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Chemical Reactions of Alkynes

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Chemical Reactions of Alkynes CHEMICAL REACTIONS OF ALKYNES 04-04-2020 SKS 1. Electrophilic Addition Reactions a. Electrophilic Addition of Bromine and Chlorine to Alkynes Alkynes show the same kind of addition reactions with chlorine and bromine that alkenes do. With alkynes the addition may occur once or twice, depending on the number of molar equivalents of halogen we employ: Anti-addition takes place: mechanism is same as the addition of Cl2 / Br2 takes place on alkenes. 04-04-2020 SKS b. Addition of Hydrogen Halides to Alkynes Alkynes react with one molar equivalent of hydrogen chloride or hydrogen bromide to form haloalkenes, and with two molar equivalents to form geminal dihalides. Both additions are regioselective and follow Markovnikov’s rule: 04-04-2020 SKS Mechanism of Electrophilic Addition of Hydrogen Halides to Alkynes: R1 R1 H - R1 R2 + H X or + X H R R 1 mole 2 2 A vinylic cation (Search for the more stable carbocation) X R X R1 1 X H R1 X or R1 H H H H R2 H R2 H R2 X R2 Resonance stabilized + X- R1 X H X H R2 04-04-2020 SKS Q1: The addition of HCl to 3,3-dimethyl-1-butyne gives the following products: 2,2-dichloro-3,3- dimethylbutane (44 %), 2,3-dichloro-2,3-dimethylbutane (18 %) and 1,3-dichloro-2,3-dimethylbutane (34 %). Account in detail for the formation of these products. Q2. The addition of 1.0 equivalent and excess HBr to 1-butyne gives 2-bromo-1-butene and 2,2- dibromobutane (a geminal dihalide), respectively. Account in detail for the formation of these products. Q3. Write the structure of the product along with the stereochemistry when 1.0 equivalent of HCl is reacted with 2-butyne. Give explanation. Q4. Give the mechanism for the formation of 1-bromo-1-butene, when 1 equivalent of HBR is reacted with 1-butyne in the presence of organic peroxide. 04-04-2020 SKS c. Addition of H2O Like alkenes, alkynes can also be hydrated. This is an example of electrophilic addition reaction. Alkynes undergo acid-catalysed addition of H2O. Terminal alkynes are less reactive than the internal alkyne towards the addition of H2O. Terminal alkynes will add water if Hg2+ is added to the acidic mixture. The Hg2+ acts as a catalyst to increase the rate of the addition reaction. Acetylene give acetaldehyde, whereas other alkynes (both internal and terminal) give ketone. 04-04-2020 SKS Mechanism Practice Problem: Q. Write the mechanism of hydration of ethyne, 1- butyne and 2-butyne. enol H H H H Ketone 04-04-2020 SKS 2. Nucleophilic addition reactions: The sp-hybridised carbon atoms of the triple-bond render alkynes more electrophilic than similarly substituted alkenes because the electronegativity of sp-hybridized orbital is more than sp2, hence pi- electrons will be more close to the carbon nucleus. - + C2H5O K HC CH H2C C OC2H5 in C2H5OH H at high temp HCN Cat. NaCN H2C C CN H Practice problem: Write the chemical reaction involved when acetylene is reacted with C2H5S-Na+ in ethanol. 04-04-2020 SKS 3. Reduction of Alkynes Reduction using Na/ Liq. NH3 (Birch Reduction): Na / Liq. NH3 or Li / Liq. NH3 is used to carry out reduction of alkynes into alkenes. Alkynes are selectively converted into trans-alkenes. Na / liq NH3 is the source of ammoniated electrons. Na in ethanol can also be used. 2-Butyne trans-2-Butene 04-04-2020 SKS Mechanism trans-Vinylic radical anion is more stable as the –ve charge and radical are at the anti- position, so no electronic repulsion. OR Mechanism 04-04-2020 SKS Reduction using Lindlar’s catalyst H2/ Pd(BaSO4) H2/ Pd(BaSO4) partially deactivated (poisoned) by quinoline is used to carry out reduction of alkynes into cis-alkenes. Alkynes are selectively converted into cis-alkenes. syn-addition of H2 takes place. 04-04-2020 SKS Mechanism: The quinoline serves to prevent complete hydrogenation of the alkyne to an alkane. N N H 04-04-2020Quinoline SKS 1,2-Dihydroquinoline Hydroboration-oxidation of alkynes: Hydroboration of internal alkynes is not a particularly useful procedure because a mixture of products will often be obtained, unless the triple-bond is symmetrically substituted. O 1. BH3 in THF H3C C C CH3 2. H2O2 / NaOH Disiamylborane is preferred over BH3 to 04-04-2020 SKS avoid the possibility of further addition Oxidative Cleavage of Alkynes Treating alkynes with hot basic potassium permanganate followed by acid, leads to cleavage at the carbon– carbon triple bond. The products are carboxylic acids. Whereas under neutral condition it gives 1,2-diketo compounds (in case of internal alkyne), b-keto acids (in case of terminal alkynes), respectively. Alkaline condition O O - hot R1 C O R1 C OH - O O - + KMnO / OH KMnO / OH + H3O + R C C R 4 4 1 2 R1 C C R2 O O - R2 C O R2 C OH hot - O O - O O KMnO4 / OH KMnO4 / OH R C C H R C C H R C C O- - KMnO4 / OH O + O H3O R C OH + CO2 R C O- + CO2 04-04-2020 SKS O Ozonolysis of alkynes KMnO4 /H2O R C C R R R Neutral Condition O O R C OH O 1 3 + O R1 C C R2 KMnO4 /H2O dil. CH3COOH O R C C H OH R R C OH Neutral Condition or H2O 2 O O O3 H3C C C CH3 H3C C OH dil. CH3COOH 2 moles or H2O O O3 H3C C C H H3C C OH + CO2 + H2O dil. CH3COOH or H2O 04-04-2020 SKS Distinction between terminal and internal alkynes Terminal alkynes react with Tollens reagent (ammoniacal silver nitrate solution) to form silver alkynides as a white precipitate, whereas internal alkynes do not react with Tollens reagent. + - R1 C C H + [Ag (NH3)2] OH R1 C C Ag + 2NH3 + H2O Tollens reacgent Silver alkynide White ppt + - R1 C C R2 + 2 [Ag (NH3)2] OH No Reaction Tollens reacgent + - H C C H + 2 [Ag (NH3)2] OH Ag C C Ag + 4NH3 + 2H2O Tollens reacgent Silver alkynide White ppt 04-04-2020 SKS + - R1 C C H + [Cu (NH3)2] OH R1 C C Cu + 2NH3 + H2O Tollens reacgent Silver alkynide Red ppt + - R1 C C R2 + [Cu (NH3)2] OH No Reaction Tollens reacgent + - H C C H + 2 [Cu (NH3)2] OH Cu C C Cu + 4NH3 + 2H2O Tollens reacgent Silver alkynide Red ppt 04-04-2020 SKS Reactions of terminal alkynes with sodamide, Sodium hydride, n-BuLi and Grignard reagent: NaNH2 / liq. NH3 R1 C C Na R1 C C Na + NH3 NaH R1 C C Na + H2 R1 C C H acidic hydrogen n-BuLI R1 C C Li + n-butane RMgX R1 C C MgX + RH 04-04-2020 SKS Conversion of terminal alkynes into internal alkynes NaNH2 / liq. NH3 R1 C C H R1 C C Na + NH3 acidic hydrogen Act as nucleophile Terminal alkyne R -X Nucleopholic substitution reaction 2 R1 C C R2 + NaX Internal alkyne Practice problems: 1. Convert 1-butyne into 2-pentyne. 2. Convert propyne into 2-butyne. 3. Convert ethyne into n-butane. 04-04-2020 SKS Reactions of ethyne (Acetylene) 1. Addition of Arsenic trichloride HgCl 2 Learn about British H C C H + ASCl3 Cl C C AsCl2 H H anti-lewisite (BAL). b-Chlorovinyldichloroarsine (trans) Lewisite 2. Dimerisation of Acetylene CuCl + NH4Cl H2 H C C H H2C C C C H H C C C CH H 2 2 Pd-BaSO4 H H Vinylacetylene 1,3-Butadiene Practice Problems: 1. What happen when ozonolysis of acetylene is carried out ? 2. What happen when acetylene is heated with hot alkaline potassium permanganate solution? 3. How will you distinguish between ethane, ethene and ethyne ? Explain. 04-04-2020 SKS.
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