STEREOISOMERS
Nabila Al-Jaber
Associate Professor of
Chemistry
Isomers
isomers are different
compounds that have the
same molecular formula.
ISOMERS
Different compounds
with the same
molecular formula
Each isomer could
have stereoisomers
STEREOISOMERS
Isomers with the same order
of attachment, but a different
configuration (3D arrangement)
of groups on one or more of
the atoms
double bond
or ring
(geometric)
cis/trans
ISOMERS
with a ring
both can apply
CONSTITUTIONAL
ISOMERS
Isomers with a different
order of attachment of
the atoms in their
molecules
ENANTIOMERS
Stereoisomers whose
molecules are nonsuperimposible mirror
images of each other
DIASTEREOMERS
Stereoisomers whose
molecules are not mirror
images of each other
TYPES OF ISOMERISM
Isomerism
Structural
isomerism
Chain
Ex. N. and
isobutane
Steroisomerism
Position
Functionl
Ex. Ortho,
meta, para
Ex.ethanol,
diethylether
Optical
Isomerism
Geometrical
Isomerism(cis
, trans isomers)
Relationship of Constitutional
and Stereoisomers
Relationships of Stereoisomers
constitutional isomers
C4H8 hydrocarbons
Structural isomers.
* A fifth possible isomer of formula C4H8 is
CH3CH=CHCH3 .
* This would be named 2-butene according
to the IUPAC rules.
* These isomers may be isolated as distinct
compounds, having characteristic and
different properties .
stereoisomers.
* The only difference being the relative •
orientation or configuration of the two
methyl groups .
* They are shown here with the designations •
cis and trans.
Configurational Stereoisomers of
Alkenes
Configurational Stereoisomers of
Alkenes
The Sequence Rule for
Assignment of Alkene
Configurations
• Assign priorities to double bond
substituents by looking at the
atoms attached directly to the
double bond carbons.
1. The higher the atomic number of the •
immediate substituent atom, the higher the
priority.
For example,H– < C– < N– < O– < Cl–. •
(priority increases left to right)
(Different isotopes of the same element are •
assigned a priority according to their atomic
mass.)
2. If two substituents have the same –
immediate substituent atom, move to
the next atom (away from the double
bond) until a difference is found.
For example, CH3– < C2H5– –
< ClCH2– < BrCH2– < CH3O–.
Cycloalkane Stereoisomers
Configurational Stereoisomers of
Cycloalkanes
Rings Conformations
Some Conformations of
Cyclohexane Rings
Conformational Energy Profile of
Cyclohexane
TC = twist chair
B = boat
TB = twist boat
C = chair
Conformational Structures of
Disubstituted Cyclohexanes
1,1-dimethylcyclohexane
1-t-butyl-1-methylcyclohexane
cis-1,2-dimethylcyclohexane
trans-1,2-dimethylcyclohexane
cis-1,3-dimethylcyclohexane
trans-1,3-dimethylcyclohexane
cis-1,4-dimethylcyclohexane
trans-1,4-dimethylcyclohexane
Ethane Conformations
Name of
Conformer
Wedge-Hatched
Bond Structure
Sawhorse
Structure
Newman
Projection
Conformational Energy Profile of
Ethane
Potential Energy Profile for Ethane
Conformers
Dihedral Angle
Bond
Repulsions in
Ethane
Conformational Isomerism
Extended Chain
Coiled Chain –
Four Conformers of Butane
Potential Energy Profile for Butane
Conformers
Potential Energy Profile for
Butane Conformers
summarize some important
aspects of conformational
stereoisomerism at this time.
summarize some important aspects
of conformational stereoisomerism
at this time.
(i) Most conformational
interconversions in simple molecules
occur rapidly at room temperature.
Consequently, isolation of pure
conformers is usually not possible.
summarize some important aspects
of conformational stereoisomerism
at this time.
(ii) Specific conformers require
special nomenclature terms such as
staggered, eclipsed, gauche and anti
when they are designated.
summarize some important aspects
of conformational stereoisomerism
at this time.
• (iii) Specific conformers may also
be designated by dihedral angles.
In the butane conformers shown
above, the dihedral angles formed
by the two methyl groups about the
central double bond are: A 180º, B
120º, C 60º & D 0º.
summarize some important aspects
of conformational stereoisomerism
at this time.
(iv) Staggered conformations about
carbon-carbon single bonds are
more stable (have a lower potential
energy) than the corresponding
eclipsed conformations. The higher
energy of eclipsed bonds is known
as eclipsing strain.
summarize some important aspects
of conformational stereoisomerism
at this time.
(v) In butane the gauche-conformer
is less stable than the anticonformer by about 0.9 kcal/mol.
This is due to a crowding of the two
methyl groups in the gauche
structure, and is called steric strain
or steric hindrance.
summarize some important aspects
of conformational stereoisomerism
at this time.
• (vi) Butane conformers B and C
have non-identical mirror image
structures in which the clockwise
dihedral angles are 300º & 240º
respectively. These pairs are
energetically the same, and have not
been distinguished in the potential
energy diagram shown here.