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Addition Reactions of Alkynes

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Addition Reactions of Alkynes ADDITIONADDITION REACTIONSREACTIONS OFOF ALKYNESALKYNES ADDITIONADDITION REACTIONSREACTIONS OFOF ALKYNESALKYNES C C two pi bonds A carbon-carbon triple bond is like two carbon-carbon double bonds. C C C C C C Most double bond reagents will react twice. ALKYNES ARE LESS REACTIVE THAN ALKENES (sp2) bonds are longer C C weaker less s character (sp) bonds are shorter C C stronger more s character SSS Shorter - Stronger - more S character RULE SIMPLESIMPLE ADDITIONSADDITIONS DO IT TWICE ! ADDITIONADDITION OFOF HClHCl TOTO ANAN ALKYNEALKYNE HCl H3C H H3C C C H C C first mole Cl H HCl second mole Cl H Follows the Markovnikoff Rule in both additions. H3C C C H Cl H Also HBr and HI ….. ADDITIONADDITION OFOF BROMINEBROMINE TOTO ANAN ALKYNEALKYNE H3C Br Br 2 anti H3C C C H C C addition first mole Br H - Br 2 Br second mole H3C C C H + Br Br Br H3C C C H Br Br Both additions proceed through a bridged intermediate and both additions are stereospecific anti. Also Cl2 and I2 ENOL-KETO TAUTOMERISM HYDRATIONHYDRATION OFOF ALKYNESALKYNES SOMETHING DIFFERENT When you add water to a triple bond, it doesn’t add twice - rather only once, usually giving a ketone as the product. ADDITIONADDITION OFOF WATERWATER ALKYNES ARE LESS REACTIVE THAN ALKENES dilute H2SO4 + H3C C CH H3C C CH H2O + H H O H reaction is slow H Water doesn’t add under the usual conditions (dil. H2SO4). + A stronger electrophile than H3O is required ……. ……. HgSO4 is added. MERCURYMERCURY ISIS LIKELIKE AA ““BIGBIG PROTONPROTON”” Hg2+ is a stronger electrophile than H+ H3C C CH H3C C CH + 2+ slower H faster Hg Hg2+ is not only a stronger Lewis acid, it is large and it forms a bridged ion. So we use a mixture of H2SO4, H2O and HgSO4 as our reagents. ( HgSO4 = mercuric sulfate ) ADDITIONADDITION OFOF WATERWATER TOTO ANAN ALKYNEALKYNE O H H2SO4 / HgSO4 H H3C C CH H3C C CH H2O 2 Hg + bridged ion, Hg+2 stronger anti opening electrophile O H OH H + H O H H3C C CH H H3C C CH exchanges Hg+ for a proton unstable enol O Follows the H3C C CH3 Markovnikoff ketone Rule ENOLSENOLS AREARE UNSTABLEUNSTABLE ((USUALLY)) discussed later ol O H ENOLS : ( have -OH attached to a double bond) Think of this combination as unstable. C C ene OH NOTE : Phenols are not “enols” and they are very stable (benzene resonance). ENOLENOL -- KETOKETO TAUTOMERISMTAUTOMERISM TAUTOMERISMTAUTOMERISM ENOL KETO O H O TAUTOMERS : species in H3C C CH2 H3C C CH2 equilibrium H that differ in unstable enol the position of a proton or other group. Most enols are not favored, To interconvert tautomers, a proton is transferred enol E from oxygen to carbon. N E Mechanism, next R slide….. G keto Y Bond Energies: C=O 179 C=C 146 O-H 111 C-H 106 they rapidly change to keto. CONVERSIONCONVERSION OFOF ANAN ENOLENOL TOTO AA KETONEKETONE ( catalyzed by small amounts of acid ) ENOL H H O O H + O H3C C CH2 H3C C CH2 H3C C CH2 + + H H H O H H H H + O H H O H H + O O H3C C CH2 H3C C CH2 H H KETONE OVERALL RESULT (NOT A STEPWISE MECHANISM) ENOL H H + H-O-H :O.. -H .. O H O H3C C CH2 H3C C CH2 H H + H-O..-H KETO HYDROGENATIONHYDROGENATION OFOF ALKYNESALKYNES HYDROGENATIONHYDROGENATION OFOF ALKYNESALKYNES Alkynes react twice with hydrogen when a metal catalyst is present. The alkene is formed first, and then the alkane. H2 CH3 CH3 CH3 C C CH3 C C syn addition Pd H H cis H2 /Pd CH3CH2CH2CH3 Since alkenes are more reactive than alkynes, it is difficult to stop the reaction after the first addition. MECHANISMMECHANISM OFOF CATALYTICCATALYTIC HYDROGENATIONHYDROGENATION REVIEW R R R R H H R R ALKENE PICKS UP R R TWO HYDROGENS .. .. syn addition . H H . H . H . H H H . metal catalyst with . H adsorbed hydrogen H atoms LINDLARLINDLAR Converting an alkyne to a cis alkene. LINDLARLINDLAR CATALYSTCATALYST There is a special deactivated catalyst, Lindlar, that allows the hydrogenation of an alkyne to stop at the alkene, after the addition of only one mole of hydrogen. Lindlar is a palladium catalyst, where the palladium is precipitated onto particles of barium sulfate from a solution which has quinoline present. Lindlar = Pd / BaSO / quinoline N 4 .. quinoline The quinoline is thought to “poison”, or reduce the activity, of the catalyst. Many sulfur and nitrogen- containing compounds act as poisons. With Lindlar catalyst, only one mole of hydrogen will add to an alkyne. As with other catalysts, hydrogen- ations with Lindlar are syn stereospecific additions. H2 CH3 CH3 CH3 C C CH3 C C Lindlar syn addition catalyst H H cis H2 CH3 C C CH3 CH3CH2CH2CH3 Pd (without Lindlar) POISONINGPOISONING MECHANISMMECHANISM ISIS UNKNOWNUNKNOWN A POSSIBLE HYPOTHESIS IS GIVEN HERE Quinoline binds to sites on the catalyst. Linear alkynes can approach and pick up hydrogens. Planar alkenes are blocked. bound quinoline alkyne fits alkene blocked QUINOLINE QUINOLINE QUINOLINE R-C C-R R R QUINOLINE QUINOLINE C=C QUINOLINE R R QUINOLINE QUINOLINE surface of QUINOLINE Lindlar catalyst ( probably over-simplified ) MAKINGMAKING AA transtrans --ALKENEALKENE Can you also convert an alkyne to a trans -alkene? YES …... SODIUMSODIUM METALMETAL ININ LIQUIDLIQUID AMMONIAAMMONIA This is a different type of reaction (not a catalytic hydrogenation) and it converts an alkyne to a trans alkene. H Na CH3 CH3 C C CH3 C C NH (liq) anti addition 3 H CH3 o - 60 C trans Just keep in mind that it is possible to make trans alkenes as well as cis alkenes, however, a different reaction is used. CONTROLLINGCONTROLLING THETHE OUTCOMEOUTCOME Chemists are always trying to either discover or design pairs of reactions that allow one to control the outcome of a change …. especially regioselectivity or stereochemistry. Consider: HBr (peroxide free) HBr (peroxides versus Markovnikoff Anti-Markovnikoff and now alkyne to cis alkene alkyne to trans alkene versus (catalytic hydrogenation) (sodium/ liquid ammonia) SYNTHESISSYNTHESIS If the organic chemist didn’t have methods giving different results, it would be difficult to do synthesis. The task of a synthetic chemist is to make molecules where each atom is arranged in the correct place and with the correct stereochemistry. There are 256 stereoisomers of cholesterol! To synthesize (construct) a molecule of cholesterol in the lab, the chemist would have to arrange the atoms in the correct order with the correct types of bonds and control the outcome at all eight stereocenters or the molecule would not be cholesterol..
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