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Chapter 9; AlkynesChapter 9; Alkynes

I.I. NomenclatureNomenclature

II.II. Physical Properties; Polarity, AcidityPhysical Properties; Polarity, Acidity

III.III. Preparation of AlkynesPreparation of Alkynes

-S-SNN2 with Acetylide Ion2 with Acetylide Ion

-Elimination of Dihalides-Elimination of Dihalides

IV.IV. Reaction of AlkynesReaction of Alkynes

- Acid/BaseAcid/Base

- Addition (HAddition (H22, HX, H, HX, H

22O, XO, X

22))

- RedoxRedox

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

1. Higher alkynes are named in much the same1. Higher alkynes are named in much the same

way as alkenes except using an -way as alkenes except using an -yneyne suffixsuffixinstead of -instead of -eneene..

HCHC CCHCCH33

PropynePropyne

HCHC CCHCCH22CHCH

33

1-Butyne1-Butyne

(CH(CH33))33CCCC CCHCCH

33

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

NomenclatureNomenclature

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NomenclatureNomenclature

2.2. Compounds with two or three triple bondsCompounds with two or three triple bonds

are named as –diynes or –triynes (retain –aare named as –diynes or –triynes (retain –a

of parent alkane)of parent alkane)3.3. If there is a double and triple bond in theIf there is a double and triple bond in the

molecule; name as an -en yne rather than –molecule; name as an -en yne rather than –

yn eneyn ene

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Acidity of AcetyleneAcidity of Acetylene

and Terminal Alkynesand Terminal Alkynes

C C CH3

CH3

C C CH3H

Internal AlkyneInternal Alkyne

Terminal AlkyneTerminal Alkyne

weakly acidic Hweakly acidic H

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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.

CompoundCompound ppK K aa

HFHF 3.23.2

HH22OO 15.715.7

NHNH33 3636

4545

CHCH44

6060

HH22CC CHCH22

HCHC CHCH 26

Acetylene Acetylene

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

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

Carbon: Hybridization and Electronegativity Carbon: Hybridization and Electronegativity

CC HH HH++ ++ppK K aa = 60= 60

sp3CC : – –

HH++ ++sp

2 HH

CC CC CC CC:

– –

ppK K aa = 45= 45

HH++ ++ spCC CC HH CC CC : – –

ppK K aa = 26= 26

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9.69.6

Preparation of AlkynesPreparation of Alkynes

1. Substitution Reactions Using the Acetylide Ion1. Substitution Reactions Using the Acetylide Ion

Effective only with 1° RX; 2°. 3° haveEffective only with 1° RX; 2°. 3° have eliminationelimination

2. Elimination Reactions2. Elimination Reactions

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Forming Internal Alkynes Using the Acetylide IonForming Internal Alkynes Using the Acetylide Ion

RR XXSSNN22

XX – –::++CC – –

::H—CH—C C—RC—RH—CH—C ++

NaNHNaNH22

NHNH33

HCHC CHCH HCHC CCNaNa

1.1. Form Nucleophile by removing acidic H fromForm Nucleophile by removing acidic H from

terminal alkyneterminal alkyne

2.2. SSNN2 Substitution with alkyl halide to form internal2 Substitution with alkyl halide to form internal

alkynealkyne

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NaNHNaNH22

NHNH33

HCHC CHCH HCHC CCNaNa

CHCH33CHCH22CHCH22CHCH22Br Br

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

HCHC CC CHCH22CHCH22CHCH22CHCH33

Example: Alkylation of AcetyleneExample: Alkylation of Acetylene

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NaNHNaNH22, NH, NH

33

CCHH(CH(CH33))22CHCHCHCH22CC

CCNaNa(CH(CH33))22CHCHCHCH

22CC

CHCH33Br Br

(81%)(81%)

C—C—CHCH33(CH(CH

33))22CHCHCHCH22CC

Example: Alkylation of a Terminal AlkyneExample: Alkylation of a Terminal Alkyne

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1. NaNH1. NaNH22, NH, NH

33

2.2. CHCH33CHCH

22Br Br

H—CH—C C—HC—H

C—HC—HCHCH33CHCH

22—C—C

(81%)(81%)

1. NaNH1. NaNH22, NH, NH

33

2.2. CHCH33Br Br

C—C—CHCH33CHCH

33CHCH22—C—C

Example: Dialkylation of AcetyleneExample: Dialkylation of Acetylene

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9.6Preparation of Alkynes

1. Substitution Reactions Using the Acetylide Ion2. Elimination Reactions

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Geminal dihalideGeminal dihalide Vicinal dihalideVicinal dihalide

XX

CC CC

XX

HH

HH

XX XX

CC CC

HHHH

The most frequent applications are in preparationThe most frequent applications are in preparation

of terminal alkynes.of terminal alkynes.

Preparation of AlkynesPreparation of Alkynes

by "Double Dehydrohalogenation" by "Double Dehydrohalogenation"

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(CH(CH33))33CCCCHH22—CH—CHClCl

22

1. 3NaNH1. 3NaNH22, NH, NH33

2. H2. H22OO

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

(CH(CH33))33CCCC CHCH

Geminal dihalideGeminal dihalide → → Alkyne Alkyne

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NaNHNaNH22, NH, NH

33

HH22OO

(CH(CH33))33CCCCHH22—CH—CHClCl

22

(CH(CH

33))

33CCCC

HH

CHCHClCl

(slow)(slow)

NaNHNaNH22, NH, NH

33

(CH(CH33))33CCCC CHCH

(slow)(slow)

NaNHNaNH22, NH, NH

33

(CH(CH33))33CCCC CNaCNa

(fast)(fast)

Geminal dihalideGeminal dihalide → → Alkyne Alkyne

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CHCH33(CH(CH

22))77CCHH—C—CHH22Br Br

Br Br

1. 3NaNH1. 3NaNH22, NH, NH

33

2. H2. H22OO

(54%)(54%)

CHCH33(CH(CH

22))77CC CHCH

Vicinal dihalideVicinal dihalide → → Alkyne Alkyne

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I.I. Acid/ Base ReactionAcid/ Base ReactionII.II. Addition ReactionsAddition Reactions

Hydrogenation (Section 9.9)Hydrogenation (Section 9.9)

Addition of Hydrogen HalidesAddition of Hydrogen Halides(Section 9.11)(Section 9.11)Hydration (Section 9.12)Hydration (Section 9.12)

Addition of Halogens (Section 9.13)Addition of Halogens (Section 9.13)

III.III. Redox ReactionsRedox ReactionsOzonolysis (Section 9.14)Ozonolysis (Section 9.14)

Reactions of AlkynesReactions of Alkynes

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RCRCHH22CCHH22R'R'catcat

catalyst = Pt, Pd, Ni, or Rhcatalyst = Pt, Pd, Ni, or Rhalkene is an intermediatealkene is an intermediate

RCRC CR'CR' ++ 22HH22

Addition Reactions Addition Reactions

Hydrogenation of Alkynes; H Hydrogenation of Alkynes; H 2 2

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Alkynes are slightly less reactive than alkenesAlkynes are slightly less reactive than alkenes

HHBr Br

Br Br

(60%)(60%)

CHCH33(CH(CH

22))33CC CHCH CHCH33(CH(CH

22))33CC CHCH22

Follows Markovnikov's RuleFollows Markovnikov's Rule

Addition Reactions

Hydrohalogenation of Alkynes; HX

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CHCH33CHCH

22CC CCHCCH22CHCH

33

2 H2 H

FF

(76%)(76%)

FF

FF

CC CC

HH

HH

CHCH33CHCH22 CHCH22CHCH33

Two Molar Equivalents of Hydrogen HalideTwo Molar Equivalents of Hydrogen Halide

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HHBr Br

regioselectivity opposite to Markovnikov's ruleregioselectivity opposite to Markovnikov's rule

CHCH33

(CH(CH22

))33

CCCHCH

(79%)(79%)

CHCH33(CH(CH

22))33CCHH

CHCHBr Br peroxidesperoxides

Free-radical Addition of HBr Free-radical Addition of HBr

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expected reaction:expected reaction:

enol enol

HH++

RCRC CR'CR' HH22OO++

OHOH

RCHRCH CR'CR'

observed reaction:observed reaction:

RCHRCH22CR'CR'

OO

HH++

RCRC CR'CR' HH22OO++

ketoneketone

Addition Reactions Addition Reactions

Hydration of Alkynes; H Hydration of Alkynes; H 2 2 OO

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enols are regioisomers or enols are regioisomers or tautomerstautomers of ketones,of ketones,

and exist in equilibrium with themand exist in equilibrium with them

keto-enol equilibration is rapid in acidic mediaketo-enol equilibration is rapid in acidic mediaketones are more stable than enols andketones are more stable than enols and

predominate at equilibriumpredominate at equilibrium

enol enol

OHOH

RCHRCH CR'CR' RCHRCH22CR'CR'

OO

ketoneketone

EnolsEnols

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OO HH

CC CC

HH++OO

HH

HH

::

....

::

Mechanism of conversion of enol to ketoneMechanism of conversion of enol to ketone

step 1; Acidic H adds across double bond to formstep 1; Acidic H adds across double bond to form

more stable carbocation. Lone pair on O stabalizesmore stable carbocation. Lone pair on O stabalizes

carbocation.carbocation.

....HHOO

CC CCHH++

::

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step 2; Carbocation is stabilized by resonancestep 2; Carbocation is stabilized by resonance

HH OO

CC CCHH

....

HH++OO

CC CCHH++

....

::

HH

HH

OO:: ::

OO

CC CCHH

....

HH++

OO

CC CCHH

....::

+ H+ H33OO

++

step 3; Acid catalyst is reformedstep 3; Acid catalyst is reformed

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HH22O, HO, H++

CHCH33(CH(CH

22))22CC C(CHC(CH22))22CHCH

33

HgHg2+2+

(89%)(89%)

OO

CHCH33(CH(CH

22))22CHCH22C(CHC(CH

22))22CHCH33

viavia

OHOH

CHCH33(CH(CH

22))22CHCH C(CHC(CH22))22CHCH

33

Example of Alkyne HydrationExample of Alkyne Hydration

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HH22O, HO, H22SOSO

44

HgSOHgSO44

CHCH33(CH(CH

22))

55CCHCCH

33

(91%)(91%)

Markovnikov's rule followed in formation of enol Markovnikov's rule followed in formation of enol

viavia

CHCH33(CH(CH

22))55CC CHCH22

OHOH

CHCH33(CH(CH

22))55CC CHCH

OO

Regioselectivity Regioselectivity

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ClCl

ClCl

CCClCl22CHCH CHCH

33+ 2+ 2 ClCl

22HCHC CCHCCH33

Addition Reactions; Halogenation; X2

Addition is anti Addition is anti

Br Br 22

CHCH33CHCH22

CHCH22

CHCH33Br Br

Br Br

CHCH33CHCH

22CC CCHCCH22CHCH

33 CC CC

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1.1. OO33

2. H2. H22OO

CHCH33(CH(CH

22))33CC CCHH

++CHCH33(CH(CH

22))33CCOHOH

(51%)(51%)

OO

HOHOCCOHOH

OO

Ozonolysis of AlkynesOzonolysis of Alkynes

gives two carboxylic acids by cleavage of triple bond gives two carboxylic acids by cleavage of triple bond

ung_alkyne

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