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8. Alkynes: AnIntroduction to Organic
Synthesis
Based on McMurrys Organic Chemistry, 7thedition
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Alkynes
Hydrocarbons that contain carbon-carbon
triple bonds
Acetylene, the simplest alkyne is produced
industrially from methane and steam at hightemperature
Our study of alkynes provides an introduction
to organic synthesis, the preparation of
organic molecules from simpler organic
molecules
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Why this chapter?
We will use alkyne chemistry to begin looking
at general strategies used in organic
synthesis
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8.1 Naming Alkynes
General hydrocarbon rules apply with -yne as a suffix
indicating an alkyne
Numbering of chain with triple bond is set so that the smallest
number possible for the first carbon of the triple bond
Multiple triple bonds are: diynes, triynes, etc Double and triple bonds are: enynes
Number nearest a multiple bond (either double or triple)
If you have a choice, double bond lower number than triple
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8.2 Preparation of Alkynes: Elimination
Reactions of Dihalides Treatment of a 1,2-dihalidoalkane with KOH or NaOH produces a two-fold
elimination of HX (double dehydrohalogenation) Vicinal dihalides are available from addition of bromine or chlorine to an
alkene
Intermediate is a vinyl halide (vinylsubstituent = one attached to C=C)
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Electronic Structure of Alkynes
Carbon-carbon triple bond results from sp orbital oneach C forming a sigma bond and unhybridized pXand pyorbitals forming bonds.
The remaining sp orbitals form bonds to other atomsat 180 to C-C triple bond.
The bond is shorter and stronger than single ordouble
Breaking a bond in acetylene (HCCH) requires 318kJ/mole (in ethylene it is 268 kJ/mole)
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Addition of Bromine and Chlorine
Initial addition usually gives transintermediate
Can often be stopped at this stage if desired (1 eq. Br2)
Product with excess reagent is tetrahalide
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Addition of HX to Alkynes Involves
Vinylic Carbocations
Addition of H-X to alkyne
should produce a vinylic
carbocation intermediate
Secondary vinyl
carbocations are aboutas stable as primary
alkyl carbocations
Primary vinyl
carbocations probably
do not form at all
Nonethelss, H-Br can add
to an alkyne to give a vinyl
bromide if the Br is not on
a primary carbon
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8.4 Hydration of Alkynes Addition of H-OH as in alkenes
Mercury (II) catalyzes Markovnikov oriented addition
Hydroboration-oxidation gives the non-Markovnikov product
Keto-enol Tautomerism
Tautomerism= Isomeric compounds that rapidily interconvert bythe movement of a proton and are called tautomers
Enols rearrange to the isomeric ketone by the rapid transfer of a
proton from the hydroxyl to the alkene carbon The keto form is usually so stable compared to the enol that only
the keto form can be observed
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Mercury(II)-Catalyzed Hydration of AlkynesAlkynes do not react with aqueous protic acids
Mercuric ion (as the sulfate) is a Lewis acid catalyst
that promotes addition of water in Markovnikovorientation
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Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes
The immediate product is a vinylic alcohol, or enol, which spontaneously
transforms to a ketone
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Hydration of Unsymmetrical Alkynes If the alkyl groups at either end of the C-C triple bond
are not the same, both products can form and this isnot normally useful
If the triple bond is at the first carbon of the chain(then H is what is attached to one side) this is calleda terminal alkyne
Hydration of a terminal alkyne always gives themethyl ketone, which is useful
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Hydroboration/Oxidation of Alkynes BH3(borane) adds to alkynes to give a vinylic borane
Anti-Markovnikov
Oxidation with H2O2produces an enol that converts to the
ketone or aldehyde
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 Unhindered terminal alkynes add two boranes
Hydroboration/oxidation converts terminal alkynes to aldehydes
because addition of water is non-Markovnikov
The product from the mercury(II) catalyzed hydration converts
terminal alkynes to methyl ketones
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8.5 Reduction of Alkynes
Addition of H2over a metal catalyst (such as
palladium on carbon, Pd/C) converts alkynes to
alkanes (complete reduction)
The addition of the first equivalent of H2produces an
alkene, which is more reactive than the alkyne so the
alkene is not observed
C i f Alk t i Alk
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Conversion of Alkynes to cis-Alkenes Addition of H2using chemically deactivated palladium on calcium
carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene
The two hydrogens add syn(from the same side of the triple bond)
The Lindlar Catalyst will not reduce double bonds
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Conversion of Alkynes to trans-Alkenes
Anhydrous ammonia (NH3) is a liquid below -33 C
Alkali metals dissolve in liquid ammonia and function
as reducing agents
Alkynes are reduced to trans alkenes with sodium or
lithium in liquid ammonia
The reaction involves a radical anionintermediate
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The trans stereochemistry is less sterically
crowded and is formed in this step
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8.6 Oxidative Cleavage of Alkynes
Strong oxidizing reagents (O3or KMnO4) cleave internal
alkynes, producing two carboxylic acids
Terminal alkynes are oxidized to a carboxylic acid and carbondioxide
Neither process is useful in modern synthesiswere used to
elucidate structures because the products indicate the
structure of the alkyne precursor
Alk A idit F ti f A t lid A i
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Alkyne Acidity: Formation of Acetylide Anions
Terminal alkynes are weak Brnsted acids (alkenes and alkanes are muchless acidic (pKa~ 25. See Table 8.1 for comparisons))
Reaction of strong anhydrous bases with a terminal acetylene produces an
acetylide ion The sp-hydbridization at carbon holds negative charge relatively close to the
positive nucleus
8 8 Alk l ti f A t lid A i
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8.8 Alkylation of Acetylide Anions
Acetylide ions can react as nucleophiles as well as bases(see Figure 8-6 for mechanism)
Reaction with a primary alkyl halide produces a hydrocarbonthat contains carbons from both partners, providing ageneral route to larger alkynes
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Limitations of Alkyation of Acetylide Ions Reactions only are efficient with 1 alkyl bromides and alkyl iodides
Acetylide anions can behave as bases as well as nucelophiles
Reactions with 2 and 3 alkyl halides gives dehydrohalogenation,
converting alkyl halide to alkene
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8.9 An Introduction to Organic
Synthesis
Organic synthesis creates molecules by design
Synthesis can produce new molecules that are
needed as drugs or materials
Syntheses can be designed and tested to improve
efficiency and safety for making known molecules
Highly advanced synthesis is used to test ideas and
methods, answering challenges
Chemists who engage in synthesis may see somework as elegant or beautifulwhen it uses novel ideas
or combinations of stepsthis is very subjective and
not part of an introductory course
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Synthesis as a Tool for Learning
Organic Chemistry
In order to propose a synthesis you must be familiarwith reactions
What they begin with
What they lead to
How they are accomplished What the limitations are
A synthesis combines a series of proposed steps togo from a defined set of reactants to a specifiedproduct
Questions related to synthesis can include partialinformation about a reaction of series that the studentcompletes
St t i f S th i
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Strategies for Synthesis
Compare the target and the starting material
Consider reactions that efficiently produce the
outcome. Look at the product and think of what canlead to it Example
Problem: prepare octane from 1-pentyne
Strategy: use acetylide coupling