ORGANIC CHEMISTRY 171

Section 201

Alkynes

Alkynes

• Hydrocarbons that contain carbon-carbon triple bonds

• Acetylene is the simplest alkyne.• Our study of alkynes provides nomenclature,physical

properties,structure and SP-hybridization,preparation and reactions.

3

Alkynes. General Formula

CnH2n-2

C2H2 H—C C—H sp => linear, 180o

acetylene

ethyne

C3H4 CH3CCH

methylacetylene

propyne

NOMENCLATURENaming Alkynes

• General hydrocarbon rules apply with “-yne” as a suffix indicating an alkyne.

• Numbering of chain with triple bond is set so that the smallest number possible include the triple bond.

C C CH2H2C CH2 CH2 CH2 CH3H3C

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3-Nonyne

nomenclature:

common names: “alkylacetylene”

IUPAC: parent chain = longest continuous carbon chain that contains the triple bond.

alkane drop –ane add -yne

prefix locant for the triple bond, etc.

CH3CH2CCCH3 2-pentyne

ethylmethylacetylene

“terminal” alkynes have the triple bond at the end of the chain:

CH3

CH3CH2CCH HCCCHCH2CH3

1-butyne 3-methyl-1-pentyne

ethylacetylene sec-butylacetylene

Substitutive nomenclature: Similar to alkenes, but with the following differences:

1. The ending "-ene" is replaced with "-yne"CH3

CH3

C C CH3

4-Methyl-2-pentyne

2. The double bond has a priority over the triple bond when numbered

CCH CH2 CH CH2

1-Pentene-4-yne

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3. There are no cis-trans-isomers, because the triple bond is linear

Enynes

• An enyne has a double bond and triple bond.• Number for an Enynes starts at the multiple bond

closest to the end (it does not matter wheather it is a double or triple bond)

CC CH2 CH CH2CH2H3C

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1-Hepten-4-yne

Diyines and Triynes

• A compound with two triple bonds is a diyine.– A triyne has three triple bonds.

• Number from chain that ends nearest of triple bond.

CC CH2 C CHCH2H3C

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1,4-Heptdiyne

Alkynes as Substituents

Alkynes as substituents are called “alkynyl”.

H3C CH2 C C

1-butynyl

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Cycloalkynes• The smallest cycloalkyne isolated is

cyclononyne– the C-C-C bond angle about the triple bond is

approximately 156°

Physical Properties Similar to alkanes and alkenes of

comparable molecular weight and carbon skeletone.

0.766174-36

0.746125-79

0.71671-132

0.69040-900.69127-32

(a gas)8-126(a gas)-23-102(a gas)-84-81

Densityat 20°C(g/mL)

Boiling Point(°C)

MeltingPoint(°C)FormulaName

1-Decyne

1-Octyne

1-Hexyne

1-Pentyne2-Butyne

1-ButynePropyne

Ethyne HC CHCH3C CH

CH3C CCH3CH3(CH2 )2C CH

CH3CH2C CH

CH3(CH2 )3C CH

CH3(CH2 )5C CH

CH3(CH2 )7C CH

physical properties:1-Weakly or non-polar and have no H-bonding.

2-Relatively low m.p. or b.p.but they increase as the molecular weight increase.

3-Water insoluble but soluble in organic solvents.

4-Relatively acidic (have relative acidic character).

Acidity• A major difference between the chemistry of

alkynes and that of alkenes and alkanes is the acidity of the hydrogen bonded to a triply bonded carbon.

AcidityAcetylene reacts with sodium amide to form

sodium acetylide

it can also be converted to its metal salt by reaction with sodium hydride or lithium diisopropylamide (LDA)

++

pKa 38pKa 25Weaker

acidWeaker

baseStronger

baseStronger

acid

NH3–NH2HC CH HC C-

Sodium hydride Lithium diisopropylamide (LDA)

Na+H– [(CH3)2CH]2N– Li+

Acidity• Water is a stronger acid than acetylene;

hydroxide ion is not a strong enough base to convert acetylene to its anion

Keq = 10-9.3

pKa 15.7pKa 25

-- ++

Stronger acid

Strongerbase

Weakerbase

Weakeracid

HC CH HC COH H2O

Electronic Structure of Alkynes• Carbon-carbon triple bond result from • sp hybridized orbital on each C forming a sigma

bond at 180º • unhybridized 2pX and 2py orbitals forming a 2

bond• The formed C-C triple bond is shorter and stronger

than single or double• Breaking a bond in acetylene (HC=CH) requires

318 kJ/mole (in ethylene it is 268 kJ/mole)

C C HH18

In other words, in alkynes: 1-One- and two - bonds form a triple bond.

2-The valence shell of the atom of carbon has one s-orbital and three p-orbitals.

3- The carbon is bonded by two-bonds, each of two p-orbitals participates in the formation of this - bond

4-The remaining one s-orbital and one p-orbital are equally involved in the formation of -bonds, giving rise to the sp-hybridization state.

So,the presence of sp-hybridized carbons is characteristic for alkynes.

- and 2-bonds in alkynes

-bondsp

s

C C HH

p

CH CH

-bond -bond

Preparation

Preparation of Alkynes1.Elimination Reactions of Dihalides or

Dihydrohaloalkenes

• Treatment of a 1,2 dihydrohaloalkene with KOH or NaOH (strong Base) produces a two-fold elimination of HX

CCH3C H

Cl CH2 CH3

1) 2 NaNH2

2) H3O+CH3C CH2 CH3

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dehydrohalogenation of vicinal dihalides

H H H | | |— C — C — + KOH — C = C — + KX + H2O | | | X X X

H | — C = C — + NaNH2 — C C — + NaX + NH3

| X

Preparation of Alkynes: Vicinal Dihalides

• Vicinal dihalides are available from addition of bromine or chlorine to an alkene.

• Intermediate is a vinyl halide.

C CH

H Br2

CH2Cl2

C CH

HBr Br

2 KOH

Ethanol+ 2 H2O + 2 KBr

A vicinal dibromide

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RR XXSSNN22

XX––::++CC

––

::H—C H—C C—RC—RH—C H—C ++

2.Alkylation of Acetylene and Terminal Alkynes

The alkylating agent is an alkyl halide, andthe reaction is nucleophilic substitution.The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne.

NaNHNaNH22

NHNH33

CHCH33CHCH22CHCH22CHCH22BrBr

(70-77%)(70-77%)

HCHC CHCH HCHC CCNaNa

HCHC CC CH2CH2CH2CH3

Example: Alkylation of Acetylene

NaNHNaNH22, NH, NH33

CHCH33BrBr

CCHH(CH(CH33))22CHCHCHCH22CC

CCNaNa(CH(CH33))22CHCHCHCH22CC

(81%)(81%)

C—CHC—CH33(CH(CH33))22CHCHCHCH22CC

Example: Alkylation of a Terminal Alkyne

1. NaNH1. NaNH22, NH, NH33

2. 2. CHCH33CHCH22BrBr

(81%)(81%)

H—C H—C C—HC—H

1. NaNH1. NaNH22, NH, NH33

2. 2. CHCH33BrBr

C—HC—HCHCH33CHCH22—C—C

C—C—CHCH33CHCH33CHCH22—C—C

Example: Dialkylation of Acetylene

Reactions of Alkynes

1. Alkyne Adition Reactions

a.Reactions of Alkynes: Addition of HX and X2

• Addition reactions of alkynes are similar to those of alkenes

• Intermediate alkene reacts further with excess reagent• Regiospecificity according to Markovnikov

CHC CH2 CH3

HBr

CH2Cl2CC

H Br

H CH2 CH3

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b.Addition of Bromine and Chlorine

• Initial addition gives trans intermediate.• Product with excess reagent is tetrahalide.

CHC CH2 CH3

Br2

CH2Cl2CC

H Br

Br CH2 CH3

Br2

CH2Cl2C

Br

H

Br

C

Br

Br

C

H

H

C

H

H

H

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c.Addition of HX to Alkynes Involves Vinylic Carbocations

• Addition of H-X to alkyne should produce a vinylic carbocation intermediate– Secondary vinyl carbocations form less readily than

primary alkyl carbocations– Primary vinyl carbocations probably do not form at all

C C H Br C CH

Br- C

H

C+

Br-

H

C C

Br

+

HC CH H Br C CH

Br- C

H

C+

Br-

C C+H

Br

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d.Hydration of Alkynes

• Addition of H-OH as in alkenes– Mercury (II) catalyzes

Markovinikov oriented addition

– Hydroboration-oxidation gives the non-Markovnikov product

CHC CH2 CH3

BH3 THF

H2O2 OH-CC

H H

HO CH2 CH3

Anti- Markinov

CHC CH2 CH3

HgSO4, H2SO4 CCH OH

H CH2 CH3H2O

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e.Mercury(II)-Catalyzed Hydration of Alkynes

• Mercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation

• The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone

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Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes

• Addition of Hg(II) to alkyne gives a vinylic cation• Water adds and loses a proton• A proton from aqueous acid replaces Hg(II)

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Keto-enol Tautomerism• Isomeric compounds that can rapidily interconvert by the

movement of a proton are called tautomers and the phenomenon is called tautomerism

• Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon

• The keto form is usually so stable compared to the enol that only the keto form can be observed

OH

H

H

Enol Tautomer

Rapid

(Less favored)

O

H

H

Enol Tautomer

(More favored)

H

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f.Hydration of Unsymmetrical Alkynes• If the alkyl groups at either end of the C-C triple bond are not the

same, both products can form and this is not normally useful• If the triple bond is at the first carbon of the chain (then H is what is

attached to one side) this is called a terminal alkyne• Hydration of a terminal always gives the methyl ketone, which is

useful

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g.Hydroboration/Oxidation of Alkynes

• BH3 (borane) adds to alkynes to give a vinylic borane

• Oxidation with H2O2 produces an enol that converts to the ketone or aldehyde

• Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration

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Comparison of Hydration of Terminal Alkynes

• Hydroboration/oxidation converts terminal alkynes to aldehydes because addition of water is non-Markovnikov

• The product from the mercury(II) catalyzed hydration converts terminal alkynes to methyl ketones

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2.Reduction of Alkynes

a.Addition of H2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction)

b.The addition of the first equivalent of H2 produces an alkene, which is more reactive than the alkyne so the alkene is not observed

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c.Conversion of Alkynes to cis-Alkenes

• Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene

• The two hydrogens add syn (from the same side of the triple bond)

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d.Conversion of Alkynes to trans-Alkenes

• Anhydrous ammonia (NH3) is a liquid below -33 ºC– Alkali metals dissolve in liquid ammonia and function as

reducing agents• Alkynes are reduced to trans alkenes with sodium or lithium

in liquid ammonia• The reaction involves a radical anion intermediate (see Figure

8-4)

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3.Oxidative Cleavage of Alkynes

• Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acids

• Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide

• Neither process is useful in modern synthesis – were used to elucidate structures because the products indicate the structure of the alkyne precursor

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4.Alkyne Acidity: Formation of Acetylide Anions• Terminal alkynes are weak Brønsted acids (alkenes and

alkanes are much less acidic (pKa ~ 25. See Table 8.1 for comparisons))

• Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion

• The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus (see figure 8-5)

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Alkylation of Acetylide Anions• Acetylide ions can react as nucleophiles as well as bases

(see Figure 8-6 for mechanism)• Reaction with a primary alkyl halide produces a

hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes

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