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Alkynes

Alkynes-hydrocarbons with a carbon-carbon triple bond.

The carbon-carbon triple bond results from the interaction of two sp hybridized carbon

atoms. 180 degree angle. Linear.The carbon-carbon triple bond is the shortest and strongest known

carbon-carbon bond. Why? It has the highest percentage of s character.

Type Hybridization % s character % p character

Alkane sp3  25 75

Alkene sp2  33 67

Alkyne sp 50 50

 Nomenclature

Alkynes are named like alkenes with the following adjustments: 1) Number the carbons

on the chain so that the triple bonded carbons receive the lowest possible numbers, 2)Indicate the location of the triple bond by writing the number of the lowest numbered

carbon making up the triple bond in front of the name of the parent chain. 3) Change theending from –ene to –yne.

Reactions to KnowPreparation of Alkynes

Synthesis of Alkynes:

1. Dehydrohalogenation of vicinal dihalides:

C C

R

H

Br 

R

HBr 

2 Alc. KOHC C RR

 

 Note: vic-dihalides are prepared from alkenes. Thus, in effect, this is a conversion ofalkene to alkyne.

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Reactions of Alkynes

1. Hydrohalogenation:

C CR R2 HX

C C

H

H

R

X

X

R

 

(Geminal Dihalide)

2. Mercury (II)-Catalyzed Hydration:

C CR H

H2O, H2SO4

  HgSO4C CR H

OH

H

C

H2

CR CH3

O

  Enol  (Ketone)

The enol is an intermediate in the reaction. However, it immediately rearranges to formthe a ketone through a process called keto-enol tautomerism.

3. Hydroboration-Oxidation: (Internal alkynes will give a ketone; terminal alkyneswill give an aldehyde).

C CR R   C CR R

OH

H

  Enol

C C

H2

R R

1) BH3

 2) H2O2, OH

O

 (Ketone)

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As in the mercury-catalyzed reaction, the enol is an intermediate which quickly

undergoes keto-enol tautomerism to form the ketone.

C CR R   C C

H

R

BR2

R

C C

H2

R R

O

C CR R

OH

H

  Enol

1) BH3  2) H2O2, OH

 Examples:

a) Internal alkyne to ketone

C CH3C CH3   C CH3C CH3

OH

H

  Enol

C C

H2

H3C CH3

1) BH3

 2) H2O2, OH

O

  b) Terminal alkyne to aldehyde

C CH CH3   C CH CH3

OH

H

  Enol

C C

H2

H CH3

1) BH3

 2) H2O2, OH

O

 4. Hydrogenation:

C CR R  2 H2/ Pd

C CR R

H

H

H

H

(Alkane)

 

5. Hydrogenation Using Lindlar Catalyst:

C CR R  H2

C C

HH

R R

(Cis Alkene)Lindlar's cat.

 

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6. Hydrogenation Using Lithium and Ammonia (Na can be used instead of Li):

C CR R Li, NH3

C C

RH

R   H

(Trans Alkene)

 

7. Cleavage of Internal Alkynes:

C CR R'

O3C

O

R   OH   C

O

R'   OH

KMnO4 or  +

(Carboxylic Acid)  

8. Cleavage of Terminal Alkynes:

C CR H

O3C

O

R   OH CO2

KMnO4 or  +

(Carboxylic Acid and CO2)  

9. Alkylation Using Acetylide Ions:

C CR H1) NaNH2, NH3

 2) R'CH2Br C CR   CH2R'

 

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Practice Problems:

1) What are the products of the following reaction:a)

C CH3C CH2CH3 Li, NH

3

?

  Answer

 b)

C CH3C H

O3

KMnO4 or 

 Answer

c)

C CH CH2CH3

1) BH3

 2) H2O2, OH

?

 Answer

Alkyne Acidity: Forming Acetylide Anions

Terminal alkynes are weakly acidic.Acidity of simple hydrocarbons:

Alkynes > Alkenes> Alkanes

Why are terminal alkynes more acidic than alkenes or alkanes? Acidity depends upon thestability of the conjugate base. Why are acetylide anions more stable than vinylic or alkylanions?

1) Hybridization of the negatively charged carbon atom:a. Acetylide anion has an sp hybridized carbon. Therefore,the negative charge resides in an orbital that has 50% s

characteristic.

 b. Vinylic anion ' sp2 hybridized. 33% s character.c. Alkyl anion ' sp3 hybridized. 25% s character.

Since s orbitals are nearer the positive nucleus and are lower in energy than p orbitals, thenegative charge is stabilized to a greater extent.

1. Formation of Acetylide Anions

RC C H  NH2 Na+

RC C  Na+  NH3Acetylide anion  

Acetylide Anions are important to organic synthesis because they can be used to make a

carbon chain longer.

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2. Alkylation of Acetylide Anions

RC C + 1° R X ! RC CR  

You’ll learn the mechanism for this reaction in a later chapter.

Practice Synthesis Problem:

2) How could you prepare

C C

H3CH2C

H

CH3

H from H3CH2CC CH and any

needed reagents?

Answer

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Answer

1) What are the products of the following reaction:a)

C CH3C CH2CH3 Li, NH

3

?

 

C CH3C CH2CH3 Li, NH3

C C

CH2CH3

HH3C

H

 Return to Problem

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Answer

1) What are the products of the following reaction: 

 b)

C CH3C H

O3

KMnO4 or 

 

C CH3C H

O3

H3C C OH

O

CO2KMnO4 or 

 Return to Problem

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Answer

1) What are the products of the following reaction: 

c)

C CH CH2CH3 1) BH3

 2) H2O2, OH?

 

C CH CH2CH3   HC CH2CH2CH3

O

1) BH3

 2) H2O2, OH  

Return to Problem

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Answer:

2) How could you prepare

C C

H3CH2C

H

CH3

H from H3CH2CC CH and any

needed reagents?

BrCH3

H3CH2CC CCH3

H3CH2CC C

H2

 NaNH2H3CH2CC CH

C C

H3CH2C

H

CH3

H Lindlar's Catalyst 

Return to Problem