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Suratmo Kimia UB 1 Alkuna Based on McMurry’s Organic Chemistry, 7 th edition

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Suratmo Kimia UB 1

Alkuna

Based on McMurry’s Organic Chemistry, 7th edition

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Alkuna

Hidrokarbon yang mengandung karbon-karbonikatan rangkap tigaAsetilena (etuna), merupakan alkuna paling sederhana, dapat dihasilkan dari kalsiumkarbida (karbid) dengan air

Alkuna dapat diubah menjadi senyawa lain.

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Struktur Elektronik AlkunaIkatan Karbon-karbon rangkap 3, triple bond results from sporbital on each C forming a sigma bond and unhybridized pXand 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 doubleBreaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)

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Electronic Structure of AlkynesCarbon-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 doubleBreaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)

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1. Penamaan LakunaSama seperti pada alkena dengan akhiran “una“.Penomoran dimulai dari ujung paling dekat ikatan rangkap 3.Ikatan rangkap tiga lebih dari satu : diuna, triuna dst..Ikatan rangkap dua dan tiga :Double and triple bonds are: enuna.

Penomoran dimulai dari ujung dekat ikatan rangkap (2 atau 3)Ikatan rangkap dua lebih diutamakan

6-metil-3-oktuna 4-metil-7-nonen-1-una

1-hepten-6-una

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2. Pembuatan Alkuna : Reaksi eliminasi DihalidaReaksi 1,2-dihalidoalkana dengan KOH atau NaOH, terjadieliminasi 2 HX (double dehydrohalogenation)Dihalida visinal dapat dihasilkan dari adisi brimin atau klorinpada alkena.

1,2-difenil etena(stilbena)

1,2-dibromo-1,2-difenil etena(dibromida visinal)

KOH, etanol

difenilsetilan

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2. Pembuatan Alkuna : Reaksi eliminasi Dihalida

Vinil halida (vinyl tersubstitusi pada ikatan rangkap C=C)

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3. Reaksi Alkuna : Adisi HX dan X2

Reaksi adisi pada alkuna sama dengan yang bterjadi pada alkena.Intermediat alkena bereaksi dengan reagen yang berlebih.Mengikuti Markovnikov

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Addition of Bromine and Chlorine

Initial addition usually gives trans intermediate

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 about as stable as primary alkyl carbocationsPrimary 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 AlkynesAddition of H-OH as in alkenes

Mercury (II) catalyzes Markovnikov oriented additionHydroboration-oxidation gives the non-Markovnikov product

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8.4 Hydration of AlkynesKeto-enol Tautomerism

Tautomerism = Isomeric compounds that rapidily interconvert by the movement of a proton and are called tautomersEnols 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 Alkynes

Alkynes do not react with aqueous protic acidsMercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation

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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 AlkynesIf 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 usefulIf the triple bond is at the first carbon of the chain (then H is what is attached to one side) this is called a terminal alkyneHydration of a terminal alkyne always gives the methyl ketone, which is useful

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Hydroboration/Oxidation of AlkynesBH3 (borane) adds to alkynes to give a vinylic boraneAnti-Markovnikov

Oxidation with H2O2 produces an enol that converts to the ketone or aldehydeProcess 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 boranesHydroboration/oxidation converts terminal alkynes to aldehydes because addition of water is non-Markovnikov

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Comparison of Hydration of Terminal Alkynes

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 H2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction)The addition of the first equivalent of H2 produces an alkene, which is more reactive than the alkyne so the alkene is not observed

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Conversion of Alkynes to cis-AlkenesAddition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkeneThe 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 ºCAlkali 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 anion intermediate

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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 (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

The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus

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Alkyne Acidity: Formation of Acetylide AnionsTerminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25. See Table 8.1 for comparisons))Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion

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8.8 Alkylation of Acetylide AnionsAcetylide ions can react as nucleophiles as well as bases (see Figure 8-6 for mechanism)

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8.8 Alkylation of Acetylide AnionsReaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes

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Limitations of Alkyation of Acetylide IonsReactions only are efficient with 1º alkyl bromides and alkyl iodidesAcetylide anions can behave as bases as well as nucelophilesReactions with 2º and 3º alkyl halides gives dehydrohalogenation, converting alkyl halide to alkene