Geometric isomerism

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

2. 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

3. 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.

4. 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
alkenes.

5. 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:

6. 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.

7. 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-Prelog (CIP) rules.

8. 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.

9. • 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.

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

11. 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.

12. 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.

14. 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).

15. 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

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. REPRESENTATION OF CONFORMATIONAL ISOMERS
WEDGE AND DASH SAWHORSE NEWMAN

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

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

25. 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 also called eclipsed interaction strain.

26. 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.

27. CONFORMATIONAL ISOMERISM IN N-BUTANE

28. 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.

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

30. 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 eclipsed)

31. • 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.

32. 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.

34. 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.

36. AXIAL AND EQUATORIAL BOND IN CYCLOHEXANE

37. HOW TO DRAW CYCLOHEXANE

39. RELATIVE STABILITY OF CONFIRMERS OF CYCLOHEXANE
Ring Flipping or Ring Inversion

40. 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.

41. Axial
hydrogens

42. • 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

43. 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.

44. Steric
interactions

45. 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.

46. 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.

47. • 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

48. 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.

49. • 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.

50. CRITERIA FOR ATROPISOMERSIM
• Neither ring must have plane of symmetry

51. 2ND CASE OF PLANE OF SYMMETRY
• The substituent in ortho position should be large enough so it can restrict
rotation around pivotal bond

52. 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.

53. 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.

54. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

55. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

56. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

57. 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.

58. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

61. 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.

62. 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.

63. 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 distereoslective.

67. 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

68. C C C
C anti-addition
C C C C syn-addition