C-O bond causes the starch molecule to fold into a helix
Chapter 1: Chirality and Enantiomerism
Some molecules are not superimposable on their mirror images. When a molecule and its mirror
image are not congruent (superimposable), they are different molecules. Any molecule or object
that cannot be superimposed on its mirror image is chiral. A chiral molecule and its mirror image
are enantiomers. The aspect of non-superimposability of molecules with their mirror
images is called enantiomers
3
, Definition
Chirality (handedness) is the characteristic of a molecule that lacks a plane of symmetry that
makes it impossible to superimpose on its mirror image.
• An Enantiomeric molecules have the same relationship as right and left hands.
Enantiomers and hands are chiral (they possess the property of chirality)
• Molecules (or other objects) that are not chiral are said to be achiral (without chirality)
How can we tell if a molecule is chiral?
There are two things to look for:
1. Look for a plane of symmetry
A plane of symmetry divides an object or molecule into two equal halves that are mirror images
of each other. An object that has a plane of symmetry is superimposable on its mirror image and
is achiral. A chiral molecule DOES NOT have a plane of symmetry.
4
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