Geometric isomerism

GEOMETRIC ISOMERISM
PREPARED BY:
MR. NADIM MR CHHIPA
ASSOCIATE PROFESSOR,
ASP & BRI, ADALAJ

DEFINITION
• The isomerism which occurs due to difference of the positions
of the substituents about a double bond or a ring due to
restricted rotation is called geometric isomerism.
• They do not rotate the plane of polarised light (unless they also
happen to be chiral), and do not have identical properties.
• Conditions for geometric isomerism
There must be a carbon-carbon double bond in the
compounds.
Each of the carbon of the double bond must be attached to two
different substituents

CIS-TRANS ISOMERSIM
Configuration of the isomeric but-2-ene shown in figure. They
differ in their names by the prefixes cis- (Latin: on same side) and
trans- (Latin across), which indicate that methyl group are in same
side or on opposite side of the molecule.
These forms are not interconvertible due to restricted rotation of
double bond.

First we will determine which of the two chair conforms of cis- 1,4-
dimethyl-cyclohexane is more stable. One chair conformer has one
methyl group in an equatorial position and one methyl group in an
axial position. The other chair conformer also has one Therefore,
both chair conformers are equally stable.
This method of denoting geometric isomerism works best when the
alkene is di-substituted. In fact, it will always work when the alkene
is di-substituted (and other conditions are fulfilled). But this
method can fail with tri-substituted or tetra-substituted

For this cis/trans method of denoting to work, there must be at
least one identical group on each carbon of the double ,bond.
For example:

Cis isomer is less stable than trans isomer
• In cis isomer, two large groups on the separate carbons are always
on the same side. Thus, these two groups are closer to each other and
repel each other. This is called steric strain.
• On the other hand, in trans isomer the two large groups are on the
opposite sides. So they are far apart. Hence they don’t repel each
other. So, the steric strain is far less.

E AND Z NOMENCLATURE
• For denoting Geometrical isomers by cis/trans, is not sufficient
when there are more than two different substituents on a
double bond. So denote them E/Z nomenclature is adopted.
• If the group of highest priority on both carbon are on the same
side, then it is Z (Z = Zusammen = Together) isomer, if they are
on opposite sides, then it is E (E = Entgegen =Opposite)
isomer.
• The letters E and Z are represented within parantheses and are
separated from rest of the name with a hyphen.
• the groups attached to each carbon of the double bond are
analyzed and then given priorities according to Cahn-Ingold-

CIP RULES FOR E/Z NAMING CONVENTION
• Substituents on any one of the two double-bonded carbon
atom is looked at.
• First, the atom which is directly attached to the double bond
carbon is looked at. This is the first atom. The group where
first atom has higher atomic number has higher priority.

• If, both groups are attached by the same first atom, then the
atomic number of the second atom (atom attached to first atom)
is looked at.
• Similarly, if the second atoms are also same, third atoms are
looked at.

If the first atoms of two groups have the same higher atomic
number substituents, one with more such substituent is given
higher priority.

If there is any double bond or triple bond within the group, it is
considered at two or three single bonds respectively. So:
If there is a phenyl group attached to first atom, then it is thought
that First atom is attached to three carbons.

If isotopes of same element are present, the higher priority is
given to the isotope with higher atomic mass. E.g. the Deuterium
isotope (H2 or D) has more priority than protium (H1 or H). The
C13 isotope has more priority than C12.

SYN-ANTI SYSTEM
• This is used for compounds which are oximes of aldehyde,
hydrazones and Semicarbazide, in which carbon is joined to nitrogen
by double bond also exhibit geometrical isomerism.
• Since H and OH group can arrange on same side or opposite sides of
the double bond.
• when hydrogen and hydroxyl group are on the same side, the isomer
is known as syn (analogous to cis) and when these groups are on the
opposite sides, the isomer is known as anti (analogous to trans).

In Aldoxime the syn isomer- in which –OH group of the
oxime is on the side of the hydrogen of the aldehyde
carbon
In Ketoxime - specify the group with respect to which the
oxime -OH group is syn

DETERMINATION OF CONFIGURATION OF
GEOMETRICAL ISOMERISM
1. Dipole Moment
Cis isomer have higher dipole moment than trans-isomer. As in
trans-isomer two bond moments are opposed because of the
symmetry of molecule, where sys isomer being non-symmetrical
has a finite dipole moment as bond moments are not opposed.

2. Melting/ Boiling Point
trans isomer have higher Melting and Boiling than cis-isomer. As
in trans-isomer molecules are more symmetrical and hence fit
more closely in the crystal lattice as compared to the molecules
of cis isomer.
Intermolecular forces work well in trans isomer and U shape of
cis isomer can not fir perfectly in crystal lattice. Poor packing is
leads to poor intermolecular forces. So they required less energy
to break.

3. Solubility.
• In general, solubility of a cis isomer is higher than that of the
corresponding trans isomer. This is due to the reason that the
molecules of a cis isomer are less tightly held in the crystal
lattice.

4. Stability
• The trans isomer is more stable than cis isomer due to steric
hindrance. Intermolecular reactions occur easily when reacting
groups are close together. Hence, the cis isomer will form cyclic
derivatives more readily as against trans derivatives. But this
reaction will take place in only those cis isomers in which the
substituent’s on two double bonded carbons are capable of
intramolecular reaction with each other.

CONFORMATIONAL ISOMERISM
• Different spatial arrangement of atoms that can be generated
or converted into one another by free rotation about single
bond is known as confirmations.
• Different confirmation of same molecule also called confirmers,
rotamers or conformational isomers.
• It can be determined by use of x-ray and electron differenction,
IR, Raman, UV and NMR, etc.
• Confirmations can be calculated by method called molecular
mechanics.

REPRESENTATION OF CONFORMATIONAL
ISOMERS
WEDGE AND DASH SAWHORSE NEWMAN

TORSIONAL OR DIHEDRAL ANGLE (Ø)
• the angle created by two intersecting planes
• In case of ethane angel between HCC and CCH plane.

CONFORMATIONAL ISOMERISM IN ETHANE
• Ethane has two confirmation known as Staggered and Eclipsed.

CONTINUED……
• There is a energy barrier of about 3kcal/mol. The potential
energy of the molecule is at minimum from the staggered
confirmations, increase with rotation, and reaches a maximum
at a eclipsed confirmation. So Ethane molecule exist as most
stable , eclipsed confirmation.
• As 3kcal energy barrier is nor too large, even at room
temperature rapid interconversion between staggered
confirmation occurs as single bond permits free rotation.
• The energy required to rotate the ethane molecule about the
carbon-carbon bond is called torsional energy. The reason for
instability of eclipsed or skew confirmation is torsional stain

WHY THE ECLIPSED CONFIRMATION IS
HIGHER IN ENERGY THAN STAGGERED
CONFIRMATION
• There is a some steric repulsion between the Hydrogen atoms
of the eclipsed confirmation that is reduced in staggered
confirmation.
• In eclipsed confirmation electron cloud of C-H bond are most
nearer so repulsion increases.
• Thus repulsion force caused torsional strain in molecule, the
more strain more will be the internal energy of molecule, less
stability.

CONFORMATIONAL ISOMERISM IN N-
BUTANE

CONTINUED…..
• Due to presence of methyl group, two new points included:
there are several staggered confirmation and one more factor
besides torsional strain affect the conformational stabilities.
• There are four different confirmation of n-Butane:
1) Fully staggered confirmation, called anti, trans or
antiperiplanner. Dihedral angle, 180°, where methyl group are far
away from each other.

2) Gauche also called syn-clinical. Dihedral angle, 60° and 300°,
where methyl group are closer to each other than in anti-
confirmation.

2) Two eclipsed confirmation known as anticlinical with Dihedral
angle, 120° and 240°. synperiplanar Dihedral angle, 0°, where
methyl group are closest to each other and also known as fully
eclipsed.
Anticlinical (Skew) synperiplanar (Fully eclip

• Anti confirmation found to be most stable, compared to gauche
by 0.9kcal.mol, both are free from torsional strain.
• In gauche confirmation methyl group are crowded together ,
that closer than their sum of van der walls radii; under these
condition, Van der walls forces are repulsive and raise the
conformational energy. So because of this repulsion Van der
walls strain generated and molecules become less stable.
• This forces not affect only relative stabilities but also the
heights of energy barrier, energy maximum is reached when
two methyl group swing past each other rather than Hydrogen
around 5 kcal/mol, but as it is not so big so rotation can
happens at even room temperature also.

REACTIONS
• E2 is an anti-elimination. They are stereo specific. The
hydrogen and the halogen must be on opposite sides of the
molecule before the E2 elimination can take place. This makes
sense as both the base and the leaving group are negatively
charged. Therefore they would try to be as far apart as
possible. In addition, the leaving group is large and there is
more room for the removal of the adjacent proton if it is on the
opposite side from the leaving group.
• If the anti-arrangement is not possible, syn-arrangement may
takeplace.

CONFORMATIONAL ISOMERISM IN
CYCLOHEXANE
• All C in Cyclohexane is SP3 Hybridized, So they will attached to
each other with bond angle 109° not by 120° as in planner
structure. So they will appear as Two different confirmation in
3D- Chair and Boat. As Cyclohexane ring is free of Angle Strain
and Torsional Strain.

AXIAL AND EQUATORIAL BOND IN
CYCLOHEXANE

RELATIVE STABILITY OF CONFIRMERS OF
CYCLOHEXANE
Ring Flipping or Ring Inversion

1. CHAIR CONFIRMATION
• C-H bonds are perfectly staggered, So Bond opposition strain is
minimum.
• ‘H’ atoms on adjacent carbon atoms have enough space for
their accommodation, So Steric strain is minimum.

• As a result of simultaneous rotation about all C-C
bonds, a chair conformation of cyclohexane can
interconvert to another chair conformation by a ring-flip
or ring-inversion.
• In the process, equatorial bonds become axial and vice
versa

2. BOAT CONFIRMATION
• 1. Bond opposition strain: C-H bonds on the sides are eclipsed.
• 2. Fp – Fp interaction: Distance between two Fp Hs is 1.84Ao,
These two strains make boat conformation highly strained.
• It has 29.71kJ/mol more energy than chair conformation.
• Therefore boat conformation is less stable than chair
conformation.

3. TWIST OR SKEW BOAT CONFORMATION:
• Less torsional strain as compared to boat conformation.
• Flag pole Hs are away from each other.
• C2, C3, C5 and C6 become non-planer.
• Energy content : 6.696kJ less than boat but 23.02kJ more than
chair.
• Therefore more stable boat but less stable than chair.

4. HALF CHAIR CONFORMATION:
• Suffers from angle strain
• It has 46.04kJ more energy than chair conformation. Maximum
energy content than any other conformation. There it is least
stable.

• Isolation of any conformation of CH is not possible because :
• At RT the average energy content of CH is more than
sufficient to overcome this small barrier.
• There exists a dynamic equilibrium between different
conformations of CH.
• Chair <> Twist Boat <> Boat<> Half Chair
• Decreasing Order of Stability
Chair > Twist Boat > Boat > Half Chair

ATROPISOMERISM
• Biphenyls are compounds whereby a phenyl ring is connected to
another through a central σ bond. Kind of confirmation isomerism.
• Atropisomers are stereoisomers resulting from hindered rotation
about one or more single bonds, where the energy barrier to
rotation is high enough to allow for the isolation of the
conformers, Kind of enantiomer (from Greek, a=not and tropos=
turn).
• Atropisomers are detectable by NMR if half lives exceed 10-2 sec.
• Atropisomers are isolatable if the half-life is above 1000 sec.

• Polynuclear aromatic systems such as binol also exist as
enantiomers.
• If bulky group on ortho position of bi-phenyl or strained ring
structural features. Bulky substituents or strained rings may
enhance the barrier to rotation between two distinct
conformations to such an extent as to allow observation of
atropisomers.
• Atropisomerism is also called axial chirality and the chirality is
not simply a centre or a plane but an axis.

CRITERIA FOR ATROPISOMERSIM
• Neither ring must have plane of symmetry

2ND CASE OF PLANE OF SYMMETRY
• The substituent in ortho position should be large enough so it
can restrict rotation around pivotal bond

3RD CASE OF PLANE OF SYMMETRY
• In the third case neither ring is symmetric there is no plane of
symmetry, and many such compounds have been resolved. This
corresponds to AB.....AB.

CONDITION FOR ATROPOISOMERISM
• 1. A rotationally stable axis
• 2. Presence of different substituents on both sides of the axis
• 3. The configurational stability of axially chiral biaryl compounds is
mainly determined by three following factors:
• i. The combined steric demand of the substituent in the combined
steric demand of the substituents in the proximity of the axis.
• ii. The existence, length and rigidity of bridges.
• iii. Atropisomerisation mechanism different from a merely physical
rotation about the axis, e.g. photo chemically or chemically induced
processes.

NOMENCLATURE FOR ASSIGNING
ATROPISOMERS

NOMENCLATURE FOR ASSIGNING
ATROPISOMERS
• Determining the axial stereochemistry of biaryl atropisomers can be
accomplished through the use of a Newman projection along the axis
of hindered rotation.
• The ortho, and in some cases meta substituents are first assigned
priority based on Cahn–Ingold–Prelog priority rules.
• Starting with the substituent of highest priority in the closest ring
and moving along the shortest path to the substituent of highest
priority in the other ring, the absolute configuration is assigned P or
Δ for clockwise and M or Λ for counterclockwise.

STEREOSPECIFIC REACTION
• A reaction in which stereo chemically different molecules reacts
differently is called a stereospecific reaction. In this case the cis-
and trans- stereoisomers give different products.
• In Stereospecific reaction, stereoisomers can:
• Yields different stereoisomers as product.
• Reacts at different rate.
• Reacts with different paths to yield quite different kind of compounds as
products.
• Its focused on reactants and their stereochemistry, as each
stereoisomer behaves specifically.
• It means, Reaction starts with one specific stereoisomer can yield a
specific isomer only as product.
• Stereospecificity towards enantiomers is called enantiospecificity.
• Stereospecificity towards distereomers is called distereospecificity.

Stereospecific' relates to the mechanism of a reaction, the best-
known example being the SN2 reaction, which always proceeds
with inversion of stereochemistry at the reacting centre.

STEREOSELECTIVE REACTION
• A stereoselective process is one in which one stereoisomer
predominates over another when two or more may be formed
as per favorable reaction pathway.
• If more than one reaction occur between a set of reactants
under the same conditions giving products that are
stereoisomers and if one product forms in greater amounts
than the other, the overall reaction is said to be stereoselective.
• Steroselectivity solely concerns with the products, and their
stereochemistry.
• Stereoselectivity towards enantiomers is called enantioselective.
• Stereoselectivity towards distereomers is called

C
C
H
CH3
H
H3C
C
C
CH3
H
H
H3C
CH3
H Br
CH3
Br H
CH3
Br H
CH3
H Br
CH3
H Br
CH3
H Br
+
Br2
Br2

C C C
C anti-addition
C C C C syn-addition

Geometric isomerism

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