ISOMERISM AND STEREOCHEMISTRY  GEOMETRIC ISOMERS

INTRODUCTION
Left handedness of a molecule is an
important property they possess or another
truth most of the people are right-handed. A
lot of plants and animals exhibit handedness
and this is essential in our study of organic
chemistry. The heart of a human body is
chiral. Most of the plants and animal
molecules are chiral too.
A. SKELETAL ISOMERS
ISOMERS
 constitutional isomers that have the
 molecules that have the same same functional groups but differ from
molecular formula, but have a different each other in the connectivity of the
arrangement of the atoms in space. carbon skeleton.
 excluding any different arrangements  EXAMPLE: Butyl alcohol (1) and
which are simply due to the molecule isobutyl alcohol (2) are constitutional
rotating as a whole, or rotating about isomers containing the same functional
particular bonds. groups, namely the alcohol group.

TYPES OF ISOMERISM
 SKELETAL ISOMERS

 POSITIONAL ISOMERS B. POSITIONAL ISOMERS

 the basic carbon skeleton remains

unchanged, but important groups are
moved around on that skeleton.
 EXAMPLE: two structural isomers
with the molecular formula C3H7Br. In
 FUNCTIONAL ISOMERS one of them the bromine atom is on
the end of the chain, whereas in the
other it's attached in the middle.
C. FUNCTIONAL ISOMERS  These two models represent exactly
the same molecule. You can get from
 the isomers contain different functional one to the other just by twisting around
groups - that is, they belong to the carbon-carbon single bond. These
different families of compounds molecules are not isomers.
(different homologous series).  If you draw a structural formula instead
 EXAMPLE: a molecular formula of using models, you have to bear in
C3H6O could be either propanal (an mind the possibility of this free rotation
aldehyde) or propanone (a ketone). about single bonds. You must accept
that these two structures represent the
same molecule:

 there are other possibilities as well for

this same molecular formula - for  What happens if you have a carbon-
example, you could have a carbon- carbon double bond - as in 1,2-
carbon double bond (an alkene) and dichloroethene?
an -OH group (an alcohol) in the same
molecule.
 molecular formula: C3H6O2. (a
carboxylic acid) and methyl ethanoate
(an ester).

 These two molecules are not the

same. The carbon-carbon double bond
won't rotate and so you would have to
take the models to pieces in order to
D. GEOMETRIC ISOMERS convert one structure into the other
one.
 these isomers occur where you have
restricted rotation somewhere in a  Drawing structural formula for the last
molecule. pair of models gives two possible
 EXAMPLES: 1,2-dichloroethane. isomers:

1. The two chlorine atoms are locked

on opposite sides of the double
bond. This is known as
the trans isomer. (trans: from Latin
meaning "across" - as in
free rotation about this single bond. transatlantic).
2. the two chlorine atoms are locked
on the same side of the double
bond. This is known as
the cis isomer. (cis: from latin
meaning "on this side")
 JEAN BAPTISTE BIOT
 1815, French Physicist
 When a beam of plane-polarized light
is passed through solutions of certain
organic molecules, the plane
polarization is rotated. Not all organic
molecules exhibit this property; those
that do rotate plane-polarized light are
said to be optically active.
POLARIMETER
 An instrument that is used to measure
the amount and direction of rotation.
 The most likely example of geometric
isomerism you will meet at an
introductory level is but-2-ene. In one
case, the CH3 groups are on opposite
sides of the double bond, and in the
other case they are on the same side.

CONFIGURATIONAL ISOMERS
 are stereoisomers that can cannot be ENANTIOMERS
converted into one another by rotation
around a single bond.  are pairs of stereoisomers that are
chiral. A chiral molecule is non-
OPTICAL ACTIVITY superimposable on its mirror image, so
 A beam of ordinary light consisting of that the mirror image is actually a
electromagnetic waves that vibrate in different molecule.
an infinite number of planes at right  is describing a comparison between
angles to the direction of light travel. two molecules; it is telling us how two
different molecules are related.
CHIRAL
 only describing what a single atom
looks like; that it has four distinct
groups attached.
ENANTIOMERISM
 The relationship between two
stereoisomers having molecules that
 When a beam of ordinary light is are mirror images of each other;
passed through a device called a enantiomers have identical chemical
polarizer, the light that passes through and physical properties in an achiral
vibrates only in a well-defined plane, environment but form different
hence its name is plane-polarized light products when reacted with other
chiral molecules and exhibit optical
activity.
  When looking at a molecule, look for
carbons that are substituted with four
different groups.

CHIRALITY
 essentially means 'mirror-image, non-
superimposable molecules', and to say
that a molecule is chiral is to say that
its mirror image (it must have one) is
not the same as it self. Whether a  Because CH3 and CH2 groups cannot
molecule is chiral or achiral depends be chiral centers, this molecule has
upon a certain set of overlapping only three carbons that could be chiral
conditions. centers. The two leftmost possibilities,
identified in the next figure, have four
CHIRAL CENTER nonidentical groups and are chiral
 An atom that has four different groups centers, but the one on the far right
bonded to it in such a manner that it has two identical methyl (CH3) groups
has a nonsuperimposable mirror and so is not a chiral center.
image. FISCHER PROJECTION FORMULA
 a convention used to depict a stereo
formula in two dimensions without
destroying the stereochemical
information, i.e., absolute
configuration, at chiral centers.
 EXAMPLE: (R)-Lactic acid

 Chiral molecules usually contain at To convert this stereo formula into a Fischer
least one carbon atom with four projection, use the following procedure:
nonidentical substituents. Such a Step 1: Hold the molecule so that the chiral
carbon atom is called a chiral center center is on the plane of the paper…
(or sometimes a stereo genic center),
using organic-speak. Any molecule 1. two bonds are coming out of the plane of
that contains a chiral center will be the paper and are on a horizontal plane,
chiral (with the exception of a meso
2. the two remaining bonds are going into the
compound).
plane of the paper and are on a vertical
plane.
  can have different physical properties
and reactivity. They have different
melting points and boiling points and
different densities. They have two or
more stereocenters.

1. Hold the molecule from the methyl

group and rotate by 90o clockwise
2. Hold the molecule from the methyl
group and turn it backwards until the
hydrogen group are on a plane
perpendicular to the plane of a paper
Step 2: Push the two bonds coming out of the
plane of the paper onto the plane of the
paper.

Diastereomers vs. Enantiomers vs.

Meso Compounds

Step 3: Pull the two bonds going into the

plane of the paper onto the plane of the
paper.

Step 4: Omit the chiral atom symbol for

convenience.

DIASTEREOMERS
 are stereoisomers that are not related
as object and mirror image and are not
enantiomers.
 unlike enatiomers which are mirror
images of each other and non-
superimposable, diastereomers are
not mirror images of each other and
non-superimposable.
MESO COMPOUNDS  Cis-2-butene has both methyl groups
on the same side of the molecule.
 To identify meso, meso compound is Trans-2-butene has the methyl groups
superimposed on its mirror image, and on opposite sides of the molecule.
has an internal plane that is symmetry
(figure 3). Meso-tartaric acid is achiral
and optically unactive

 However, that the presence of a

double bond does not necessarily lead
to cis-trans isomerism. We can draw
two seemingly different propenes:

 Different views of the propene

CIS-TRANS ISOMERISM IN CYCLIC molecule (flip vertically). These are not
COMPOUNDS isomers.

 (geometric isomerism) can arise from There are two requirements for cis-trans
the different orientations of isomerism:
atoms/groups on a carbon-carbon 1. Rotation must be restricted in the
double bond or on a cyclic structure molecule.
(ring).
2. There must be two nonidentical groups
on each doubly bonded carbon atom.
 In these propene structures, the
second requirement for cis-trans
isomerism is not fulfilled. One of the
doubly bonded carbon atoms does
have two different groups attached, but
the rules require that both carbon
atoms have two different groups.
In general, the following statements hold true
 Consider the alkene with the in cis-trans isomerism:
condensed structural formula
CH3CH=CHCH3. We could name it 2-  Alkenes with a C=CH2 unit do not
butene, but there are actually two such exist as cis-trans isomers.
compounds; the double bond results in  Alkenes with a C=CR2 unit, where the
cis-trans isomerism. two R groups are the same, do not
exist as cis-trans isomers.
 Alkenes of the type R–CH=CH–R can
exist as cis and trans isomers; cis if
the two R groups are on the same side
of the carbon-to-carbon double bond,
and trans if the two R groups are on
opposite sides of the carbon-to-carbon
double bond.
If a molecule has a C=C bond with one non- STAGGERED CONFORMATION
hydrogen group attached to each of the
carbons, cis/trans nomenclature descried 1. Ball and Stick
above is enough to describe it. However, if 2. Sawhorse Projection
you have three different groups (or four), then 3. Newman Projection
the cis/trans approach is insufficient to
describe the different isomers, since we do
not know which two of the three groups are
being described.

EXAMPLE:
 If you have a C=C bond, with a methyl
group and a bromine on one carbon,
BALL
1 AND SAWHORSE
2 NEWMAN
3
and an ethyl group on the other, it is PROJECTIO
neither trans nor cis, since it is not
clear whether the ethyl group is trans
to the bromine or the methyl. This is ECLIPSE CONFORMATION
addressed with a more advanced E/Z
nomenclature discussed elsewhere.

 Cis-trans isomerism also occurs in 1. Ball and Stick

cyclic compounds. In ring structures, 2. Sawhorse Projection
groups are unable to rotate about any
of the ring carbon–carbon bonds.
3. Newman Projection
Therefore, groups can be either on the
same side of the ring (cis) or on
opposite sides of the ring (trans) MELVIN S. NEWMAN
CONFORMATION IN OPEN CHAIN  Newman projection views the molecule
SYSTEM from one end directly along the
carbon-carbon axis and represents.
We can depict conformers in two ways:  Sawhorse projection views the carbon-
carbon bond from an oblique angle
1.) Sawhorse projection and indicates spatial orientation by
2.) Newman projection showing all of the carbon-hydrogen
bonds.
CONFORMATIONS IN CYCLIC
SYSTEMS
CYCLOPROPANE

 The most stable conformation of

cyclohexane is the chair conformation.
As you can see from the Newman
projections, all the bonds in the chair
NEWMAN PROJECTION conformation are completely staggered
ALONG A C-C BOND whereas in the boat, which is less
stable by 7.1kcal/mol, these bonds are
completely eclipsed. In addition, in the
 For cyclopropane, the three carbon boat conformation there is repulsion
atoms lie in a plane forming a very between the “flagpole” hydrogens
rigid ring which cannot adopt a which are very close to each other.
nonplanar conformation. As a
consequence of this, there is eclipsing
of C-H bonds on neighboring carbons.
 All cycloalkanes larger than
cyclopropane undergo puckering to
yield nonplanar conformations in which
eclipsing of C-H bonds are relieved.
Depicted below are the puckered
nonplanar conformations of AXIAL AND EQUATORIAL BONDS IN
cyclobutane and cyclopentane. CYCLOHEXANE
 These are the two kinds of C-H bonds
BUTTERFLY-WING in the chair conformation of
CONFORMATION OF cyclohexane - axial bonds and
CYCLOBUTANE
equatorial bonds.

ENVELOPE
CONFORMATION OF
CYCLOPENTANE

 Each carbon atom in the chair

 cyclohexane can undergo puckering to
conformation has one axial and one
give conformations in which all the
equatorial bond.
bond angles are tetrahedral (109.5o).
Below are two conformations of
cyclohexane calling the boat and chair
conformation.
 APPLICATION
 Fruits are natural source of organic
compound, such as tartaric acid
C4H6O6. A picture of the vine
represents the handedness of the
substances and the leaves represent
chirality.
 Chirality is a phenomenon that govern
the universe. The human body is
structured to be chiral. All 20 amino
acids are chiral and all of them are
 Cyclohexane has two chair classified as left-handed.
conformations which are mirror images
SUMMARY OF TERMS/CONCEPTS
of each other and rapidly
interconvertible through ring inversion  Stereochemistry - study of chemistry
or ring flip. During inversions, the axial that deals with the molecular
bonds are converted to equatorial arrangement and connectivity in space
bonds and vice versa. of the substances.
 Stereoisomers - contains same
connectivity but different arrangement
configurational and conformational
isomers
 Enantiomers - stereoisomers that are
non-superimposable mirror images,
properties that differ are direction (+, or
-) of optical rotation. Occur only on
chiral molecules, not identical with its
CONFORMATIONS OF MONOSUBSTITUTED mirror image.
CYCLOHEXANES  Diastereomers - stereoisomers that
 A monosubstituted cyclohexane can are not mirror image they have
have two chair conformations, one with different physical and a different
the substituent in an equatorial compound.
position, the other with the substituent
in an axial position.