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Alkynes

Alkynes. Structure. sp hybridization. Acidity of Terminal Alkynes. Stronger base. Weaker base. Other strong bases that will ionize the terminal alkyne:. Not KOH. Important Synthetic Method: Dehydrohalogenation. 1. Dehydrohalogenation…

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Alkynes

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  1. Alkynes

  2. Structure sp hybridization

  3. Acidity of Terminal Alkynes Stronger base Weaker base Other strong bases that will ionize the terminal alkyne: Not KOH

  4. Important Synthetic Method: Dehydrohalogenation 1. Dehydrohalogenation… An alkyl halide can eliminate a hydrogen halide molecule, HX, to produce a pi bond. Recall that HX can be added to a double bond to make an alkyl halide. HX can also be removed by strong base, called dehydrohalogenation. Preparation of alkene Strong base RCH=CHR+ HX RCHXCH2R Or rewriting base RCHBrCH2R RCH=CHR Also, if we start with a vinyl halide and a very strong base (vinyl halides are not very reactive). NaH RCH=CHBr RCCH

  5. Synthetic planning (Retrosynthesis) Work Backwards….. Trace the reactions sequence from the desired product back to ultimate reactants. Starting reactant Target molecule. Overall Sequence converts alkene alkyne But typical of synthetic problems side reaction occurs to some extent and must be taken into account. C

  6. More Sythesis: Nucleophilic Substitution Use the acidity of a terminal alkyne to create a nucleophile which then initiates a substitution reaction. Note that we still have an acidic hydrogen and, thus, can react with another alkyl group in this way to make RCCR’ Alkyl halides can be obtained from alcohols

  7. Reactions: alkyne with halogen RCCR + Br2 RBrC=CBrR No regioselectivity with Br2. Stereoselective for trans addition.

  8. Reactions: Addition of HX The expected reaction sequence occurs, formation of the more stable carbocation. Markovnikov orientation for both additions. Now for the mechanism….

  9. Mechanism The expected reaction sequence occurs, formation of the more stable carbocation.

  10. Addition of the second mole, another example of resonance.

  11. Reactions: Acid catalyzed Hydration (Markovnikov). Markovnikov addition, followed by tautomerism to yield, usually, a carbonyl compound.

  12. Reactions: Anti Markovnikov Hydration of Alkynes, Regioselectivity Step 2 Step 1 Similar to formation of an anti-Markovnikov alcohol from an alkene Step 1, Internal Alkyne: addition to the alkyne with little or no regioselectivity issue. Alternatively Asymmetric, terminal, alkyne if you want to have strong regioselectivity then use a borane with stronger selectivity for more open site of attack. Less exposed site. Aldehyde not ketone. More exposed site. sia2BH

  13. Tautomerism, enol  carbonyl Step 2, Reaction of the alkenyl borane with H2O2, NaOH would yield an enol. Enols are unstable and rearrange (tautomerize) to yield either an aldehyde or ketone. Overall… internal alkyne   ketone (possibly a mixture, next slide) Terminal alkyne   aldehyde

  14. Examples Used to insure regioselectivity. As before, for a terminal alkyne. But for a non-terminal alkyne frequently will get two different ketones Get mixture of alkenyl boranes due to low regioselectivity.

  15. Reduction, Alkyne  Alkene 1. Catalytic Hydrogenation If you use catalysts which are also effective for alkene hydrogenation you will get alkane. You can use a reduced activity catalyst (Lindlar), Pd and Pb, which stops at the alkene. You obtain a cisalkene. Syn addition

  16. Reduction - 2 2. Treatment of alkenyl borane with a carboxylic acid to yield cis alkene. Instead of H2O2 / NaOH Alkenyl borane 3. Reduction by sodium or lithium in liquid ammonia to yield the trans alkene.

  17. Plan a Synthetic SequenceRetrosynthesis YES! Synthesize butan-1-ol from ethyne. Work backward from the target molecule. A big alkyne can be formed via nucleophilic substitution. This is the chance to make the C-C bond we need. Is read as “comes from”. Major problem: make big from small. Be alert for when the “disconnect” can be done. Catalytic Lindlar reduction • BH3 • H2O2, NaOH Convert ethyne to anion and react with EtBr. Do a “disconnect” here. Target molecule Catalytic reduction Lindlar Addition of HBr. Now, fill in the “forward reaction” details Can we get an alkyne from smaller molecules? Not yet! So how can we get it? How about joining molecules to get an alkene? Not yet!! So how can we get an alkene? Ask yourself! Do we know how to join any two molecules together to yield an alcohol?

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