introduction

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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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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. =>

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Name these:

CH3 CH

CH3

CH2 C C CH

CH3

CH3

CH3 C C CH2 CH2 Br

CH3 C CH

propyne

5-bromo-2-pentyne

2,6-dimethyl-3-heptyne =>

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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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Examples

CH2 CH CH2 CH

CH3

C CH

4-methyl-1-hexen-5-yne

CH3 C C CH2 CH

OH

CH3

4-hexyn-2-ol

=>

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Common Names

Named as substituted acetylene.

CH3 C CH

methylacetylene

CH3 CH

CH3

CH2 C C CH

CH3

CH3

isobutylisopropylacetylene=>

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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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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. =>

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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. =>

*

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

Bond angle is 180, so linear geometry. =>

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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. =>

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Acidity Table

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Migration of Triple Bond

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Forming Acetylide Ions

• H+ can be removed from a terminal alkyne by sodium amide, NaNH2.

CH3 C C H + NaNH2 CH3 C C:- Na

++ NH3

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

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Qualitative Test

• Reagent is AgNO3 or CuNO3 in alcohol, orammonia is added to form the complex ion.

• The solid is explosive when dry.• Copper tubing is not used with acetylene.

=>

CH3 C C H + Cu+

CH3 C C Cu + H+

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Bimolecular Nucleophilic Substitution

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Internal Alkynes from Acetylides

• Acetylide ions are good nucleophiles.

• SN2 reaction with 1 alkyl halides lengthens the alkyne chain.

++CH3 C C:- Na

+CH3CH2 Br CH3 C C CH2 CH3 NaBr

=>

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Alkyl Halide Must be 1

• Acetylide ions can also remove H+

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

CH3 C C:- Na

++ CH3 CH

Br

CH3 CH3 C C H H3C CH CH2+

=>

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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

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Formation of Primary Alcohol

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 H

H

H

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Reaction of Aldehydes

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 OH

HHCH3 C C C O H

CH3

H

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Reaction of Ketones

Product is a tertiary alcohol.

+ C OCH3

CH3

CH3 C C CH3 C C C

CH3

CH3

O

=>

+H2O OH

HHCH3 C C C O H

CH3

CH3

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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. =>

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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°C

KOH (fused)CH3 CH CH CH2 CH3

Br Br

=>

, 150°CCH3 CH2 C CH

H2O2)

NaNH21)CH3 CH2 CH2 CHCl2

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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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Addition of hydrogen: three different reactions:

• Reduction to alkane

• Reduction to cis-alkene.

• Reduction to trans-alkene. =>

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Reduction to Alkane

•Add lots of H2 with metal catalyst (Pd, Pt, or Ni) to reduce alkyne to alkane, completely saturated.

CH2

C CH CH2 CH2 CH2 CH2 CH3Pt

CH2 CH2 CH2 CH2 CH3CH2CH3

CH2 CCH2CH3 C CH2 CH3 CH2 CH2CH2CH3Pd

H2

H2

CH2 CH3

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Lindlar’s Catalyst - Poisoned

• Powdered BaSO4 coated with Pd, poisoned with quinoline.

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

=>

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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-

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Mechanism

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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 CH3C

BrC

Br

CH3

+CH3

CBr

CCH3

Br

Br2

CH3 C

Br

Br

C

Br

Br

CH3

=>

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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

=>

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HBr with Peroxides

Anti-Markovnikov product is formed with a terminal alkyne.

CH3 C C H CH3 C C

H H

Br

HBr

ROOR

HBrCH3 C C

H

H

H

Br

BrROOR

=>

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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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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+

C

O+

H H

H2OCH3 CH

Hg+

C

OH

H3O+

CH3 CH

HC

OH

an enol =>

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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

H

H

H2O

CH3 C C

O

H

H

H

CH3 CH

HC

OH

H3O+

CH3 C C

OH

H

H

H

A methyl ketone

=>

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Hydroboration Reagent

• Di(secondary isoamyl)borane, called disiamylborane.

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

• Only one mole can add.

=>

BCH

CH

H

CH3

CHCH3H3C

H3C

HC CH3H3C

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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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Enol to Keto (in Base)

• H+ is removed from OH of the enol.

• Then water gives H+ to the adjacent carbon.

CH3 CO

HC

H

HOH

CH3 CO

HC

H

H

OHCH3 C

OH

HC

H

CH3 CO

HC

H

An aldehyde =>

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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. =>

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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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Ozonolysis

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

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

=>

HO C

O

CH2 CH3CH3 C

O

OHH2O(2)

O3(1)CH3 C C CH2 CH3 +

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Product of KMnO4 oxidation

KMnO4, KOH

heat, HOH

O

O-?1 2

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1 23

4

KMnO4, KOH

heat, HOH?

O O-O

O-1 2

34+

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