Alkynes

Structure sp hybridization

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

Important Synthetic Method: Dehydrohalogenation 1. Dehydrohalogenation… An alkyl halide can eliminate a hydrogen halide molecule, HX, to produce a pi bond. RCH=CHR + HX RCHXCH 2 R Strong base Also, if we start with a vinyl halide and a very strong base (vinyl halides are not very reactive). RCH=CHBr RCCH NaH Or rewriting RCHBrCH 2 R RCH=CHR base 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

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

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

Reactions: alkyne with halogen No regioselectivity with Br 2. Stereoselective for trans addition. RCCR + Br 2  RBrC=CBrR

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

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

Addition of the second mole, another example of resonance.

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

Reactions: Anti Markovnikov Hydration of Alkynes, Regioselectivity 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. sia 2 BH Less exposed site. More exposed site. Step 1 Step 2 Aldehyde not ketone.

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

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

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

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

Plan a Synthetic Sequence Retrosynthesis Synthesize butan-1-ol from ethyne. Work backward from the target molecule. Target molecule Is read as “comes from”. A big alkyne can be formed via nucleophilic substitution. This is the chance to make the C-C bond we need. Do a “disconnect” here. Catalytic Lindlar reduction Catalytic reduction Lindlar Addition of HBr. Convert ethyne to anion and react with EtBr. 1.BH 3 2.H 2 O 2, NaOH Now, fill in the “forward reaction” details Major problem: make big from small. Be alert for when the “disconnect” can be done. Ask yourself! Do we know how to join any two molecules together to yield an alcohol? 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? Can we get an alkyne from smaller molecules? YES!