John E. McMurry
www.cengage.com/chemistry/mcmurry
Chapter 9
Alkynes: An Introduction to
Organic Synthesis
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Learning Objectives
 Naming alkynes
 Preparation of alkynes: Elimination reactions of
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dihalides
Reactions of alkynes: Addition of HX and X2
Hydration of alkynes
Reduction of alkynes
Oxidative cleavage of alkynes
Alkyne acidity: Formation of acetylide anions
Alkylation of acetylide anions
An introduction to organic synthesis
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Alkynes
 Hydrocarbons that contain carbon–carbon triple
bonds
 Acetylene is the simplest alkyne
 Polyynes - Linear carbon chains of sp-hybridized
carbon atoms
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Naming Alkynes
 General hydrocarbon rules apply
 Suffix -yne indicates an alkyne
 Number of the first alkyne carbon in the chain is used to
indicate the position of the triple bond
 Alkynyl groups are also possible
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Worked Example
 Name the following alkynes:
A)
B)
2,5-Dimethyl-3-hexyne
3,3-Dimethyl-1-butyne
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Table: Priority of Functional Groups in Nomenclature
Compounds with more than one functional group.
Functional Group
Carboxylic acid
Ester
Amide
Nitrile
Aldehyde
Ketone
Alcohol
Thiol
Amine
Alkene
Alkyne
Alkane
Ether
Halides
Nitro
Name as Main Group
–oic acid
–oate
–amide
–nitrile
–al
–one
–ol
–thiol
–amine
–ene
–yne
–ane
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Name as Substituent
Carboxy
Alkoxycarbonyl
Amido
Cyano
Formyl
Oxo
Hydroxy
Thio
Amino
Alkenyl
Alkynyl
Alkyl
Alkoxy
Halo
Nitro
Preparation of Alkynes:
Elimination Reactions of Dihalides
 Treatment of a 1,2-dihaloalkane with KOH or NaOH
produces a two-fold elimination of HX
 Vicinal dihalides are available by addition of bromine
or chlorine to an alkene
 Intermediate is a vinyl halide
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Electronic Structure of Alkynes
 Carbon–carbon triple bond results from sp hybrid
orbitals of carbon forming a σ bond and unhybridized
2py and 2pz orbitals forming two π bonds
 Remaining sp orbitals form bonds to other atoms at
180° from the C–C bond giving linear molecule
 Breaking a π bond in acetylene (HCCH) requires 202
kJ/mole (in ethylene it is 269 kJ/mole)
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Reactions of Alkynes:
Addition of HX and X2
 Addition reactions of electrophiles with alkynes are
similar to those with alkenes
 Regiochemistry according to Markovnikov’s rule
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Reactions of Alkynes: Addition
of HX and X2
 Addition products are formed when bromine and
chlorine are added
 Initial addition gives trans intermediate
 Intermediate alkene reacts further with excess
reagent producing tetrahalide
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Similarities in Alkene and Alkyne
Additions
 Mechanisms of alkene and alkyne additions reactions are
similar but not identical
 Addition of HX to an alkene is a two step process with an
alkyl carbocation as intermediate
 Addition of HX to an alkyne is also a two step process with
vinylic carbocation as the intermediate
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Figure 9.2 - Vinylic Carbocation
 Comprises an sp-hybridized carbon and forms
less readily than an alkyl carbocation
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Worked Example
 Identify the product of following reaction
 Solution:
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Hydration of Alkynes
 Mercury (II)-catalyzed hydration of alkynes
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Alkynes undergo hydration readily in the presence of
mercury (II) sulfate as a Lewis acid catalyst
Reaction occurs with Markovnikov chemistry
Enol intermediate rearranges into a ketone through the
process of keto–enol tautomerism
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Keto-Enol Tautomerism
 Enols rearrange to the isomeric ketone by the rapid transfer of
a proton from the hydroxyl to the alkene carbon
 Isomeric compounds that can rapidly interconvert by the
movement of a proton are called tautomers
 The keto form is usually so stable compared to the enol that
only the keto form can be observed
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Figure 9.3 - Mechanism of Mercury
(II)-Catalyzed Hydration of Alkyne to
Yield a Ketone
 Addition of Hg(II) to
alkyne gives a vinylic
cation
 Water adds and loses
a proton
 A proton from aqueous
acid replaces Hg(II)
 keto–enol tautomerism
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Hydration of Unsymmetrical Alkynes
 Hydration of an unsymmetrically substituted internal
alkyne (RC≡CR’) results in a mixture of both possible
ketones
 Hydration of a terminal alkyne always gives the
methyl ketone
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Worked Examples
 What products are obtained by hydration of:
 Solution:
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This symmetrical alkyne yields one product
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Hydroboration-Oxidation of Alkynes
 Borane adds to alkynes to give a vinylic borane
 Oxidation with H2O2 produces an enol that converts to
the ketone or aldehyde by tautomerization
 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
 Mercury(II)-catalyzed hydration is Markovnikov and
converts terminal alkynes to methyl ketones
 Hydroboration-oxidation converts terminal alkynes to
aldehydes because the addition is non-Markovnikov
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Worked Examples
 Name an alkyne that can be used in the preparation
of the following compound by a hydroborationoxidation reaction
 Solution:
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Reduction of Alkynes
 Accomplished by addition of H2 over a metal catalyst
(such as palladium on carbon, Pd/C). It is a two-step
reaction
 The addition of the first equivalent of H2 produces an
alkene intermediate
 The alkene immediately reacts with another equivalent
of H2 and yields alkane
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Reduction of Alkynes to cis-Alkenes
 Addition of H2 using deactivated palladium on carbon
catalyst (poisoned with traces of lead or CaCO3 and
quinolone, called Lindlar catalyst) produces a cis alkene
 The two hydrogens add syn (from the same side of the triple
bond
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Reduction of Alkynes to trans-Alkenes
 Alkynes can also be converted to alkenes using
sodium or lithium metal as the reducing agent in
liquid ammonia as the solvent (-33 ºC), called
metal-ammonia catalyst.
 This method produces trans alkenes
 The reaction involves a radical anion intermediate
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Mechanism of Li/NH3 Reduction of an Alkyne
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Worked Example
 Use any alkyne to prepare trans-2-Octene
Solution:
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Using the correct reducing agent results in a double
bond with the desired geometry (cis or trans)
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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
 The method was used to elucidate structures
because the products indicate the structure of the
alkyne such as internal or terminal
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Alkyne Acidity:
Formation of Acetylide Anions
 Terminal alkynes are weak Brønsted acids, pKa ~ 25
(alkenes, pKa ~ 44 and alkanes, pKa ~ 60, are much
less acidic)
 Reaction of a strong base with a terminal alkyne
results in the removal of the terminal hydrogen and
the formation of an acetylide anion
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Acidity of Hydrocarbons
 Methane and ethylene
do not react with a
common base
 Acetylene can be
deprotonated by any
anhydrous base with a
pKa higher than 25
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Acidity is Based on the
Percentage of s Character
 Percentage of “s character” is based on the sp-
hybridized carbon atom
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Acetylide anions possess an sp-hybridized carbon
with 50% s character
Vinylic anions have an sp2-hybridized carbon with
33% s character
Alkyl anions have an sp3-hybridized carbon with 25%
s character
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Comparison of Alkyl, Vinylic, and
Acetylide Anions
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Worked Example
 The pKa of acetone, CH3COCH3, is 19.3
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Which of the following bases with given solvents is
strong enough to deprotonate acetone?
A) KOH/H2O (pKa of H2O = 15.7)
B) Na+ -C≡CH/NH3 (pKa of C2H2 = 25, NH3 = 36)
C) NaHCO3/H2O (pKa of H2CO3 = 6.4)
D) NaOCH3/CH3OH (pKa of CH3OH = 15.6)
 Solution:
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Na+ -C≡CH adequately deprotonates acetone
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Alkylation of Acetylide Anions
 Acetylide anion can react as nucleophile as well as base due to
the negative charge and unshared electron pair on carbon
 Reaction of acetylide anion with a primary alkyl halide produces
a terminal alkyne providing a general route to larger alkynes.
Further interaction of terminal alkyne with an alkyl halide gives
an internal alkyne product
Acetylide anion
Alkynyl anion
 Any terminal alkyne can be used for making larger alkynes
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Alkylation Reaction of Acetylide Anion
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Alkylation of Acetylide Ions
 Acetylide ions cause elimination instead of
substitution upon reaction with secondary and
tertiary alkyl halides
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Worked Example
 Prepare cis-2-butene using propyne, an alkyl halide
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Use any other reagents needed
 Solution:
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Hydrogenation of an alkyne, which can be synthesized
by alkylation of a terminal alkyne, forms a cis bond
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Introduction to Organic
Synthesis
 Organic synthesis is vital to pharmaceutical
industries, chemical industries, and academic
laboratories
 Planning a successful multistep synthetic sequence
involves:
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Utilizing available knowledge of chemical reactions
Organizing knowledge into a workable plan
 Working in a retrosynthetic direction is important
in planning an organic synthesis
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Working backward
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Worked Example
 Use 4-Octyne as the only source of carbon in the
synthesis of Butanal
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Use any inorganic compounds necessary
 Solution:
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Butanal can be synthesized from either cis-4-octene
or trans-4-octene
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Worked Example
 Synthesize decane using an alkyne and any alkyl
halide needed
 Solution:
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Using retrosynthetic logic:
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H2/Pd can reduce 1-decyne C8H17C≡CH to decane C10H22
Alkalylation of HC≡C:-Na+ by C8H17Br, 1-bromooctane
produces C8H17C≡CH
Treatment of HC≡CH with NaNH2, NH3 gives HC≡C:-Na+
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Summary
 Alkynes are hydrocarbons comprising a carbon–carbon
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triple bond
Enols are formed when alkynes react with aqueous
sulfuric acid in the presence of mercury (II) catalyst
Tautomerization is a process in which an enol yields a
ketone
Reduction of alkynes can produce alkanes and alkenes
Acetylide anions are formed when a strong base removes
alkyne hydrogen
In an alkylation reaction, an acetylide anion acts as a
nucleophile and displaces a halide ion from a primary
alkyl halide
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