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CHAPTER 04 ALKENES AND ALKYNES

CHEM 240: Fall 2019

Prof. Greg Cook

cook.chem.ndsu.nodak.edu/chem240

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Alkenes

• Compounds that contain double bonds. These are unsaturated hydrocarbons.

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C CH

H

H

HEthylene (Ethene)

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Structure and Bonding

• A double bond is sp2 hybridized (pi bond)

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Structure of Alkynes

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C CH H

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Alkene Stereoisomers

• Double bonds have restricted rotation

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pi-bond broken

rotateC C C C

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Alkene Stereoisomers

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C CH

H

CH2CH3

HC C

H

H

CH3

CH3

C CH

H3C

CH3

HC C

H

H3C

H

CH3

but-1-ene 2-methyl-propene

trans-but-2-ene cis-but-2-ene

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Substituent Effects

• steric strain is also important - trans alkenes more stable than cis alkenes

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C C

C

H

C

H

H

HH

H

H H

C C

C

H

HH

HH

CH

H

H

trans isomer is less crowded

H+ cat.heat

24% 76%

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Alkene Isomerization

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β-carotenefrom carrots

metabolized

OHvitamin A

metabolized

trans-retinal

O

cis-retinal O

hν

When trans-retinal absorbs a photon of light, it isomerizes one of the double bonds to the cis isomer. This changes the shape of the molecule, and that changes the shape of the rhodopsin protein that it is buried in. This sends a signal to the brain that the eye has been struck by a photon. This is critical for vision.

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Alkene Examples

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hex-1-ene(1-hexene)

2-methylbut-2-ene(2-methyl-2-butene)

3-methylhexa-1,5-diene(3-methyl-1,5-hexadiene)

Cl

4-chlorocyclohexene

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Common Names

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Alkene and Alkyne Nomenclature

• Step 1 - find the longest chain containing the double or triple bonds

• Step 2 - number the chain from the end nearest the multiple bond. Multiple bonds have priority over alkyl and halide substituents but not alcohols

• Step 3 - name the molecule using -ene as the suffix to replace the -ane for alkenes and -yne for alkynes

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Naming Alkynes

• Alkynes are named similar to alkenes

• They have equal priority

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oct-1-ene-6-yne

propyne but-1-yne but-2-yne

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Naming Examples

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Alkene Stereoisomers

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C CH

H

CH2CH3

HC C

H

H

CH3

CH3

C CH

H3C

CH3

HC C

H

H3C

H

CH3

but-1-ene 2-methyl-propene

trans-but-2-ene cis-but-2-ene

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E-Z Naming System

• Cahn-Ingold-Prelog rules for assigning priorities

• Look at the atom directly connected to the carbon of the double bond. Rank the atoms according to their atomic number. The higher atomic number gets priority over the lower.

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C CCl

H

Br

FC C

Cl

H

F

Br

Z (zusammen, together) E (entgegen, opposite)

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E-Z Naming System

• If the distinction cannot be made at the first atom, look at the next level of atoms attached to those groups. Only look out as far as you need to make a distinction and NO FURTHER.

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C CCl

H

CH3

CH2CH3

An E alkene

C CCl

H

CH2OH

CH2CH2CH2CH3

A Z alkene

C CCl

H

CH2CH2OH

CH(CH3)2

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E-Z Naming System

• Multiple bonds are equivalent to the same number of single bonds IN BOTH DIRECTIONS.

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C CCl

H

CH

CH(CH3)2

A Z alkene

CC

HC

CC

HC

CH CH2

CH2 CHH

H

CHH H

H

H

H H

CH2

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E-Z Naming System

• Number alkene as carbon-1 in the direction that provides the lowest number for the substituents.

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Naming Examples

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Physical Properties

• Alkenes and alkynes are generally non-polar hydrocarbons. They do not dissolve in water.

• Alkenes and Alkynes have more reactivity than alkanes as we will see in the next chapter.

• The pi-bonds of alkenes and alkynes are sites of higher electron density and thus are the locations of reactivity.

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Acidity of Terminal Alkynes

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C CH

HH

H

HH C C

H

H

H

HC CH H

pKa 62 45 26

base base base

C CH

HH

HH C C

H

H HC CH

carbanion carbanion carbanion

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Deprotonation of Alkynes

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C CH HpKa 26

C CH+ H O H + H O H

H

pKa -1.7

C CH HpKa 26

C CH+ O H + H O HpKa 15.7

C CH HpKa 26

C CH+ N H +pKa 36

H H N H

H

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Acidity of Alkynes

• Why are alkynes more acidic than alkanes and alkenes?

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Alkylation of Alkynes

• Terminal alkynes can be alkylated to make new carbon-carbon bonds. Reaction proceeds via a SN2 substitution.

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NaNH2H NaH3C Br

δ+ δ-

CH3