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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: dphillips@hsc.edu.kw

Tel: 24986070

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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.

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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.

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Geometric Isomerism in cyclic compounds and alkenes

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).

Chloro atoms are Chloro atoms are on same side of db on opposite sides of db

CC

H

Cl

H

Cl

CC

H

Cl

H

Cl

pi orbitals

OR CC

Cl

H

H

Cl

CC

H

Cl

H

Cl

HH

sigma (single bond) bond free rotation

double bond restricted rotation

is different from

(A) (B)

groups groups

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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:

CC

H

Cl

H

Cl

CC

Cl

H

H

Cl

is different from

(A) (B)

cis-1,2-Dichloroethene trans-1,2-Dichloroethene bp 60 oC bp 48 oC

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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.

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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.

cis-2-pentene trans-2-pentene

CC

CH2CH3

H

H3C

H

CC

H

CH2CH3

H3C

His Diffrent from

CC

CH3

H

Cl

H

CC

CH3

H

H

Cl

is Diffrent from

cis-1-chloropropene trans-1-chloropropene

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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:

CC

CH2CH3

Cl

H

H

CC

Cl

CH2CH3

H

His Same as

CC

CH2CH3

CH2CH3

Cl

H

CC

CH2CH3

CH2CH3

H

Clis Same as

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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.

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.

CCF

Cl

Br

I

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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”).

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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:

CC

F

Cl

Br

I

Atom: F Cl Br IAtomic Number: 9 17 35 53

Increasing Priority

CC

F

Cl

Br

I

CC

Cl

F

Br

I

I higher prioritythan Br

Cl higher prioritythan F

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

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

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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.

For e.g. for naming:

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)

C C

CH3

H

H3C

CH3CH2

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Sequence Rules in Geometric Isomerism E / Z System

“Sequence Rules” for Order of group priority:

1. for Atoms and group of atoms:

2. for Isotopes:

Atom: F Cl Br IAtomic Number: 9 17 35 53

Increasing Priority

(53)(35)(17)(16)(8)(7)(6)(1)

Increasing priority

-H -CH3 -NH2 -OH -SH -Cl -Br -IAtom No:Atoms:

Isotope: 11H or H 21H or D hydrogen deuterium

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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:

CC

CH3

H

C

C

Three H's: lower priority

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

C

H

H

H

H

H

HH

H

C C

CH3

H

H3C

CH3CH2

(E)-3-Methyl-2-petene

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

C C

CH2CH2CH2Cl

CH2CH2CH3

H3CH2C

CH3

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

-pentene

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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:

< <<<<<<

R C

O

R R C

O

R

O

R C

O

OH R C

O

OH

O

R C

O

OR R C

O

OR

O

R C N R C

N

N

N

Is same as

Is same as

Is same as

Is same as

-CH=CR2 -CN -CH2OH C

O

OHC

O

C

O

H C

O

OR< < << < <

increasing priority

Structure Equivalent for priority determination

Structure Equivalent for priority determination

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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

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Geometric Isomerism in Unsaturated cyclic compounds configuration of the double bond in cyclopropene (3-

member 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

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

Cyclopentene Cyclohexene derivative

CH3

CH3

CH3

CH3

trans-Cyclooctene cis-Cyclooctene

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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

CH3

H

CH3

H

H

HH

H

HH

H

H

CH3

H3C

H

HH

HH

H

CH3

trans-1,2-Dimethyl-cyclopentane

cis-1,2-Dimethyl-cyclopentane

H3CH3C CH3

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Geometric Isomerism in Saturated cyclic compounds

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

trans-1,4-Dimethylcyclohexane

cis-1,4-Dimethylcyclohexane

H

H3C

CH3

H

H

H3C

H

CH3

CH3

H3C

H3C

CH3

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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.

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Conformation of open-chain compounds.

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

Formulas (i) and (v) are 3-Dimensional representations of the molecule.

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.

(i) (ii) (iii) (iv) (v) Three-dimensional Ball-and-stick Newman projection Sawhorse projection Fischer projectionformula formula formula formula formula

C

H

H

H C

H

H

HH

H

H

H

H

HH

H HH

HHH

H H

H

H HH

H HH

HH

(dash & wedge)

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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)

H

H H

H H

H

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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:

H

H H

H

HH

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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

CH3

HCH3

H H

H

H3C

H

CH3

H H

H

H

H3C

CH3

H H

H3C

H

H

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

Gauched conformation Eclipsed methyls highest energy - least stable

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Conformer Potentials of Butane about the Central C-C Bond

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Questions - Sequence Rules in Geometric Isomerism

1. List the following atoms or groups in Order of Increasing Priority (Lowest priority first): -NH2; -H;

-CH3; -Cl;

2. List the following groups in Order of Increasing Priority (Lowest to highest): -CO2H; -CH2OH; -CO2CH3; -OH; -H;

3. Determine whether the following compounds is (E) and (Z) isomer:

CC

CH2CH2CH3

Cl

H

H3C

CC

CH3

Cl

H3C

Cl

(i) (ii)

CC

CH2CH3

D

D

H3C

CC

C

Cl

C

ClH2C

(iii) (iv)

O O

CH2ClH3C

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Question: Geometric isomerism

Name the following compounds by the (E) and (Z) System of Nomenclature:

CC

CH2CH2CH3

Cl

H

H3CCC

CH3

Cl

H3C

Cl(i) (ii)

CC

H

SCH3

Br

Cl

(iii)

C

CH CH2N

CH3

CH3

N N

C

CH CH2N

CH3

CH3(iv) (v) Zimeldine (antidepressant)