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CHEM0901

ORGANIC CHEMISTRY

lecture 7

email: nadale.downer@uwimona.edu.jm

N. K. Downer-Riley

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Double Bond Equivalents (DBE)

2

The double bond equivalent is a value that indicates thedegree of unsaturation of an organic compound.

1 DBE = 1 degree of unsaturation = 1double bond

or 1 ring

In other words, it represents the number of double bonds

or rings which are present in the organic compound.

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CH2

CH2

CH2

CH2

CH2

CH3

C

C

C

CH3

H

H

H

H

CHCH2

CH

CH2

CH2

CH2

C

C

C

CH3

H

H

H

C5H101 DBE

C5H8

2 DBE

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Calculating the DBE

DBE = 1 + n4 + ( n3 n1) = 1 + #C + ( #N n1)2 2

n1 = monovalent element e.g. hydrogen, halogens

n3 = trivalent element e.g. nitrogen

n4 = tetravalent element e.g. carbon

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Calculate DBEs

C6H12O6

C6H3Cl3O

C46

H58

N4O

9

DBE = 1 + #C + ( #N n1)

2

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General features of Alkenes

General formula CnH2n

Contain C=C bonds

Unsaturated hydrocarbons

Orbitals are sp2 hybridised

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Bonding in Alkenes

This hybridization results in

3 sp2

hybrid orbitals which lie in one plane with an angle of 120

between each other (trigonalplanargeometry). one unhybridized p orbital which is positioned at a right angle

(90) to the plane of the hybrid orbitals.

Valence

shell

electrons

orbitalscombine

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sp2 Hybridisation

In ethene:

Bonds in ethene:4 C-H -bonds, 1 C-C -bond, 1 C-C -bond

Double bond = bond + -bond

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Geometric Isomerism

C atoms joined by single bonds rotate freely. However Catoms joined by a double bond cannot rotate.

The relative positions of the groups attached to thesecarbons are therefore fixed.

This leads to geometric (cis/trans) isomerism.

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Geometric isomerism

Geometric isomers are compounds with the same structuralformula but different spatial arrangement of atoms around adouble bond.

cis-trans isomerism is observed only when the two carbon

atoms of the double bond have two identical groupsattached to them. Cis like groups lie on the same side

Trans like groups lie on opposite sides

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Geometric Isomerism in Cycloalkanes

Recall cycloalkanes and alkenes share the same generalformula CnH2n

Rotation about C-C bonds exists in open chains but NOT inrings.

Lack of rotation results in the formation of geometricisomers

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Cis/trans isomerism cannot be displayed by an alkenewhich has 2 identical groups bonded to one carbon of

the double bond.

C C

H

H

CH3

Br

CH3

C

CH3

C

H

Cl

X

C

Y

C

R1

R2

When X = Y, no cis/trans isomerism is possible

e g .

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The isomers shown here can neither be described ascisnor trans

When 3 or 4 unlike groups are attached around theC=C bond, the E-Zsystem is used.

Using this system, groups attached to the C atoms ofthe double bond are assigned a priority according toa set of rules.

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Using the E/Z system

Consider each carbon of the C=C individually Determine which of the two substituent has the highest

priority The higher atomic mass of the atom attached, the

higher the priority Hydrogen always has the least priority

If the two atoms attached are identical, the atomicnumbers of the next atoms are used to assign thepriority. Priority is determined at the first point of difference alongthe chain if the atoms have the same atoms directly attached.Therefore the larger the alkyl group, the higher the priority

If the higherpriority atoms or groups are

-on opposite sides of the pi bond, E isomer [E= across].-on the same side, Z isomer [Z= together].

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C C

Cl

H

CH3

Br

CH3

C

CH2

C

H

ClCH3

a b

1

1

1

1

Carbon a: chloro group is ofhigher priority than hydrogenCarbon b: bromo group is of

higher priority than methylhigher priority groupsopposite each other thereforeE isomer.

Carbon a: ethyl group is ofhigher priority than methylCarbon b: chloro group is ofhigher priority than methylhigher priority groups onsame side therefore Z isomer.

a b

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Nomenclature of alkenes

Identify longest chain thatcontains the double bond. Basename is derived from numberof carbons and suffix -ene

Number this chain so that C

atoms of the double get thelowest possible numbers

Indicate the position of thedouble bond using the # of the

1st

atom of the double bond asthe prefix.

Base name = pentene

CH

C

CH2

CH

CH3

ClCH3

CH3

1 2

34

5

2-pentene

Indicate the location of thesubstituent groups by the # of the Catom.

4-chloro-3-ethyl-2-pentene

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Nomenclature contd

12

3

4

5

If there are more than onedouble bonds present, thealkene is named as diene,

triene etc. with thenumbers indicating theposition of the doublebonds.

1,3-pentadiene

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Nomenclature contd

Br

F H

Cl

Substituents are denotedalphabetically.

For cyclic alkenes, number the

Cs of the double bond 1 and 2,while giving the substituents thelowest possible number at thefirst point of difference.

In the case of geometricisomerism, E-Zor cis-trans, theword or letter precedes the nameof the compound.

Cl

1

2

(Z)-1-bromo-2-chloro-1-fluoroethene

3-chloro-1-methylcyclohexene

H2C CH CH CH CH

1 2

3 4 5

Br

CH3

CH2

Cl

CH36 7

4-bromo-1-chloro-5-methyl-2-heptene

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