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

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1 Alkuna Based on McMurry s Organic Chemistry, 7 th edition 1

2 Alkuna Hidrokarbon yang mengandung karbon-karbon ikatan rangkap tiga Asetilena (etuna), merupakan alkuna paling sederhana, dapat dihasilkan dari kalsium karbida (karbid) dengan air Alkuna dapat diubah menjadi senyawa lain. 2

3 Struktur Elektronik Alkuna Ikatan Karbon-karbon rangkap 3, triple bond results from sp orbital on each C forming a sigma bond and unhybridized p X and p y 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) 3

4 Electronic Structure of Alkynes Carbon-carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized p X and p y 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) 4

5 1. Penamaan Lakuna Sama 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 5

6 2. Pembuatan Alkuna : Reaksi eliminasi Dihalida Reaksi 1,2-dihalidoalkana dengan KOH atau NaOH, terjadi eliminasi 2 HX (double dehydrohalogenation) Dihalida visinal dapat dihasilkan dari adisi brimin atau klorin pada alkena. 1,2-difenil etena (stilbena) 1,2-dibromo-1,2-difenil etena (dibromida visinal) KOH, etanol difenilsetilan 6

7 2. Pembuatan Alkuna : Reaksi eliminasi Dihalida Vinil halida (vinyl tersubstitusi pada ikatan rangkap C=C) 7

8 3. Reaksi Alkuna : Adisi HX dan X 2 Reaksi adisi pada alkuna sama dengan yang bterjadi pada alkena. Intermediat alkena bereaksi dengan reagen yang berlebih. Mengikuti Markovnikov 8

9 Addition of Bromine and Chlorine Initial addition usually gives trans intermediate Can often be stopped at this stage if desired (1 eq. Br 2 ) Product with excess reagent is tetrahalide 9

10 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 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 10

11 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 11

12 8.4 Hydration of Alkynes Keto-enol Tautomerism Tautomerism = Isomeric compounds that rapidily interconvert by the 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 12

13 Mercury(II)-Catalyzed Hydration of Alkynes Alkynes do not react with aqueous protic acids Mercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation 13

14 Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone 14

15 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 attached to one side) this is called a terminal alkyne Hydration of a terminal alkyne always gives the methyl ketone, which is useful 15

16 Hydroboration/Oxidation of Alkynes BH 3 (borane) adds to alkynes to give a vinylic borane Anti-Markovnikov Oxidation with H 2 O 2 produces an enol that converts to the ketone or aldehyde Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration 16

17 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 17

18 Comparison of Hydration of Terminal Alkynes The product from the mercury(ii) catalyzed hydration converts terminal alkynes to methyl ketones 18

19 8.5 Reduction of Alkynes Addition of H 2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction) The addition of the first equivalent of H 2 produces an alkene, which is more reactive than the alkyne so the alkene is not observed 19

20 Conversion of Alkynes to cis-alkenes Addition of H 2 using 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 20

21 Conversion of Alkynes to trans-alkenes Anhydrous ammonia (NH 3 ) 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 reaction involves a radical anion intermediate 21

22 The trans stereochemistry is less sterically crowded and is formed in this step 22

23 8.6 Oxidative Cleavage of Alkynes Strong oxidizing reagents (O 3 or KMnO 4 ) 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 23

24 Alkyne Acidity: Formation of Acetylide Anions The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus 24

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

26 8.8 Alkylation of Acetylide Anions Acetylide ions can react as nucleophiles as well as bases (see Figure 8-6 for mechanism) 26

27 8.8 Alkylation of Acetylide Anions Reaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes 27

28 Limitations of Alkyation of Acetylide Ions Reactions only are efficient with 1º alkyl bromides and alkyl iodides Acetylide anions can behave as bases as well as nucelophiles Reactions with 2º and 3º alkyl halides gives dehydrohalogenation, converting alkyl halide to alkene 28

Electrophilic Addition Reactions

Electrophilic Addition Reactions Electrophilic addition reactions are an important class of reactions that allow the interconversion of C=C and C C into a range of important functional groups. Conceptually,