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5.1

Alkene Nomenclature

characterized by molecular formula CnH

2n have a C=C double bond

said to be "unsaturated or olefins

Chapter 5

Structure and Preparation of Alkenes:

Elimination Reactions

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Chapter 5; Alkanes and Elimination Reactions

1. Nomenclature2. Geometric Isomers

i. Cis/trans and E/Z

ii. Priority Rules

3. Physical Properties4. Alkene Stability

5. Elimination Reactions

i. E1 and E2 Mechanisms

ii. :H and :R shifts6. Dehydrohalogenation Reactions

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1) Find the longest continuous chain thatincludes the double bond.

2) Replace the -aneending of the unbranched

alkane having the same number of carbons

by -ene.

3) Number the chain in the direction that gives

the lowest number to the doubly bonded

carbon.

H2C CHCH2CH3 1-Butene

Alkene Nomenclature

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4) If a substituent is present, identify its position

by number. The double bond takes

precedence over alkyl groups and halogens

when the chain is numbered.

The compound shown above is

4-bromo-3-methyl-1-butene.

H2C CHCHCH2Br

CH3

Alkene Nomenclature

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4) If a substituent is present, identify its position

by number. Hydroxyl groups take

precedence over the double bond when the

chain is numbered.

The compound shown above is

2-methyl-3-buten-1-ol.

H2C CHCHCH2OH

CH3

Alkene Nomenclature

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Alkene Nomenclature

5. If more than one double bond is present;

name as adiene ortriene. Use numbers

to indicate multiple bond positions.

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Cycloalkene Nomenclature

1) Replace the -aneending of the cycloalkane

having the same number of carbons by -ene.

Cyclohexene

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1) Replace the -aneending of the cycloalkane

having the same number of carbons by -ene.2) Number throughthe double bond in the

direction that gives the lower number to the

first-appearing substituent.

6-Ethyl-1-methylcyclohexeneCH3

CH2CH3

Cycloalkene Nomenclature

1

23

4

5 6

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Planar

Restricted C-C Bond Rotation

Structure of Ethylene

C C

H

H

H

H

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Isomers

Constitutional isomers Stereoisomers

different atom connectivity same atom connnectivity;

different arrangement

of atoms in space

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Requirements for Geometric Isomers

(Type of Stereoisomer)

1. Restricted Bond Rotation

Cycloalkane

Multiple Bond

2. Two Different Carbon Atoms that Have Two

Different Substituents Attached to Them

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2-Methylpropene1-Butene

cis-2-Butene

C C

H

H H

CH2CH3

H

CH3

C C

H3C

H

C C

H

HH3C

H3C

Constitutional Isomers

of C4H8

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H3C

C C

CH3

H

HH

CH3

C C

H3C

H

Stereoisomers of C4H8

Also known as Geometric isomers, cis/trans isomers, or

E/Z isomers

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C C

E: higher ranked substituents on opposite sides

Z: higher ranked substituents on same side

Entgegen

higher

higherlower

lower

C C

Zusammen

lower

higher

lower

higher

The E-Z Notational System

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(1) Higher atomic number outranks loweratomic number

Br > F Cl > H

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

higher

lower

Br

F

Cl

H

higher

lower

C C

Table 5.1 CIP Rules

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(2) When two atoms are identical, compare theatoms attached to them on the basis of their

atomic numbers. Precedence is established

at the first point of difference.

CH2CH3outranks CH3

C(C,H,H)

Table 5.1 CIP Rules

C(H,H,H)

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(3) Work outward from the point of attachment,comparing all the atoms attached to a

particular atom before proceeding further

along the chain.

CH(CH3)2outranks CH2CH2OH

C(C,C,H) C(C,H,H)

Table 5.1 CIP Rules

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(4) Evaluate substituents one by one.Don't add atomic numbers within groups.

CH2OH outranks C(CH3)3

C(O,H,H) C(C,C,C)

Table 5.1 CIP Rules

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(5) An atom that is multiply bonded to another

atom is considered to be replicated as a

substituent on that atom.

CH=O outranks CH2OH

C(O,O,H) C(O,H,H)

Table 5.1 CIP Rules

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Alkene Polarity and Boiling Point

CC

ClH

Cl H

CC

ClCl

H H

Dipoles cancel;

Trans isomer nonpolar

Lower boiling pt

Dipoles reinforce;

Cis isomer polar

Higher boiling pt

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Double bonds are classified according to

the number of carbons attached to them.

H

C C

R

H

H

monosubstituted

R'

C C

R

H

H

disubstituted

H

C C

R

H

R'

disubstituted

H

C C

R H

R'

disubstituted

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Double bonds are classified according to

the number of carbons attached to them.

R'

C C

R

H

R"

trisubstituted

R'

C C

R

R"'

R"

tetrasubstituted

In general; the more substituted the alkene is

the more stable it is.

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Electronic (larger effect)

disubstituted alkenes are more stable

than monosubstituted alkenes

Steric (smaller effect)

For disubstituted alkenes;transalkenes are more stable than cisalkenes

Substituent Effects on AlkeneStability

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+ 6O2

4CO2+ 8H2O

2707 kJ/mol

2717 kJ/mol

2710 kJ/mol

Figure 5.4 Heats of

combustion of C4H8

isomers.

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cis-2-butene trans-2-butene

van der Waals strain

due to crowding ofcis-methyl groups

Figure 5.5

cis and trans-2-Butene

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Give the structure or make a molecular model of

the most stable C6H12alkene.

C C

H3C

H3C CH3

CH3

Problem 5.8