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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Chapter 4 Alkenes and Alkynes:Structure and Preparation

Examples:

Isobutylene, -Pinene, Farnesene, Cicutoxin

CH3

CH3

CH2H

CH3 CH3

CH3

CH3CH2

CH3CH3

CH3

CH3

OH

OH

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Nomenclature:Replace the aneending of the parent hydrocarbon with ene. The longest

continuous chain which includes the double bond forms the base name., and

the chain is numbered in such a way, so that the alkene carbons obtain the

lowest possible numbers.

CH2

CH3

CH3

CH3

CH3

CH3

CH3

but-1-ene(2E)-pent-2-ene

(2E)-5-methylhex-2-ene

C-C double bonds outrank alkyl groups and halogens in determining the

main carbon chain. Hydroxyl groups, however, outrank the double bond.

CH3

Br

CH2

1

2

34

5

6

OH

CH2

1

2

34

56

6-bromo-3-ethylhex-1-ene

4-ethylhex-5-en-1-ol

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Cycloalkenes and their derivatives are named by adapting cycloalkane

terminology.

Cl CH3

CH3 CH3

1,3-dimethylcycloheptene

3-chloro-1-methylcyclohexene

cyclopentene

OH

cyclohex-2-en-1-ol

Structure and Bonding in Alkenes

C C

H

H H

H

1.33 A

1.08 A

o

o o

o

116.6

121.7

C C

H

HHH

HH

1.54 A

1.09 A

o

o

Alkene Alkane

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Hybridization at C: sp2

CC -bondCC -bond

Isolated double bonds: conjugated double bonds

CH2CH2

CH3

CH2

Delocalized systems are more stable !!!

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

ISOMERISM IN ALKENES.

There are four possible Isomers of butene:

CH3

CH2

CH3 CH2

CH3

CH3

CH3

CH3CH3

but-1-ene

2-methylprop-1-ene

(2E)-but-2-ene(2Z)-but-2-ene

trans cis

Naming by the E-Znotational system

Prioritize substituents:

1. Higher atomic number, higher priority (e.g., NH2shows higher priority than CH3).

2. If the group has the same atomic number, go to thenext atom (e.g, CH2CH3has higher priority than CH3).

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

When the two groups of higher priority, each ondifferent carbon atoms of the double bond, are on thesame side of the molecule, the name of the molecule

is prefixed with (Z) (from the German zusammen,together).

When the groups of higher priority are on oppositesides of the molecule, the designation (E) (from theGerman entgegen, across) is used.

Examples:

Br CH3

Cl H

high

low

high

low

Br

Cl

OH

CH3

CH3

high

high

low

low

CH3

H

CH3

CH3

O

OH

low

lowhigh

high

(1Z)-1-bromo-1-chloroprop-1-ene

(3E)-4-bromo-4-chloro-3-isopropylbut-3-en-1-ol

(3E)-3-(1-hydroxyethyl)pent-3-en-2-one

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Stabilities:

We distinguish mono- di- tri- tetrasubstituted alkenes. Generally, the higher

the substitution order the more stable the alkene.

CH2

CH3

CH3

CH3

CH2

CH3

CH3

CH3CH3

CH3

CH3CH3

CH3CH3

Preparation of Alkenes

Y

H

H

H

H

X

H

H

H

H

+ XY

Alkene formation requires X and Y-substituents to be on adjacent carbons.

Principal methods would be deydration of alcohols (X=H, Y=OH),

dehydrohalogenation of alkylhalides (X=H, Y= Cl, Br, I)

Dehydration of alcohols

An acid catalyst is needed for the reaction to proceed.

H

H

H

H

H

OH

HH

H H

H+

alcohol alkene

additional product would be water (dehydration)

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

other examples:

OH

CH3

CH3

CH3

OH

CH3

CH3

CH2

CH3

CH3

OH

CH3

CH3

CH3CH3

+

CH3

CH2CH3

90% 10%

From the last example we see, that there is a preference to from the highest

substituted alkene. We call this behaviorregioselective. The first to look into

this regioselectivity was Zaitsev, therefore the rule to determine the mainproduct is called Zaitsev rule.

OH

CH3 CH3 CH3 CH3 +

CH3

CH3

75%

25%

In case we have a chance to produce cis and trans products, than there is a

tendency to form the more stable trans product. This behavior is called

stereoselective.

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Mechanism:CH3

CH3 CH3

OHCH3

CH3

CH2

CH3

CH3 CH3

O

H

H

O+

HH

H

OH

H

CH3

CH3 CH3

O+

H

H

CH3

CH3 CH3

O+

H

CH3

C+

CH3 CH3H

O

H

CH3

C+

CH3

H

++

+

+H

O

H

CH3

CH3 CH2

+ H

O+

HH

H2

SO4

For primary carbons, there is no carbocation intermediate. Two steps, bond

cleavage and bond formation, occur simultaneously

H

OH

H

H

O+

H

CH2 CH2+ +H

O+

H

HH

O H

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Dehydrohalogenation of Alkyl Halides

Means elimination of a hydrogen and a halogen, which are bound to two

different carbon atoms.

The regioselectivity follows Zaitsev rules and overall the reaction is

regioselective, meaning the most stable alkene is formed in the elimination

process.

E2- Reaction Mechanism

In the E2 mechanism the two leaving groups have to be in an anti

configuration, which leads to a stereospecific reaction.

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Mechanism for E2

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

:

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Alkynes

Hydrocarbons with a triple bond are named alkynes general formula CnH2n-2.

Follow the hydrocarbon nomenclature end substitute anewith yne.

The carbons in a triple bond are sp-hybridized.

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Chapter 4 Alkenes and Alkynes: Structure and Preparation

Preparation:

a) double dehydrohalogenation of geminal alkyl dihalides

CH3

Cl

Cl

CH3

CH

3 NaNH2/ NH

3

b) double dehydrohalogenation of vicinal alkyl dihalides

CH3

Br

Br

CH3

CH

3 NaNH2/ NH

3

Important Diynes : tumor inhibitors:

Dynemicin A:

OH

OHNHO

O OH

O

CH3 O

OH

O

CH3