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Organic Chemistry 5th Edition L G Wade Jr Hi H Chapter 9 Alkynes Jo Blackburn Richland College Dallas TX Dallas County Community College District 2003 Prentice Hall HiH Introduction Alkynes contain a triple bond General formula is CnHZn2 Two elements of unsaturation for each triple bond Some reactions are like alkenes addition and oxidation Some reactions are specific to alkynes gt Chapter 9 2 Nomenclature IUPAC Find the longest chain containing the triple bond Change ane ending to yne Number the chain starting at the end closest to the triple bond Give branches or other substituents a number to locate their position gt Chapter 9 3 Name these CH3 CECH propyne CH3 CE BI 5br0m02pentyne 13 13 26dime thyl 3he ptyne gt Chapter 9 4 Additional Functional H H Groups i All other functional groups except ethers and halides have a higher priority than alkynes For a complete list of naming priorities look inside the back cover of your text gt Chapter 9 5 Examples EH3 4methyl1hexen5yne H 4hexyn201 Chapter 9 Common Names Named as substituted acetylene CH3 methylacetylene 13 EH3 CH3CHCHZCHCH3 isobutylisopropylacetylene Chapter 9 7 Nonpolar insoluble in water Soluble in most organic solvents Boiling points similar to alkane of same size Less dense than water Up to 4 carbons gas at room temperature gt Physical Properties Chapter 9 8 Acetylene Acetylene is used in welding torches ln pure oxygen temperature of flame reaches 280000 It would violently decompose to its elements but the cylinder on the torch contains crushed firebrick wet with acetone to moderate it Chapter 9 9 Synthesis of Acetylene Hi H Heat coke with lime in an electric furnace to form calcium carbide Then drip water on the calcium carbide 3C CaO gt CaCz CO coke lime CaC2 2 H20 gt H CEC H CaOH2 gtkThis reaction was used to produce light for miners lamps and for the stage gt Chapter 9 10 Electronic Structure The sigma bond is spsp overlap H C C s f m l i l The two pi bonds a e unhybridi ed p overlaps at 90 which blend into a cylindrical shape overlap ofp orbitals cylinder of electron density 11 Bond Lengths i More 8 Character so shorter length Three bonding overlaps so shorter H 1543 133 A 120A gtC Cmf gtcclt H CEC H H 39H H H H 1091 108 A 106A ethane ethene ethyne Bond angle is 180 so linear geometry gt Chapter 9 12 Acidity of Alkynes ii H Terminal alkynes RCECH are more acidic than other hydrocarbons Acetylene gt acetylide by NHZ39 but not by OH39 or R0 More 8 character so pair of electrons in anion is held more closely to the nucleus Less charge separation so more stable gt Chapter 9 13 Acidity Table Compound Conjugate Base Hybridization 5 Character pKa H H H H l I l l weakest H H H ZltD sp3 25 50 mi H H H H HC CH HC C 33 7 44 2 SP 0 H H H H iNH3 ammonia 35 H CEC H H CECQ Sp 50 25 stronger R OH R Qi alcohols 16 18 acid Chapter 9 14 gt Forming Acetylide Ions H can be removed from a terminal alkyne by sodium amide NaNH2 CH3 CEC H NaNH2 gt CH3 CEC39Na NH3 NaNH2 is produced by the reaction of altimmonia with sodium mletal H N H H N 2 H2 Na Fegt Na 1 3 quot6 gt Chapter9 15 Heavy Metal Acetylides Terminal alkynes form a precipitate with Agl or Cul salts Internal alkynes do not react Two uses gtQualitative test for terminal alkyne gtSeparation of a mixture of terminal and internal alkynes Chapter 9 16 Qualitative Test i CH3 CEC H Cu gt CHS CEc Cul H Reagent is AgNO3 or CuNO3 in alcohol or ammonia is added to form the complex ion The solid is explosive when dry Copper tubing is not used with acetylene gt Chapter 9 17 Separation of Mixtures Eli H CHs CEC CHs i N 0 reaction Cu CH3 CH2 CEC H gt CH3 CH2 CECI Cu redbrown preCIpItate Filter the solid to separate then regenerate the terminal alkyne by adding dilute acid CH3 CH2 CEC Cu HCl gt CH3 CH2 CEC H CuCl gt Chapter 9 18 Alkynes from Acetylides Acetylide ions are good nucleophiles 8N2 reaction with 1 alkyl halides lengthens the alkyne chain CH3 CEC39Na CH3CH2 1 gt CH3 CEC CH2 CH3 NaBr Chapter 9 19 Must be 10 iii H Acetylide ions can also remove H If backside approach is hindered elimination reaction happens via E2 Br CH3 CEC Na CH3 CH CH3 gt CH3 CEC H H3C CHCH2 Chapter 9 20 Addition to Carbonyl Acetylide ion carbonyl group yields an alkynol alcohol on carbon adjacent to triple bond e e aH H20 R CEC lj Q H lt H H R CE Chapter 9 21 Add to Formaldehyde H Product is a primary alcohol with one more carbon than the acetylide H H e I e CHs CEC C6 gt CH3 CEC C O H I H gt A H20 CH3 CEC E m o Chapter 9 22 Add to Aldehyde ii H Product is a secondary alcohol one R group from the acetylide ion the other R group from the aldehyde H3 CH 9 3 S e CHs CEC5C3 gt CH3 CEC Q H gt H H3 A e u H20 CH3 EC Ij g H lt 0 H H H gt Chapter 9 23 Add to Ketone Product is a tertiary alcohol CH 9 3 m e CH3 CEC1 C gt CH3 CEC C O CH3 EH3 gt EH3 A H H20 CH3 EC Ij Q H lt CH3 Chapter 9 24 Synthesis by Elimination Removal of two molecules of HX from a vicinal or geminal dihalide produces an alkyne First step HX is easy forms vinyl halide Second step removal of HX from the vinyl halide requires very strong base and high temperatures gt Chapter 9 25 Reagents for H H Elimination i l3r l3r KOH fused Molten KOH or alcoholic KOH at 200 C favors an internal alkyne Sodium amide NaNHZ at 150 C followed by water favors a terminal alkyne 1 NaNHz 1500C CH3 CH2 CH2 CHC12 gt CH3 CH2 CECH 2 H20 2 gt Chapter 9 26 Migration of Triple Bond H H4 H 1 R3 R ALCEC R PM 2 Ex H l 1 MC Fl I39IC C CEC R H R 1 HKCCC R W4 UB D mum L csluh ilimxl mrhnn inn R534 R KCCC 33 High R quotl If 5 an ullcnc H l H H R CEC C R B 2 R C LC t r R C39EC C H7413 H R R uquot immmimd Emma res311 mmc sl limd curbunic39n gt 27 Chapter 9 Addition Reactions Similar to addition to alkenes Usually exothermic One or two molecules may add TABLE 93 Approximate BUl ld Energies of Carbon Carbon Bonds Hal I Pi bond becomes two sigma bonds Chris39s 1939181392iid Apprarirmlw ung allmne sigma bond 83 coal quot347 1 J allacne pl bond 33 Real 392 I54 Ids second allkynr pi bond 54 kcal 23926 kl l Chapter 9 Addition of Hydrogen H Three reactions Add lots of H2 with metal catalyst Pd Pt or Ni to reduce alkyne to alkane completely saturated Use a special catalyst Lindlar s catalyst to convert an alkyne to a cisalkene React the alkyne with sodium in liquid ammonia to form a transalkene gt Chapter 9 29 Lindlar s Catalyst H Powdered BaSO4 coated with P poisoned with quinoline H2 adds syn so cisalkene is formed R R CEC R CZC R RH H CEC H H Pd partially poisoned gt Chapter 9 30 E R Na in Liquid Ammonia Eli H Use dry ice to keep ammonia liquid 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 gt NH3e39 Na gt Chapter 9 31 M e c h a n IS m Step I An electron adds to the alkyne forming a radical anion R 0 R C C R e gtQCCRI radical anion alkyne Step 2 The radical anion is protonated to give a radical R O H R O H U H H R H vinyl radical Step 3 An electron adds to the radical forming an anion R R Q e gt Rz H R most stable trans Vinyl anion Step 4 Protonation of the anion gives an alkene R Q H R H gt C H N gt CC N I H R H 32 C H R V H trans alkene H Addition of Halogens H CI2 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 Brz CH Br CH CH3 CH3 CEC CH3 gt 3czc 3gtCC Br CH3 Br Br Bl 2 Br Br I CH3 C CH3 Br Br Chapter 9 33 gt Addition of HX Fi H HCI HBr and HI add to alkynes to form vinyl halides Forterminal alkynes Markovnikov product is formed If two moles of HX is added product is a geminal dihalide Br Br HBr I B H gt gt CH3 C CH3 Chapter 9 34 gt HBr with Peroxides Hi AntiMarkovnikov product is formed with a terminal alkyne H H Br HBr H HBr I I CH3 CEC H gt m CH3 Ij Ij H ROOR Br H Br Chapter 9 35 Hydration of Alkynes Mercuric sulfate in aqueous sulfuric acid adds HOH to one pi bond with a Markovnikov orientation forming a vinyl alcohol enol that rearranges to a ketone Hydroborationoxidation adds HOH with an antiMarkovnikov orientation and rearranges to an aldehyde Chapter 9 35 Mechanism for Mercu ration Mercuric ion ng is electrophile Vinyl carbocation forms on mostsub C Water is the nucleophile A H 2 Hg Hg CH3 CEC H gtg CH3 CCH gt CH3 CH k 0 H H H26 H3O Hg k CH3 CCH lt CH3 CH H20 OH OH an enol Chapter 9 37 Enol to Keto in Acid Add H to the CC double bond Remove H from OH of the enol H H CH3 cc H30 G9 I H CHs C CH 4 CH3 C C H OH OH H OH H H26 H A methyl ketone CHs e H 0 H gt Chapter 9 38 Hydroboration Reagent Disecondary isoamylborane called H3C CH3 disiamylborane HC CH3 Bulky branched reagent HscCHB CH adds to the least H hindered carbon H3C CH3 Only one mole can add gt Chapter 9 39 Hydroboration Oxidation B and H add across the triple bond Oxidation with basic H202 gives the enol CH C C H Sia BH C C CH H202 H 3 2 Hs CH CC H BSiaz NaOH 3 14 OH gt Chapter 9 40 Enol to Keto in Base H is removed from OH of the enol Then water gives H to the adjacent carbon C C C H C C C H CH Q C H H3 Hegt H3 elt gt 3 OH 0 0 0 H V H H t H Him An aldehyde SHE S Geo Chapter 9 41 Oxidation of Alkynes Similar to oxidation of alkenes Dilute neutral solution of KMnO4 oxidizes alkynes to a diketone Warm basic KMnO4 Cleaves the triple bond Ozonolysis followed by hydrolysis Cleaves the triple bond Chapter 9 42 Reaction with KMnO4 Mild conditions dilute neutral 0 KMnO4 gt H20 neutral Harsher conditions warm basic 0 O KMnO4 KOH 9 e H CH3 CC CH2 CH3 gt CH3 C O 0 C CH2 CH3 H20 warm gt Chapter 9 43 Ozonolysis Ozonolysis of alkynes produces carboxylic acids Alkenes gave aldehydes and ketones 1 03 CH3 CC CH2 CH3 gt CH3 C OH HO C CHz CH3 2 H20 Used to find location of triple bond in an unknown compound Chapter 9 44 CCCCCC r9