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Chapter 9- Alkynes: Reactions and Synthesis
Ashley Piekarski, Ph.D.
Alkynes
• What exactly is an alkyne? • Our study of alkynes will provide an introducCon or organic synthesis • the preparation of organic molecules from
simpler organic molecules • we are building up our toolbox of useful
organic reactions! J
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Naming Alkynes
• General hydrocarbon rules apply with “-‐yne” as a suffix indicaCng an alkyne
• Numbering of chain with triple bond is set so that the smallest number possible for the first carbon of the triple bond
Let’s practice!
(Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
(Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
(Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
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Preparation of Alkynes: Elimination Reactions of Dihalides
• Treatment of a 1,2-‐dihalidoalkane with KOH or NaOH produces a two-‐fold eliminaCon of HX
• Vicinal dihalides are available from addiCon of bromine or chlorine to an alkene
vicinal-‐ stands for two funcConal groups bonded to two adjacent atoms
Reactions of Alkynes: Addition of HX and X2
• AddiCon reacCons of alkynes are similar to those of alkenes
• Intermediate alkene reacts further with excess reagent
• Regiospecificity according to Markovnikov
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Electronic Structure of Alkynes
• Carbon-‐carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized pX and py orbitals forming π bonds.
• The remaining sp orbitals form bonds to other atoms at 180º to C-‐C triple bond.
• The bond is shorter and stronger than single or double • Breaking a π bond in acetylene (HCCH) requires 318 kJ/mole
(in ethylene it is 268 kJ/mole)
Addition of Bromine and Chlorine
• IniCal addiCon gives trans intermediate • Product with excess reagent is tetrahalide
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Addition of HX to Alkynes Involves Vinylic Carbocations
• AddiCon of H-‐X to alkyne should produce a vinylic carbocaCon intermediate
• vinyl groups are derivaCves of ethene, with one hydrogen atom replaced with some other group
Learning check
(Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
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Hydration of Alkynes
• AddiCon of H-‐OH as in alkenes • Mercury (II) catalyzes Markovinikov oriented addition • Hydroboration-oxidation gives the non-Markovnikov
product
Mercury(II)-Catalyzed Hydration of Alkynes
• Alkynes do not react with aqueous proCc acids
• Mercuric ion is a Lewis acid catalyst that promotes addiCon of water in Markovnikov orientaCon
• The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone
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Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes
Keto-enol Tautomerism
• Isomeric compounds that can rapidly interconvert by the movement of a proton are called tautomers and the phenomenon is called tautomerism
• 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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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 is not normally useful
• If the triple bond is at the first carbon of the chain (then H is what is agached to one side) this is called a terminal alkyne
• HydraCon of a terminal always gives the methyl ketone, which is useful
Hydroboration/Oxidation of Alkynes
• BH3 (borane) adds to alkynes to give a vinylic borane • OxidaCon with H2O2 produces an enol that converts to the
ketone or aldehyde • Process converts alkyne to ketone or aldehyde with
orientaCon opposite to mercuric ion catalyzed hydraCon
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Comparison of Hydration of Terminal Alkynes
• HydroboraCon/oxidaCon converts terminal alkynes to aldehydes because addiCon of water is non-‐Markovnikov
• The product from the mercury(II) catalyzed hydraCon converts terminal alkynes to methyl ketones
Learning check
(Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
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Learning check (Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
Reduction of Alkynes
• AddiCon of H2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reducCon)
• The addiCon of the first equivalent of H2 produces an alkene, which is more reacCve than the alkyne so the alkene is not observed
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Conversion of Alkynes to cis-Alkenes
• AddiCon of H2 using chemically deacCvated 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)
Alkyne hydrogenation application
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Conversion of Alkynes to trans-Alkenes
• Anhydrous ammonia (NH3) is a liquid below -‐33 ºC • 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 reacCon involves a radical anion intermediate
Mechanism- Conversion of Alkynes to trans-Alkenes
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Learning check
(Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
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
• Neither process is useful in modern synthesis – were used to elucidate structures because the products indicate the structure of the alkyne precursor
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Alkyne Acidity: Formation of Acetylide Anions
• Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic
Alkyne Acidity: Formation of Acetylide Anions
• Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25. See Table 8.1 for comparisons))
• ReacCon of strong anhydrous bases with a terminal acetylene produces an acetylide ion
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Alkyne Acidity: Formation of Acetylide Anions
• The sp-‐hydbridizaCon at carbon holds negaCve charge relaCvely close to the posiCve nucleus
Alkylation of Acetylide Anions
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Alkylation of Acetylide Anions
• Acetylide ions can react as nucleophiles as well as bases
• ReacCon with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes
Limitations of Alkyation of Acetylide Ions
• ReacCons only are efficient with 1º alkyl bromides and alkyl iodides
• Acetylide anions can behave as bases as well as nucleophiles
• ReacCons with 2º and 3º alkyl halides gives dehydrohalogenaCon, converCng alkyl halide to alkene
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Learning check
(Z)-3-methyloct-3-en-5-yne
cycloocta-1,4-diyne
5-ethyldeca-1,9-diyne
1 eq HBr
2 eq Br2
H2O, H2SO4
HgSO4
BH3, THF
H2O2, H2O, NaOH
H
O
O
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 some work as
elegant or beau3ful when it uses novel ideas or combinaCons of steps – this is very subjecCve 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 familiar with reacCons • What they begin with • What they lead to • How they are accomplished • What the limitations are
• A synthesis combines a series of proposed steps to go from a defined set of reactants to a specified product • Questions related to synthesis can include partial
information about a reaction of series that the student completes
Strategies for Synthesis
• Compare the target and the starCng material • Consider reacCons that efficiently produce the outcome.
Look at the product and think of what can lead to it Example • Problem: prepare octane from 1-pentyne • Strategy: use acetylide coupling
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Learning check
• Synthesize 2-‐bromopentane from acetylene
Learning check
• Synthesize butanal from 4-‐octyne
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Learning check
• Synthesize 2-‐heptanone from acetylene
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