Chapter 9Alkynes

Organic Chemistry, 6th EditionL. G. Wade, Jr.

Chapter 9 2

Introduction• Alkynes contain a triple bond.• General formula is CnH2n-2.• Two elements of unsaturation for each

triple bond.• Some reactions are like alkenes:

addition and oxidation.• Some reactions are specific to alkynes.

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Chapter 9 3

Nomenclature: IUPAC

• Find the longest chain containing the triple bond.

• Change -ane ending to -yne.• Number the chain, starting at the end

closest to the triple bond.• Give branches or other substituents a

number to locate their position. =>

Chapter 9 4

Name these:

CH3 CH

CH3

CH2 C C CH

CH3

CH3

CH3 C C CH2 CH2 Br

CH3 C CHpropyne

5-bromo-2-pentyne5-bromopent-2-yne

=>

2,6-dimethyl-3-heptyne2,6-dimethylpept-3-yne

Chapter 9 5

Additional Functional Groups

• All other functional groups, except ethers and halides have a higher priority than alkynes.

• For a complete list of naming priorities, look inside the back cover of your text.

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Chapter 9 6

Examples

CH2 CH CH2 CH

CH3

C CH

4-methyl-1-hexen-5-yne4-methylhex-1-en-5-yne

CH3 C C CH2 CH

OH

CH3

4-hexyn-2-ol hex-4-yn-2-ol

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Chapter 9 7

Common Names

Named as substituted acetylene.

CH3 C CHmethylacetylene(terminal alkyne)

CH3 CH

CH3

CH2 C C CH

CH3

CH3

isobutylisopropylacetylene(internal alkyne) =>

Chapter 9 8

Physical Properties

• Nonpolar, insoluble in water.• Soluble in most organic solvents.• Boiling points similar to alkane of same

size.• Less dense than water.• Up to 4 carbons, gas at room temperature.

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Chapter 9 9

Acetylene

• Acetylene is used in welding torches.• In pure oxygen, temperature of flame

reaches 2800C.• It would violently decompose to its

elements, but the cylinder on the torch contains crushed firebrick wet with acetone to moderate it. =>

Chapter 9 10

Synthesis of Acetylene

• Heat coke with lime in an electric furnace to form calcium carbide.

• Then drip water on the calcium carbide.

H C C H Ca(OH)2CaC2 + 2 H2O +

C CaO3 + +CaC2 COcoke lime

*This reaction was used to produce light for miners’ lamps and for the stage. =>

*

Chapter 9 11

Electronic Structure• The sigma bond is sp-sp overlap.

• The two pi bonds are unhybridized p overlaps at 90, which blend into acylindrical shape.

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Chapter 9 12

Bond Lengths• More s character, so shorter length.• Three bonding overlaps, so shorter.

Bond angle is 180, so linear geometry. =>

Chapter 9 13

Acidity of Alkynes

• Terminal alkynes, R-CC-H, are more acidic than other hydrocarbons.

• Acetylene acetylide by NH2-, but not

by OH- or RO-.• More s character, so pair of electrons in

anion is held more closely to the nucleus. Less charge separation, so more stable. =>

Chapter 9 14

Acidity Table

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Chapter 9 15

Forming Acetylide Ions• H+ can be removed from a terminal alkyne by sodium

amide, NaNH2.

• NaNH2 is produced by the reaction of ammonia with sodium metal.

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Chapter 9 16

Alkynes from Acetylides

• Acetylide ions are good nucleophiles.• SN2 reaction with 1 alkyl halides

lengthens the alkyne chain.

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Chapter 9 17

Must be 1

• Acetylide ions can also remove H+

• If back-side approach is hindered, elimination reaction happens via E2.

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Chapter 9 18

Addition to Carbonyl

Acetylide ion + carbonyl group yields an alkynol (alcohol on carbon adjacent to triple bond).

+H2OO

H

HHR C C C O H

=>

C O+R C C R C C C O

Chapter 9 19

Add to Formaldehyde

Product is a primary alcohol with one more carbon than the acetylide.

+ C OH

HCH3 C C CH3 C C C

H

H

O

=>

+H2O OH

HHCH3 C C C O HH

H

Chapter 9 20

Add to Aldehyde

Product is a secondary alcohol, one R group from the acetylide ion, the other R group from the aldehyde.

+ C OCH3

HCH3 C C CH3 C C C

CH3

H

O

=>+H2O O

H

HHCH3 C C C O HCH3

H

Chapter 9 21

Add to Ketone

Product is a tertiary alcohol.

+ C OCH3

CH3

CH3 C C CH3 C C CCH3

CH3

O

=>

+H2O OH

HHCH3 C C C O HCH3

CH3

Chapter 9 22

Synthesis by Elimination

• Removal of two molecules of HX from a vicinal or geminal dihalide produces an alkyne.

• First step (-HX) is easy, forms vinyl halide.

• Second step, removal of HX from the vinyl halide requires very strong base and high temperatures. =>

Chapter 9 23

Reagents for Elimination

• Molten KOH or alcoholic KOH at 200C favors an internal alkyne.

• Sodium amide, NaNH2, at 150C, followed by water, favors a terminal alkyne.

CH3 C C CH2 CH3200°CKOH (fused)

CH3 CH CH CH2 CH3

Br Br

=>

, 150°CCH3 CH2 C CH

H2O2)

NaNH21)CH3 CH2 CH2 CHCl2

Chapter 9 24

Migration of Triple Bond

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Chapter 9 25

Addition Reactions• Similar to addition to alkenes.• Pi bond becomes two sigma bonds.• Usually exothermic.• One or two molecules may add.

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Chapter 9 26

Addition of Hydrogen• Three reactions:• Add lots of H2 with metal catalyst (Pd, Pt, or

Ni) to reduce alkyne to alkane, completely saturated.

• Use a special catalyst, Lindlar’s catalyst, to convert an alkyne to a cis-alkene.

• React the alkyne with sodium in liquid ammonia to form a trans-alkene. =>

Chapter 9 27

Lindlar’s Catalyst• Powdered BaSO4 coated with Pd,

poisoned with quinoline.• H2 adds syn, so cis-alkene is formed.

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Chapter 9 28

Na in Liquid Ammonia

• Use dry ice to keep ammonia liquid.• As sodium metal dissolves in the

ammonia, it loses an electron.• The electron is solvated by the

ammonia, creating a deep blue solution.

NH3 + Na + Na+NH3 e- =>

Chapter 9 29

MechanismStep 1: An electron adds to the alkyne, forming a radical anion

Step 2: The radical anion is protonated to give a radical

Step 3: An electron adds to the alkyne, forming an anion

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Step 4: Protonation of the anion gives an alkene

Chapter 9 30

Addition of Halogens• Cl2 and Br2 add to alkynes to form vinyl

dihalides.• May add syn or anti, so product is

mixture of cis and trans isomers.• Difficult to stop the reaction at dihalide.CH3 C C CH3

Br2 CH3 CBr

CBr

CH3

+CH3 C

BrC

CH3

BrBr2

CH3 C

Br

Br

C

Br

Br

CH3

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Chapter 9 31

Addition of HX• HCl, HBr, and HI add to alkynes to form

vinyl halides.• For terminal alkynes, Markovnikov

product is formed.• If two moles of HX is added, product is

a geminal dihalide.

CH3 C C H CH3 C CH2

BrHBr HBr

CH3 C CH3

Br

Br =>

Chapter 9 32

HBr with Peroxides

Anti-Markovnikov product is formed with a terminal alkyne.

HBrCH3 C C

H

H

H

Br

BrROOR

=>

CH3 C C H CH3 C C

H H

BrHBr

ROORmixture of E and Z isomers

Chapter 9 33

Hydration of Alkynes

• Mercuric sulfate in aqueous sulfuric acid adds H-OH to one pi bond with a Markovnikov orientation, forming a vinyl alcohol (enol) that rearranges to a ketone.

• Hydroboration-oxidation adds H-OH with an anti-Markovnikov orientation, and rearranges to an aldehyde.

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Chapter 9 34

Mechanism for Mercuration

• Mercuric ion (Hg2+) is electrophile.• Vinyl carbocation forms on most-sub. C.• Water is the nucleophile.

CH3 C C H CH3 C+ CHg+

HHg+2

H2O

CH3 CH

Hg+

CO+

H H

H2OCH3 CH

Hg+

C

OH

H3O+

CH3 CH

HC

OH

an enol =>

Chapter 9 35

Enol to Keto (in Acid)• Add H+ to the C=C double bond.• Remove H+ from OH of the enol.

CH3 C C

OH

H

HH

H2O

CH3 C C

O

H

HH

CH3 CH

HC

OH

H3O+

CH3 C C

OH

H

HH

A methyl ketone

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Chapter 9 36

Hydroboration Reagent

• Di(secondary isoamyl)borane, called disiamylborane.

• Bulky, branched reagent adds to the least hindered carbon.

• Only one mole can add.

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BCH

CHH

CH3

CHCH3H3C

H3C

HC CH3H3C

Chapter 9 37

Hydroboration - Oxidation

• B and H add across the triple bond.• Oxidation with basic H2O2 gives the enol.

CH3 C C H CH3 CH

C

H BSia2

Sia2 BH CH3 COH

HC

H

H2O2

NaOH

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Chapter 9 38

Enol to Keto (in Base)

• H+ is removed from OH of the enol.• Then water gives H+ to the adjacent

carbon.

CH3 CO

HC

HHOH

CH3 CO

HC

H

H

OHCH3 C

OH

HC

HCH3 C

O

HC

H

An aldehyde =>

Chapter 9 39

Oxidation of Alkynes

• Similar to oxidation of alkenes.• Dilute, neutral solution of KMnO4

oxidizes alkynes to a diketone.• Warm, basic KMnO4 cleaves the triple

bond.• Ozonolysis, followed by hydrolysis,

cleaves the triple bond. =>

Chapter 9 40

Reaction with KMnO4

• Mild conditions, dilute, neutral

• Harsher conditions, warm, basic

CH3 C

O

C

O

CH2 CH3H2O, neutral

KMnO4CH3 C C CH2 CH3

O C

O

CH2 CH3CH3 C

O

O +H2O, warm

, KOHKMnO4CH3 C C CH2 CH3

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Chapter 9 41

Ozonolysis

• Ozonolysis of alkynes produces carboxylic acids (alkenes gave aldehydes and ketones).

• Used to find location of triple bond in an unknown compound.

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HO C

O

CH2 CH3CH3 C

O

OHH2O(2)

O3(1)CH3 C C CH2 CH3 +

Chapter 9 42

End of Chapter 9