#alkyne

Hi friends! I finally got around to finishing the notes for this guide today! I would’ve posted yesterday but I was stuck at the hospital for around seven hours, so I was extra exhausted. Lowkey should’ve slept in more haha but it’s all good. 

So with these alkyne reactions, a lot of them are just repetitions of the alkene reactions covered in the previous guides. I would definitely refer to the mechanisms in the first guide, since the mechanisms are a lot more structured there. Some of the vocabulary here is reviewed in the second guide as well :)

Alkyne Hydrogenation: Catalytic Conditions

H2, Pd/C is a reagent used in organic reduction and results in with syn hydrogenation

Catalytic conditions refer to how H2, Pd/C is a catalyst 

This reagent is used in 2 equivalents, which is why this reaction goes from alkyne to alkane

Alkyne Hydrogenation: Stop at Alkene 

Sometimes we don’t want the alkyne to end up as an alkane. Rather, we want the product to be an alkene

That’s where Lindlar’s catalyst comes in, which is “poisoned” to prevent the alkyne from becoming an alkane 

Lindlar’s catalyst: Pd/BuSO4, quinoline, CH3OH

H2 and Lindlar’s catalyst prefers syn hydrogenation (aka cis alkene)

To get the trans alkene, just use Na (metal), NH3 (liquid) (or opt for Li metal)

Electrophilic Additions: Addition of HX

HX is used twice to go from alkyne to alkene to alkane (thus excess HX)

X refers to the halogens Br, Cl, and I

This reaction follows Markovnikov’s rule, where X is added to the MORE substituted carbon

The product here is a geminal dihalide: both halogens on the same carbon

Electrophilic Additions: Addition of X2

X2 is added twice to go from alkyne to alkane (thus excess X2)

Note that I2 wouldn’t work here

Markovnikov regioselectivity

Anti stereospecificity (aka anti-addition)

The product here is a vicinal dihalide: halogens on adjacent carbons

Alkyne Hydration: Oxymercuration 

Note that we don’t have demercuration in alkyne hydration

The reagents are H2O, H2SO4 with either HgSO4 or Hg(OAc)2 the as catalyst

The product here is shown in the keto form, but in the enol form the OH would attach onto the MORE substituted carbon, thus Markovnikov regiochemistry

Alkyne Hydroboration/ Oxidation

The product has “anti Markovnikov” regiochemistry, because the doubled-bond O attaches onto the LESS substituted carbon in the keto form (in the enol form, the OH attaches onto the less substituted carbon)

The reagents are BH3, THF and H2O2, OH

Keto-Enol Mechanism 

Tautomers: structural isomers differing only in the position of an H

Enol and keto forms

Essentially, the OH in the enol form becomes a double-bonded O in the keto form

This is used with H3O+ as the initial electrophile and H2O as the second nucleophile (note that the forms change with the equilibrium reaction)

Resonance structures are formed between the enol and keto forms 

The examples featured underneath are condensed mechanisms of the oxymercuration and hydroboration/oxidation reactions featured above. We’re familiar with the enol forms (as seen in the alkene reactions), but the keto forms are drawn more often

Carbon Nucleophiles

With carbon nucleophiles, you’re changing the carbon skeleton of the starting material

The reagents for this are Na+ and NH2- in the solvent NH3 (liquid)

The second step is the tricky part, because you have to come up with a reagent that establishes the new carbon-carbon bond shown in the product

Often times, I usually have to work backwards, meaning that the product is shown, so I need to figure out what that second reagent is

I do this by comparing the starting material and product to see what the new carbon-carbon bond(s) are 

Then, I’ll draw the new bond(s) in the second reagent and then add an extra bond with a good leaving group (think SN2 reaction!)

So when the leaving group leaves, the resulting positive charge is attracted to the negative charge of the carbon nucleophile

And BAM, a product with a new carbon skeleton!

It’s so important to number your carbons for these reactions!!

My professor in first-semester organic chemistry was so helpful that he made “cheat sheets” for us. Bless his heart, I work with him now and he’s the best in the department here. So below I posted the four* roadmaps that relate to the first semester of organic chemistry.

Alcohol Synthesis Roadmap

Covers the different synthesis on alcohols and at the bottom right-hand corner introduces ether synthesis.

Alkyne Synthesis Roadmap

Alkene Synthesis Roadmap

Epoxide Synthesis Roadmap