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Chapter 9 Alkynes Introduction Nomenclature: IUPAC Name These Organic Chemistry, 5th Edition L. G. Wade, Jr. Introduction • Alkynes contain a triple bond. • General formula is CnH2n-2 Chapter 9 • Two elements of unsaturation for each triple bond. Alkynes • Some reactions are like alkenes: addition and oxidation. • Some reactions are specific to alkynes. => Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2003, Prentice Hall Chapter 9 2 Nomenclature: IUPAC Name these: • Find the longest chain containing the CH3 CCH triple bond. propyne • Change -ane ending to -yne. CH3 CCCH2 CH2 Br • Number the chain, starting at the end 5-bromo-2-pentyne closest to the triple bond. • Give branches or other substituents a CH3 CH3 CH CH CH CCCHCH number to locate their position. 3 2 3 => 2,6-dimethyl-3-heptyne => Chapter 9 3 Chapter 9 4 Additional Functional Examples Groups • All other functional groups, except CH3 ethers and halides have a higher priority CH2 CH CH2 CH CCH than alkynes. 4-methyl-1-hexen-5-yne • For a complete list of naming priorities, OH look inside the back cover of your text. CH3 CCCH2 CH CH3 => 4-hexyn-2-ol => Chapter 9 5 Chapter 9 6 1 Common Names Physical Properties Named as substituted acetylene. • Nonpolar, insoluble in water. • Soluble in most organic solvents. CH3 CCH • Boiling points similar to alkane of same methylacetylene size. CH3 CH3 • Less dense than water. CH CH CH CCCHCH 3 2 3 • Up to 4 carbons, gas at room temperature. isobutylisopropylacetylene => => Chapter 9 7 Chapter 9 8 Acetylene Synthesis of Acetylene • Acetylene is used in welding torches. • Heat coke with lime in an electric • In pure oxygen, temperature of flame furnace to form calcium carbide. reaches 2800°C. • Then drip water on the calcium carbide. • It would violently decompose to its 3 CCaO+ CaC2 + CO elements, but the cylinder on the torch coke lime contains crushed firebrick wet with acetone to moderate it. * CaC2 +2H2O HCCH+ Ca(OH)2 => *This reaction was used to produce light for miners’ lamps and for the stage. => Chapter 9 9 Chapter 9 10 Electronic Structure Bond Lengths • The sigma bond is sp-sp overlap. • More s character, so shorter length. • Three bonding overlaps, so shorter. • The two pi bonds are unhybridized p overlaps at 90°, which blend into a cylindrical shape. Bond angle is 180°, so linear geometry. => => Chapter 9 11 Chapter 9 12 2 Acidity of Alkynes Acidity Table • Terminal alkynes, R-C≡C-H, are more acidic than other hydrocarbons. - • Acetylene → acetylide by NH2 , but not by OH- or RO-. • More s character, so pair of electrons in anion is held more closely to the nucleus. Less charge separation, so more stable. => Chapter 9 13 Chapter 9 14 => Forming Acetylide Ions Heavy Metal Acetylides •H+ can be removed from a terminal • Terminal alkynes form a precipitate with alkyne by sodium amide, NaNH2. Ag(I) or Cu(I) salts. - + CH3 CCH+ NaNH2 CH3 C C: Na + NH3 • Internal alkynes do not react. • Two uses: •NaNH2 is produced by the reaction of ammonia with sodium metal. ¾Qualitative test for terminal alkyne ¾Separation of a mixture of terminal and internal alkynes. Chapter 9 15 Chapter 9=> 16 Qualitative Test Separation of Mixtures Cu+ CH3 CCCH3 No reaction + + CH3 CCH+ Cu CH3 CCCu + H + Cu+ CH3 CH2 CCH CH3 CH2 CCCu red-brown precipitate • Reagent is AgNO3 or CuNO3 in alcohol, or ammonia is added to form the complex ion. Filter the solid to separate, then regenerate the terminal • The solid is explosive when dry. alkyne by adding dilute acid. • Copper tubing is not used with acetylene. CH3 CH2 CCCu ++HCl CH3 CH2 CCH CuCl => => Chapter 9 17 Chapter 9 18 3 Alkynes from Must be 1° Acetylides • Acetylide ions are good nucleophiles. • Acetylide ions can also remove H+ •SN2 reaction with 1° alkyl halides • If back-side approach is hindered, lengthens the alkyne chain. elimination reaction happens (via E2). - + Br CH3 C C: Na + CH3CH2 Br CH3 CCCH2 CH3 + NaBr - + CH3 C C: Na + CH3 CH CH3 CH3 CCH + H3CCH CH2 => => Chapter 9 19 Chapter 9 20 Problem Addition to Carbonyl • How would you synthesize the following Acetylide ion + carbonyl group yields an compounds, using acetylene and any alkynol (alcohol on carbon adjacent to suitable alkyl halides as your starting triple bond). materials? RCC + CO RCCC O • a) 1-hexyne • b) 2-hexyne H O RCCC O H H H2O + H => Chapter 9 21 Chapter 9 22 Add to Formaldehyde Add to Aldehyde Product is a primary alcohol with one Product is a secondary alcohol, one R more carbon than the acetylide. group from the acetylide ion, the other R H H group from the aldehyde. CH3 CH3 CC + CO CH3 CCC O CH3 H H CH3 CC + CO CH3 CCC O H H H H CH H O + CH CCC O H O 3 H 2 3 H H2O + CH3 CCC O H O H H H H H => => Chapter 9 23 Chapter 9 24 4 Synthesis by Add to Ketone Elimination Product is a tertiary alcohol. • Removal of two molecules of HX from a vicinal or geminal dihalide produces an CH3 CH3 alkyne. CH3 CC + CO CH3 CCC O CH3 • First step (-HX) is easy, forms vinyl CH3 CH halide. 3 H H O + CH CCC O H O 2 3 H • Second step, removal of HX from the H CH3 vinyl halide requires very strong base and high temperatures. => => Chapter 9 25 Chapter 9 26 Reagents for Migration of Triple Bond Elimination Br Br KOH (fused) CH3 CH CH CH2 CH3 CH CCCH CH 200°C 3 2 3 • Molten KOH or alcoholic KOH at 200°C favors an internal alkyne. • Sodium amide, NaNH2, at 150°C, followed by water, favors a terminal alkyne. 1) NaNH2 , 150°C CH3 CH2 CH2 CHCl2 CH3 CH2 CCH => 2) H2O => Chapter 9 27 Chapter 9 28 Addition Reactions Addition of Hydrogen • Similar to addition to alkenes • Three reactions: • Pi bond becomes two sigma bonds. • Add lots of H2 with metal catalyst (Pd, • Usually exothermic Pt, or Ni) to reduce alkyne to alkane, completely saturated. • One or two molecules may add. • Use a special catalyst, Lindlar’s catalyst to convert an alkyne to a cis-alkene. • React the alkyne with sodium in liquid => ammonia to form a trans-alkene. => Chapter 9 29 Chapter 9 30 5 Lindlar’s Catalyst Na in Liquid Ammonia • Powdered BaSO4 coated with Pd, poisoned with quinoline. • Use dry ice to keep ammonia liquid. •H adds syn, so cis-alkene is formed. 2 • As sodium metal dissolves in the ammonia, it loses an electron. • The electron is solvated by the ammonia, creating a deep blue solution. - + NH3 + Na NH3 e + Na => => Chapter 9 31 Chapter 9 32 Mechanism Addition of Halogens •Cl2 and Br2 add to alkynes to form vinyl dihalides. • May add syn or anti, so product is mixture of cis and trans isomers. • Difficult to stop the reaction at dihalide. Br CH3 Br2 CH3 CH3 CH3 CCCH3 C C + C C Br CH3 Br Br Br2 Br Br CH C C CH => 3 3 Br Br Chapter 9 33 Chapter 9 34 => Addition of HX HBr with Peroxides • HCl, HBr, and HI add to alkynes to form vinyl halides. Anti-Markovnikov product is formed with a terminal alkyne. • For terminal alkynes, Markovnikov H H Br HBr H HBr product is formed. CH CCH CH3 CCH CH3 CC 3 Br ROOR • If two moles of HX is added, product is ROOR H Br a geminal dihalide. Br Br HBr HBr CH3 CCH CH3 CCH2 CH3 CCH3 Br => Chapter 9 35 => Chapter 9 36 6 Problem Hydration of Alkynes • Show how 1-hexyne might be • Mercuric sulfate in aqueous sulfuric acid converted to adds H-OH to one pi bond with a Markovnikov orientation, forming a vinyl a) 2-bromo-1-hexene alcohol (enol) that rearranges to a b) 1-bromo-1-hexene ketone. c) 2-bromohexane • Hydroboration-oxidation adds H-OH d) 2,2-dibromohexane with an anti-Markovnikov orientation, and rearranges to an aldehyde. Chapter 9 37 Chapter 9=> 38 Mechanism for Enol to Keto (in Acid) Mercuration • Add H+ to the C=C double bond. • Mercuric ion (Hg2+) is electrophile. • Remove H+ from OH of the enol. • Vinyl carbocation forms on most-sub. C. • Water is the nucleophile. H + H H H3O CH3 C C CH3 CCH + + H CH3 CCH +2 Hg Hg OH Hg + OH H CH3 CCH CH3 C C CH3 C C OH H H H O+ H H O H2O HH 2 + CH CCH H + Hg A methyl ketone 3 H3O CH3 C C CH3 C C H2O O H H H => OH OH => an enol Chapter 9 39 Chapter 9 40 Hydroboration - Hydroboration Reagent Oxidation • Di(secondary • B and H add across the triple bond. isoamyl)borane, called H3C CH3 • Oxidation with basic H2O2 gives the enol. disiamylborane. HC CH3 CH H H • Bulky, branched reagent B CH H2O2 H C CH3 CCH Sia2 BH 3 CH3 C C CH3 C C adds to the least CH BSia NaOH OH H H 2 H CH3 hindered carbon. H3C => • Only one mole can add. => Chapter 9 41 Chapter 9 42 7 Enol to Keto (in Base) Oxidation of Alkynes •H+ is removed from OH of the enol. • Similar to oxidation of alkenes. + • Then water gives H to the adjacent • Dilute, neutral solution of KMnO4 carbon. oxidizes alkynes to a diketone. H H H • Warm, basic KMnO cleaves the triple CH3 C C CH3 C C CH3 C C 4 OH OH O O H H H bond. H HOH H • Ozonolysis, followed by hydrolysis, CH3 C C An aldehyde O cleaves the triple bond.
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