Chapter 6. Reactions of Alkynes

7/29/2019 Chapter 6. Reactions of Alkynes

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Organic Chemistry

4th Edition

Paula Yurkanis Bruice

Chapter 6

Reactions

ofAlkynes

Introduction to

Multistep Synthesis

Irene Lee

Case Western Reserve UniversityCleveland, OH

2004, Prentice Hall


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Alkynes are hydrocarbons that contain a carboncarbon

triple bond

General formula: CnH2n2 (acyclic); CnH2n4 (cyclic)


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Nomenclature

In common nomenclature, alkynes are named as

substituted acetylenes


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CH3CHCHC

Cl Br

CCH2CH2CH3

1 2 3 4 5 6 7 8

3-bromo-2-chloro-4-octyne

not 6-bromo-7-chloro-4-octyne

because 2 < 6

A substituent receives the lowest number if there is no

functional group suffix,

or if the same number for the functional group suffix is

obtained in both directions

CH3CHC

CH3

CCH2CH2Br

123456

1-bromo-5-methyl-3-hexyne

not 6-bromo-2-methyl-3-hexyne

because 1 < 2


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The Structure of Alkynes

A triple bond is composed of a s bond and two p bonds


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Addition Reactions of Alkynes


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Sequential Addition of HCl


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An alkyne is less reactive than an alkene


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Relative Stabilities of Carbocations


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Addition of Hydrogen Halides

In the presence of 1 mole equivalent of HBr

CH3CH2C CH CH3CH2C CH

H

Br-

CH3CH2C CH

HBr

CH3CH2C CH

H

CH3CH2CH CH

The secondary vinylic cation

is more stable


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In the presence of excess HBr,

CH3CH2C

Br

CH3 CH3CH2C

Br

CH3

CH3CH2C CH2

Br

CH3CH2CCH3

Br

Br

HBr

a carbocation is formed by adding the electrophile to the

sp2

carbon bonded to the greater number of hydrogen


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A secondary vinylic cation is about as stable as a primary

cation,

therefore a pi-complex may be the actual reactionintermediate

HC CH

H

Cl

+

-

Many (but not all) alkyne addition reactions are

stereoselective


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CH3CH

2C CH CH

3CH

2CH CHBr+ HBr

peroxide


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Addition of Halogens to Alkynes

CH3CH2C CCH3 CH3CH2C CCH3

Cl

Cl

CH3CH2C CCH3

Cl

Cl

Cl

Cl

Cl2

CH2Cl2

Cl2

CH2Cl2

CH3C CH CH3C CH

Br

BrCH3C CCH3

Br

Br Br

Br

Br2

CH2Cl2

Br2

CH2Cl2


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Addition of Water

RCH2 C R

O

RCH C

OH

R

keto tautomer enol tautomer

tautomerization


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Examples of Water Addition


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Hg2+ is added to increase the rate of water addition to

terminal alkynes


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HydroborationOxidation of Internal

Alkynes


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HydroborationOxidation of Terminal

Alkynes

CH3CH2C CH CH3CH

CH3

CH

CH3

2

BH+

CH3CH

CH3

CH

CH3

2

B

C C

H CH2CH3

H

CCH

HO

CH2CH

3

H

CH3CH2CH2CH

O

HO-

, H2O2

H2O


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Formation of Ketone versus Aldehyde


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Addition of Hydrogen

Formation of Cis Alkene


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Conversion of Internal Alkynes to Trans

Alkenes


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Reason for trans addition:

A idi f H d B d d


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Acidity of a Hydrogen Bonded to an sp

Hybridized Carbon

HC CH H2C CH2 CH3CH3

pKa = 25 pKa = 44 pKa = 50

El t ti iti A id


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Electronegativities versus Acid

Strengths

CH3CH2- H2C CH

-H2N

- HC C- HO- F-> > > > >

relative base strength

weakest

basestrongest

base

Th t th id th k it j t b


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The stronger the acid, the weaker its conjugate base

top 252

S th i U i A t lid I


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Synthesis Using Acetylide Ions:

Formation of CC Bond

D i i S th i


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Designing a Synthesis

CH3CH2C CH CH3CH2CCH2CH2CH3

O?

Example 1


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Example 2


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Example 3


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Commercial Use of Ethyne


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Commercial Use of Ethyne

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Chapter 6. Reactions of Alkynes