Chapter 9 Notes - DR. SCHAEFAR PDF

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Chapter 9. Alkynes: An Introduction to Organic Synthesis •

CC (triple bonds)

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C is sp hybridized – linear, 180° bond angle, 2 p bonds – CC is shorter and stronger than C-C and C=C

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Most reactions are similar to those of alkenes

9.1 Naming Alkynes •

“-yne” is the suffix

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Smallest number on the first carbon of the alkyne

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Compounds with both double and triple bonds are called “enynes” – Tiebreaker: double bond gets lower number

9.2 Preparation of Alkynes: Elimination Reactions of Dihalides •

1,2-dihaloalkane (vicinal dihalide) with strong base gives double elimination of HX – Alkene + X2 → Vicinal dihalide

Preparation of Alkyne by double elimination •

Can stop after 1 elimination

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Preparation of Alkynes from Vinyl Halide • •

Elimination of H-X from a vinyl halide yields an alkyne Reagents: 2 eq NaNH2, then H3O+

8.3 Reactions of Alkynes: Addition of HX and X2 •

Addition reactions similar to alkenes

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Intermediate alkene reacts further with excess reagent

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Regiospecificity is Markovnikov

Addition of HX to Alkynes: Vinylic Carbocations •

Addition of H-X to alkyne produces a vinylic C+ intermediate – 2° vinyl C+ is more stable than a 1° vinyl C+

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Get Markovnikov addition

Addition of X2 to Alkynes •

X2 adds in similar fashion (halonium ion) – One addition gives a trans-alkene – Two additions give a tetrahaloalkane (4 halogens)

Practice Questions: Which products would you expect from the following reactions?

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8.4 Hydration of Alkynes Addition of H-OH •

Alkynes do not react with water, protic acids – No H2SO4, H2O, heat!

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Mercury (II) gives Markovnikov addition

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Hydroboration-oxidation gives non-Markovnikov product

Mercury(II)-Catalyzed Hydration of Alkynes •

Hg+2 (as HgSO4) promotes Markovnikov addition of water

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The first product is a vinylic alcohol, or enol, which transforms to a ketone (keto form)

Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes •

Addition of Hg(II) to alkyne gives a vinylic cation (no Mercurium ion!)

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Water adds and loses a proton

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A proton from aqueous acid replaces Hg(II) (no NaBH4)

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Keto-enol Tautomerism •

Isomers that interconvert by the movement of a proton are tautomers (tautomerism )

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Enols rearrange to ketones

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The keto- form is usually more stable, isolated product

Hydration of Unsymmetrical Alkynes •

Hg- hydration of an asymmetric internal alkyne gives 2 different products

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Hg- hydration of a terminal alkyne always gives a methyl ketone

(Mercury, Markovnikov, Methyl ketone)

Hydroboration/Oxidation of Alkynes •

Same reaction as on alkenes only with the tautomerizaion step

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non-Markovnikov addition

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Get ketone (internal) or aldehyde (terminal)

Comparison of Hydration of Terminal Alkynes •

Hg+2 hydration converts terminal alkynes to methyl ketones

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Hydroboration/oxidation converts terminal alkynes to aldehydes

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Practice Question How would you prepare the following carbonyl compounds starting from a terminal alkyne?

9.5 Reduction of Alkynes •

Addition of H2 with metal catalyst (Pd/C) converts alkynes to alkanes – 2 eq H2 added

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First equivalent of H2 produces an alkene – Alkenes are more reactive than alkynes so the reaction doesn’t stop

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Addition of H2 with a weaker catalyst gives a cis-alkene – Pd on CaCO3 with Pb(OAc)2 (Lindlar catalyst)

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Addition is syn

Conversion of Alkynes to trans-Alkenes •

Ammonia (NH3) is a liquid below -33 °C – Alkali metals dissolve in NH3 , act as reducing agents – “Dissolving metal” reduction (Li, Na)

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Yields trans- alkenes

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The reaction involves a radical anion intermediate

Lithium/Ammonia Reduction of an Alkyne •

Radical process – Metal gives 1 e– NH3 gives H

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2 eq. Na and 2 eq. NH3 are consumed

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9.6 Oxidative Cleavage of Alkynes •

Oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acids –

For alkenes, O3 gives ketones/aldehydes

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Terminal alkynes yield carboxylic acid and CO2

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Historically important, little practical use

9.7 Alkyne Acidity: Formation of Acetylide Anions •

Terminal alkynes are weak Brønsted acids (donate H+)

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Reaction of a terminal alkyne with strong anhydrous base produces an acetylide anion

Why are terminal alkynes more acidic than alkenes or alkanes? – More “s character” on alkyne C, e- closer to nucleus so the anion is more stable

Acetylide ions react as nucleophiles in substitution reactions C-C bond forming reaction More discussion of reaction mechanism in Chapter 11

Alkylation Reactions with Acetylide Anions •

Reaction with 1 ° alkyl halide gives a longer alkyne – Terminal becomes internal

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Limitations of Alkylation Reaction •

Acetylide alkylation reactions only work with 1° alkyl bromides and iodides

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Reactions with 2° and 3° alkyl halides give elimination – Acetylide anions are also bases

Sample Questions Show the terminal alkyne and alkyl halide from which the following products can be obtained.

9.9 An Introduction to Organic Synthesis •

Organic synthesis creates molecules by design – Produces new molecules that are needed as drugs or materials – Improves efficiency and safety over existing syntheses – Tests ideas and methods, answering challenges

Synthesis as a Tool for Learning Organic Chemistry •

Must know reactions first – Starting materials (functional groups) – Products obtained – How they happen (mechanism) –

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A synthesis is several reaction steps to lead from a defined reactant to a specified product

– Don’t look at the starting material and ask “What reactions might this undergo?” –

Look at the product and ask, “What is an immediate precursor of this product?”

Strategies for Synthesis •

Compare target and starting material

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Consider reactions that produce the outcome. – Look at the product and ask “what can lead this?”

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Example – Prepare octane from 1-pentyne

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Page 9 of 10

Practice Example How would you prepare the following alkyl halide from acetylene and an alkyl halide?

Working Backwards:

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Chapter 9 Summary •

Alkynes: CC sp hybridized C atom, 2  bonds.

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Preparation:

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Double elimination of H-X on a vicinal dihalide

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Alkylation of an acetylide anion R-CC-

Reactions: Electrophilic addition, most are similar to those of alkenes –

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Hydration Reactions, useful for terminal alkynes: •

HgSO4, H2SO4 gives ketones after tautomerization

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BH3, H2O2 gives aldehydes after tautomerization

Reduction: several methods •

H2, Pd/C: yields alkane

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H2/Lindlar: yields cis-alkene

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Li, NH3: yields trans-alkene

Alkyne anion (acetylide anion) terminal alkynes are weakly acidic •

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Alkylation of acetylide anion: •

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Terminal alkyne, NaNH2/NH3

Reaction with 1° alkyl iodide or alkyl bromide

Retrosynthesis: Designing a synthesis by working backwards from target to a simpler starting material

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