Dr. Wolf's CHM 201 & 202 9-1

Chapter 9Chapter 9AlkynesAlkynes

Dr. Wolf's CHM 201 & 202 9-2

9.19.1Sources of AlkynesSources of Alkynes

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AcetyleneAcetyleneAcetyleneAcetylene

Industrial preparation of acetylene isIndustrial preparation of acetylene isby dehydrogenation of ethyleneby dehydrogenation of ethylene

CHCH33CHCH33

800°C800°C

1150°C1150°C

cost of energy makes acetylene a morecost of energy makes acetylene a moreexpensive industrial chemical than ethyleneexpensive industrial chemical than ethylene

HH22CC CHCH22

HH22CC CHCH22 HCHC CHCH

HH22++

HH22++

Dr. Wolf's CHM 201 & 202 9-4

9.29.2NomenclatureNomenclature

Dr. Wolf's CHM 201 & 202 9-5

HCHC CHCHAcetylene and ethyne are both acceptableAcetylene and ethyne are both acceptableIUPAC names forIUPAC names for

NomenclatureNomenclatureNomenclatureNomenclature

Higher alkynes are named in much the sameHigher alkynes are named in much the sameway as alkenes except using an -way as alkenes except using an -yneyne suffix suffixinstead of -instead of -eneene..

HCHC CCHCCH33

PropynePropyne

HCHC CCHCCH22CHCH33

1-Butyne1-Butyne

(CH(CH33))33CCCC CCHCCH33

4,4-Dimethyl-2-pentyne4,4-Dimethyl-2-pentyne

Dr. Wolf's CHM 201 & 202 9-6

9.39.3Physical Properties of AlkynesPhysical Properties of Alkynes

The physical properties of alkynes are The physical properties of alkynes are similar to those of alkanes and alkenes.similar to those of alkanes and alkenes.

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9.49.4Structure and Bonding in Alkynes:Structure and Bonding in Alkynes:

spsp Hybridization Hybridization

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StructureStructureStructureStructure

linear geometry for acetylenelinear geometry for acetylene

CC CCHH HH

120 pm120 pm

106 pm106 pm 106 pm106 pm

CC CCCHCH33 HH

121 pm121 pm

146 pm146 pm 106 pm106 pm

Dr. Wolf's CHM 201 & 202 9-9

Cyclononyne is the Cyclononyne is the smallest cycloalkyne smallest cycloalkyne stable enough to be stable enough to be stored at room temperaturestored at room temperaturefor a reasonable length for a reasonable length of time. of time.

Cyclooctyne polymerizesCyclooctyne polymerizeson standing.on standing.

Dr. Wolf's CHM 201 & 202 9-10

22ss

22pp

22spsp

Mix together (hybridize) the 2s orbital Mix together (hybridize) the 2s orbital and and oneone of the three 2p orbitals of the three 2p orbitals

22pp

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

Dr. Wolf's CHM 201 & 202 9-11

22spsp

Mix together (hybridize) the 2s orbital Mix together (hybridize) the 2s orbital and and oneone of the three 2p orbitals of the three 2p orbitals

22pp

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

Each carbon has two Each carbon has two half-filled half-filled spsp orbitals orbitalsavailable to form available to form bonds. bonds.

Dr. Wolf's CHM 201 & 202 9-12

Bonds in AcetyleneBonds in Acetylene Bonds in AcetyleneBonds in Acetylene

Each carbon isEach carbon is

connected to aconnected to a

hydrogen by ahydrogen by a

bond. The two bond. The two

carbons are connectedcarbons are connected

to each other by ato each other by a

bond and two bond and two bonds. bonds.

Dr. Wolf's CHM 201 & 202 9-13

Bonds in AcetyleneBonds in Acetylene Bonds in AcetyleneBonds in Acetylene

One of the twoOne of the two

bonds in bonds in

acetylene isacetylene is

shown here.shown here.

The second The second bond is at rightbond is at right

angles to the first.angles to the first.

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Bonds in AcetyleneBonds in Acetylene Bonds in AcetyleneBonds in Acetylene

This is the secondThis is the second

of the twoof the two

bonds in bonds in

acetylene.acetylene.

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The region of highest The region of highest

negative charge lies abovenegative charge lies above

and below the molecular and below the molecular

plane in ethylene.plane in ethylene.

The region of highest The region of highest

negative charge encirclesnegative charge encircles

the molecule around itsthe molecule around its

center in acetylene.center in acetylene.

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C—C distanceC—C distanceC—C distanceC—C distance

C—H distanceC—H distanceC—H distanceC—H distance

H—C—C anglesH—C—C anglesH—C—C anglesH—C—C angles

C—C BDEC—C BDEC—C BDEC—C BDE

C—H BDEC—H BDEC—H BDEC—H BDE

% % ss character character% % ss character character

ppKKaappKKaa

153 pm153 pm153 pm153 pm

111 pm111 pm111 pm111 pm

111.0°111.0°111.0°111.0°

368 kJ/mol368 kJ/mol368 kJ/mol368 kJ/mol

410 kJ/mol410 kJ/mol410 kJ/mol410 kJ/mol

spsp33spsp33

25%25%25%25%

62626262

134 pm134 pm134 pm134 pm

110 pm110 pm110 pm110 pm

121.4°121.4°121.4°121.4°

611 kJ/mol611 kJ/mol611 kJ/mol611 kJ/mol

452 kJ/mol452 kJ/mol452 kJ/mol452 kJ/mol

spsp22spsp22

33%33%33%33%

45454545

120 pm120 pm120 pm120 pm

106 pm106 pm106 pm106 pm

180°180°180°180°

820 kJ/mol820 kJ/mol820 kJ/mol820 kJ/mol

536 kJ/mol536 kJ/mol536 kJ/mol536 kJ/mol

spspspsp

50%50%50%50%

26262626

hybridization of Chybridization of Chybridization of Chybridization of C

Table 9.1 Comparison of ethane, ethylene, and acetyleneTable 9.1 Comparison of ethane, ethylene, and acetylene

Ethane Ethylene Ethane Ethylene AcetyleneAcetylene

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9.59.5

Acidity of Acetylene andTerminal AlkynesAcidity of Acetylene andTerminal Alkynes

HH

CC CC

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In general, hydrocarbons are In general, hydrocarbons are exceedingly weak acidsexceedingly weak acids

In general, hydrocarbons are In general, hydrocarbons are exceedingly weak acidsexceedingly weak acids

CompoundCompound ppKKaa

HFHF 3.23.2

HH22OO 1616

NHNH33 3636

4545

CHCH44 6060

HH22CC CHCH22

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Acetylene is a weak acid, but not nearlyAcetylene is a weak acid, but not nearlyas weak as alkanes or alkenes.as weak as alkanes or alkenes.

Acetylene is a weak acid, but not nearlyAcetylene is a weak acid, but not nearlyas weak as alkanes or alkenes.as weak as alkanes or alkenes.

CompoundCompound ppKKaa

HFHF 3.23.2

HH22OO 1616

NHNH33 3636

4545

CHCH44 6060

HH22CC CHCH22

HCHC CHCH 26262626

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Electronegativity of carbon Electronegativity of carbon increases with its increases with its ss character character

Electronegativity of carbon Electronegativity of carbon increases with its increases with its ss character character

CC HH HH++ ++

HH++ ++

HH++ ++

1010-60-60

1010-45-45

1010-26-26

sp3sp3CC :

sp2sp2

spsp

HH

CC CC

CC CC HH

CC CC

CC CC :

:

Electrons in an orbital with more Electrons in an orbital with more ss character are closer to the character are closer to the

nucleus and more strongly held.nucleus and more strongly held.

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Objective:Objective:Prepare a solution containing sodium acetylidePrepare a solution containing sodium acetylide

Will treatment of acetylene with NaOH be effective?Will treatment of acetylene with NaOH be effective?

Objective:Objective:Prepare a solution containing sodium acetylidePrepare a solution containing sodium acetylide

Will treatment of acetylene with NaOH be effective?Will treatment of acetylene with NaOH be effective?

NaCNaC CHCH

HH22OONaOHNaOH ++ HCHC CHCH NaCNaC CHCH ++

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No. Hydroxide is not a strong enough No. Hydroxide is not a strong enough base to deprotonate acetylene.base to deprotonate acetylene.

No. Hydroxide is not a strong enough No. Hydroxide is not a strong enough base to deprotonate acetylene.base to deprotonate acetylene.

HH22OONaOHNaOH ++ HCHC CHCH NaCNaC CHCH ++

––HOHO

....

.... ::

HH CC CHCH HOHO HH....

....++ ++ CC CHCH::––

weaker acidweaker acidppKKaa = 26 = 26

stronger acidstronger acidppKKaa = 16 = 16

In acid-base reactions, the equilibrium lies toIn acid-base reactions, the equilibrium lies tothe side of the weaker acid.the side of the weaker acid.

Dr. Wolf's CHM 201 & 202 9-23

Solution: Use a stronger base. Sodium amideSolution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide.is a stronger base than sodium hydroxide.

Solution: Use a stronger base. Sodium amideSolution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide.is a stronger base than sodium hydroxide.

NHNH33NaNaNHNH22 ++ HCHC CHCH NaCNaC CHCH ++

––HH22NN

....::

HH CC CHCH HH....

++ ++ CC CHCH::––

stronger acidstronger acidppKKaa = 26 = 26

weaker acidweaker acidppKKaa = 36 = 36

Ammonia is a weaker acid than acetylene.Ammonia is a weaker acid than acetylene.The position of equilibrium lies to the right.The position of equilibrium lies to the right.

HH22NN

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9.69.6Preparation of Alkynes Preparation of Alkynes

bybyAlkylation of Acetylene and Terminal AlkynesAlkylation of Acetylene and Terminal Alkynes

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Preparation of AlkynesPreparation of AlkynesPreparation of AlkynesPreparation of Alkynes

Carbon-carbon bond formationCarbon-carbon bond formationalkylation of acetylene and terminal alkynesalkylation of acetylene and terminal alkynes

Functional-group transformationsFunctional-group transformationseliminationelimination

There are two main methods for the preparationThere are two main methods for the preparationof alkynes:of alkynes:

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Alkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynes

H—C H—C C—HC—H

RR—C —C C—HC—H

RR—C —C C—C—RR

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RR XXSSNN22

Alkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynes

XX––::++CC––::H—C H—C C—RC—RH—C H—C ++

• The alkylating agent is an alkyl halide, andThe alkylating agent is an alkyl halide, andthe reaction is nucleophilic substitution.the reaction is nucleophilic substitution.

• The nucleophile is sodium acetylide or the The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) sodium salt of a terminal (monosubstituted) alkyne.alkyne.

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Example: Example: Alkylation of acetylene Alkylation of acetylene Example: Example: Alkylation of acetylene Alkylation of acetylene

NaNHNaNH22

NHNH33

CHCH33CHCH22CHCH22CHCH22BrBr

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

HCHC CHCH HCHC CCNaNa

HCHC CC CHCH22CHCH22CHCH22CHCH33

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NaNHNaNH22, NH, NH33

CHCH33BrBr

Example: Example: Alkylation of a terminal alkyne Alkylation of a terminal alkyne Example: Example: Alkylation of a terminal alkyne Alkylation of a terminal alkyne

CCHH(CH(CH33))22CHCHCHCH22CC

CCNaNa(CH(CH33))22CHCHCHCH22CC

(81%)(81%)

C—CHC—CH33(CH(CH33))22CHCHCHCH22CC

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1. NaNH1. NaNH22, NH, NH33

2. 2. CHCH33CHCH22BrBr

(81%)(81%)

Example: Example: Dialkylation of acetyleneDialkylation of acetyleneExample: Example: Dialkylation of acetyleneDialkylation of acetylene

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

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LimitationLimitationLimitationLimitation

Effective only with primary alkyl Effective only with primary alkyl halideshalides

Secondary and tertiary alkyl halides Secondary and tertiary alkyl halides undergo eliminationundergo elimination

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E2 predominates over SE2 predominates over SNN2 when alkyl 2 when alkyl

halide is secondary or tertiaryhalide is secondary or tertiary

E2 predominates over SE2 predominates over SNN2 when alkyl 2 when alkyl

halide is secondary or tertiaryhalide is secondary or tertiary

HH CC

C— C— XX

E2E2

CC––::H—C H—C

++CCH—C H—C ——HH CC CC XX––::++

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9.79.7Preparation of Alkynes byPreparation of Alkynes by

Elimination ReactionsElimination Reactions

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Preparation of Alkynes by Preparation of Alkynes by "Double" Dehydrohalogenation"Double" Dehydrohalogenation

Preparation of Alkynes by Preparation of Alkynes by "Double" Dehydrohalogenation"Double" Dehydrohalogenation

Geminal dihalideGeminal dihalide Vicinal dihalideVicinal dihalide

XX

CC CC

XX

HH

HH

XX XX

CC CC

HHHH

The most frequent applications are in preparation The most frequent applications are in preparation of terminal alkynes.of terminal alkynes.

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Geminal dihalide Geminal dihalide Alkyne AlkyneGeminal dihalide Geminal dihalide Alkyne Alkyne

(CH(CH33))33CCCCHH22—CH—CHClCl22

1. 3NaNH1. 3NaNH22, NH, NH33

2. H2. H22OO

(56-60%)(56-60%)

(CH(CH33))33CCCC CHCH

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NaNHNaNH22, NH, NH33

NaNHNaNH22, NH, NH33

NaNHNaNH22, NH, NH33HH22OO

Geminal dihalide Geminal dihalide Alkyne AlkyneGeminal dihalide Geminal dihalide Alkyne Alkyne

(CH(CH33))33CCCCHH22—CH—CHClCl22

(CH(CH33))33CCCCHH CHCHClCl

(CH(CH33))33CCCC CHCH

(CH(CH33))33CCCC CNaCNa

(slow)(slow)

(slow)(slow)

(fast)(fast)

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CHCH33(CH(CH22))77CCHH—C—CHH22BrBr

BrBr

Vicinal dihalide Vicinal dihalide Alkyne AlkyneVicinal dihalide Vicinal dihalide Alkyne Alkyne

1. 3NaNH1. 3NaNH22, NH, NH33

2. H2. H22OO

(54%)(54%)

CHCH33(CH(CH22))77CC CHCH

Dr. Wolf's CHM 201 & 202 9-38

9.89.8Reactions of AlkynesReactions of Alkynes

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Reactions of AlkynesReactions of AlkynesReactions of AlkynesReactions of Alkynes

Acidity (Section 9.5)Acidity (Section 9.5)Hydrogenation (Section 9.9)Hydrogenation (Section 9.9)Metal-Ammonia Reduction (Section 9.10)Metal-Ammonia Reduction (Section 9.10)Addition of Hydrogen Halides (Section 9.11)Addition of Hydrogen Halides (Section 9.11)Hydration (Section 9.12)Hydration (Section 9.12)Addition of Halogens (Section 9.13)Addition of Halogens (Section 9.13)Ozonolysis (Section 9.14)Ozonolysis (Section 9.14)

Acidity (Section 9.5)Acidity (Section 9.5)Hydrogenation (Section 9.9)Hydrogenation (Section 9.9)Metal-Ammonia Reduction (Section 9.10)Metal-Ammonia Reduction (Section 9.10)Addition of Hydrogen Halides (Section 9.11)Addition of Hydrogen Halides (Section 9.11)Hydration (Section 9.12)Hydration (Section 9.12)Addition of Halogens (Section 9.13)Addition of Halogens (Section 9.13)Ozonolysis (Section 9.14)Ozonolysis (Section 9.14)