Organic Chemistry

Organic Chemistry
Course Number: PCH 1120-217
Lecture # 8
Tuesday September 24, 2013
Geometric Isomerism in Cyclic Compounds &
Alkenes, and Conformations of Open-Chain
Compounds
Prof. Oludotun A. Phillips
Room # 2-81, 2nd Floor Pharmacy Building
Email:
Tel: 24986070


Learning Objectives
At the end of the class students should be able to:
 describe geometric isomerism.
 identify and assign configuration of geometric isomers
in alkenes and cyclic compounds (cis / trans
designations).
 assign configuration of geometric isomers (E / Z
nomenclature) using the sequence rules.
 discuss conformation in open-chain compounds.


Geometric Isomerism in cyclic compounds and alkenes
 Geometric Isomerism: Results from rigidity, due to
restricted rotation in molecules.
 It occurs in two classes of compounds, namely, the
Alkenes and Cyclic Compounds.
 Generally, the atoms and groups in molecules that are
attached to single bonds (sigma bonds) can rotate

such that the molecular structure is in a state of
continuous change.
 However, atoms or groups attached to a double bond
cannot rotate around the C=C double bond without
the pi bond being broken.


Geometric Isomerism in cyclic compounds and alkenes

H

H

H

C

C

Cl

Cl

H

H

Cl

H

C

OR

C

sigma (single bond) bond
free rotation

double bond
restricted rotation

pi orbitals
H
C

Cl

H

Cl

H
is different
from

C
Cl

(A)

Chloro atoms
groups are
on same side of db

Cl
C

C

Cl

H
(B)
groups are
Chloro atoms
on opposite sides of db

The double bonds are rigid hence the chloro groups attached to the
sp2-hybridized carbons are fixed in relation to one another.
Therefore, molecule (A) is not readily inter-convertible with molecule (B).


Geometric Isomerism in cyclic compounds and alkenes
 The molecule is designated as cis (Latin, “on the side”)
when the two groups are on the same side of the double
bond (pi-bond).
 The molecule is designated as trans (Latin, “across”)
when the two groups are on opposite sides of the double
bond (pi-bond).


The cis or trans designation is therefore incorporated into the
Nomenclature as italized Prefix:
H
C
Cl

H

H

(A)

C

is different from

C
Cl

cis-1,2-Dichloroethene
bp 60 oC

Cl

Cl

C

(B)

H

trans-1,2-Dichloroethene
bp 48 oC


Geometric Isomerism in cyclic compounds and alkenes
 cis or trans stereoisomers are two different compounds,
having different physical properties, such as b.p, m.p etc
 They are classified as “Stereoisomers” because they
differ in the arrangement of the atoms in space, and
belong to the specific category of “Geometric Isomers”
(designated: cis-trans isomers).
 Geometric Isomers: are Stereoisomers that differ by
groups being on the same side or opposite sides in a
rigid molecule.


Geometric Isomerism cis or trans Designation
 A requirement for Geometric Isomerism in Alkenes is
that each Carbon Atom involved in the pi bond (double
bond) have Two different groups attached to it, such
as H and Cl or CH3 and Cl.
H3C

H3C

CH2CH3
C

C

H

is Diffrent from
H

cis-2-pentene

Cl
H

C

C
H

cis-1-chloropropene

CH2CH3

H

CH3
C

is Diffrent from
H

C

trans-2-pentene

CH3
C

H

Cl

C
H

trans-1-chloropropene


Geometric Isomerism cis or trans Designation
 However, if one of the Carbon atoms of the double
bond has two identical groups, such as two H atoms, or
two CH3 groups, geometric isomerism is not possible.
e.g. the following molecules are not Geometric Isomers:
H
C

C

H

is Same as

Cl

Cl
C

is Same as
CH2CH3

Cl
C

C

H

CH2CH3

H

CH2CH3
C

H

H

CH2CH3

CH2CH3
C

Cl

C
CH2CH3


Geometric Isomerism E / Z System of Nomenclature
 The cis/trans designation becomes somewhat difficult to
assign when there are Three or Four Different Groups
attached to the Carbon Atoms of a Double Bond: e.g.
Br

F
C

I

C
Cl

 In the structure above, we can say that Br and Cl are
trans to each other or that I and Cl are cis to each other.
 It is difficult to designate the structure as either cis
or trans isomer.
 To resolve this ambiguity, the E / Z System of
Nomenclature is used.


Geometric Isomerism E / Z System of Nomenclature

 The E / Z System of assigning the configuration of an
isomer is based on Assignment of priorities to the
atoms or groups attached to each Carbon of the
Double Bond.
 The Isomer is (E) if the Higher-priority Atoms or
Groups are on Opposite Side of the Double Bond.
 While the Isomer is (Z) if the Higher-priority Atoms or
Groups are on the Same Side of the Double Bond.
 The Letter (E) is from the German word entgegen,
(“across”); and (Z) is from the German zusammen,
(“together”).


Geometric Isomerism E / Z System of Nomenclature



Priority is based on the Atomic Numbers of the Atoms
directly attached to the Double Bond carbons.
Atoms with the Higher Atomic Number receives the
Higher Priority:
Br
F
I higher priority
than Br
Atom:
Atomic Number:

Br

F
C

I

F
9

C
I

Cl higher priority
than F

C
Cl

Cl
Br
17
35
Increasing Priority

I
53

Br

C

Cl
C

Cl

(Z)-1-Bromo-2-chloro2-fluoro-1-iodoethene

I

C
F

(E)-1-Bromo-2-chloro2-fluoro-1-iodoethene


Sequence Rules in Geometric Isomerism E / Z System

 Note! that the Determination of Priorities by Atomic
Number alone cannot resolve / handle all the cases
encountered in Organic Chemistry.
H3C

 For e.g. for naming:

CH3
C

CH3CH2

C

H

 Priority Order can be determined using the “Sequence
Rules”
according
to
the
Cahn-Ingold-Prelog
Nomenclature System (discussed under the R/S
System of Nomenclature)


Sequence Rules in Geometric Isomerism E / Z System
“Sequence Rules” for Order of group priority:
1. for Atoms and group of atoms:
Atom:
Atomic Number:

F
9

Cl
17

Br
35

I
53

Increasing Priority
Atom No:
Atoms:

(1)

(6)

(7)

(8)

(16)

(17)

(35)

(53)

-H

-CH3

-N H2

- OH

- SH

- Cl

- Br

-I

Increasing priority

2.

for Isotopes:

Isotope:

1

1H or H
hydrogen

2

1H

or D
deuterium


Sequence Rules in Geometric Isomerism E / Z System
“Sequence Rules” for Order of group priority:
3. if Atoms are identical, the Atomic Number of the Next Atoms

are used for Priority Assignment:
Three H's:
lower priority
H
H
H3C

CH3
C

H

C

CH3CH2

H
H

(E)-3-Methyl-2-petene
-pentene

H3CH2C

CH2CH2CH2Cl
C

CH3

C

CH3
C

H
C

H

H

C

C
H

H
Two H's and One C: gives
this group the higher priority

Two H's and One C: gives
this group the higher priority

C
CH2CH2CH3

(Z)-isomer

(Z)-7-Chloro-4-propyl-3-methylhept-3-ene

Sequence Rules in Geometric Isomerism E / Z System
“Sequence Rules” for Order of group priority:
4. Atoms attached by double bond or triple bond are given
single bond equivalencies:
O

O

-CH=CR2

<
<

<
<

-CN

<<

-CH2OH

<<

C H

<
<

O

<

C

O

C OH

<
<

C OR

increasing priority

Structure

Equivalent for
priority determination

C

R

Is
same
as

R

C

R

Equivalent for
priority determination
O

O

O

O
R

Structure

R

C

OH

Is
same
as

R

R

C

N

O
OR

Is
same
as

R

C
O

OH

O

O
O

C

OR

R

C

Is
N same
as

R

C
N

N


Geometric Isomerism in cyclic compounds

 Cis-trans isomers occur in cyclic compounds having:
 the same molecular formula
 the same connectivity of their atoms
 an arrangement of atoms in space that cannot be
inter-converted by rotation about sigma bonds


Geometric Isomerism in Unsaturated cyclic compounds
 configuration of the double bond in cyclopropene (3member ring) through cycloheptene (7-member ring)
must be “cis”; these rings are not large enough to
accommodate a “trans” double bond - due to ring strain
CH3

CH3

CH3

CH3

Cyclopentene



Cyclohexene derivative

Cyclooctene (8-member ring) is the smallest cycloalkene
that can accommodate a trans double bond:

t ra ns - Cy clo octe n e

ci s -Cycl oo cte n e


Geometric Isomerism in Saturated cyclic compounds


In Cyclopentanes (5-membered ring hydrocarbon)



Dashed line represents groups or atoms Below the Plane of
the Ring in the molecule

Solid line represents groups or atoms Above the Plane of the
Ring in the molecule




Geometric Isomerism in Saturated cyclic compounds



In Cyclohexanes (6-membered ring hydrocarbon):
Cyclohexanes may be viewed as planar hexagons
H
H3 C

CH3
H

CH3
H3 C

t ra ns-1,4-D i me th yl c ycl o he xan e

H
H3 C

H
CH3

H3 C

ci s-1,4-D i m e thy l cy cl o he xane

CH3


Conformation of open-chain compounds.


In Open-Chain Compounds, atoms or groups attached
by Sigma Bonds can Rotate around these bonds.



Hence the atoms or group can assume an Infinite
Number of Positions in Space Relative to one another.



Conformation: any three-dimensional arrangement of atoms
in a molecule that results from rotation about a single bond.



Because of Rotation around the Sigma Bonds, a
Molecule can assume any number of Conformations.


Conformation of open-chain compounds.


Conformations can be represented in following Types of
Formulas e.g. for Ethane:

(dash & wedge)

 Formulas
(i)
and
(v)
are
representations of the molecule.

3-Dimensional

 Newman projection (iii) is an end-on view of only Two
Carbon Atoms in the molecule. The bond joining the two
carbon is not visible.


Conformation of open-chain compounds.


Conformers: are different structures formed by Bond Rotations.

1.

Staggered conformation: a conformation about a carbon-carbon
single bond where the atoms on one carbon are as far apart as
possible from the atoms on an adjacent carbon: (is the most
stable and lowest energy conformation)

Conformation of open-chain compounds.
2.

Eclipsed conformation: a conformation about a carbon-carbon
single bond in which the atoms on one carbon are as close as
possible to the atoms on an adjacent carbon - least stable conformer
(highest energy)

.NOTE!! the lowest energy conformation of an alkane is a fully
staggered conformation:


Conformation of open-chain compounds.


Rotation of the groups around the carbon-2 and carbon-3 of
Butane – CH3CH2CH2CH3 will give the following conformers:
CH3

H

H

H

H

H

CH3

H

H
H3C

CH3

Staggered conformation
"anti" conformer
lowest energy - most stable
CH3

H

Eclipsed

H3C CH3

H

H3C

H

H
H

Gauched conformation

H
H

H

H

Eclipsed methyls
highest energy - least stable


Conformer Potentials of Butane about the Central C-C Bond