Ch. 10 - Addition ReactionsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Alkynes contain two π-bonds, so when they are exposed to electrophiles, they do exactly what you would expect them to do: they react TWICE with them. 

Halogens Add Twice to Alkynes

Concept #1: General properties of double addition reactions to alkynes.

Transcript

It turns out that alkynes can react with halogens a lot the same way that double bonds can. Except for the fact that double bonds, remember, have one pi bond and triple bonds have two. What that means is that some reagents that we expose to alkyne are actually going to react twice. And that's what we're going to study right here.
Basically, there's two different reactions that alkynes will react twice with and these are going to produce double addition products. That just means that anything I was expecting to get from my double bond, just double that and that's going to be my expected product for a triple bond.
Now keep in mind, if we have carbocations in any of these mechanisms, which we will, that vinyl carbocations – now remember what the work vinyl means. It means something directly on a double bond. That means a carbocation that looks like this. Vinyl carbocations cannot easily rearrange. What that means is that don't be thinking about shifts in these mechanisms because they're really just not going to happen. 

Keep in mind that for some of these reactions, vinyl carbocations will be created. While unstable, these cannot rearrage, so don’t worry about carbocation rearrangements when reacting with alkynes!

Concept #2: Double hydrohalogenation of alkynes.

Transcript

Let's go ahead and look into the first one. The first one is a hydrohalogenation of an alkyne. Now a hydrohalogenation, remember, is just an HX with a triple bond. If I were to react that one time, I would expect to get a Markovnikov halogen added. But it turns out if you react this twice with alkynes, what you're going to wind up getting is actually gem-dihalides. Now remember that the word gem stands for geminal and that means that they're both on the same carbon.
Let's go ahead and see how this works. If I were to react this with one equivalent, I would expect to get that the triple bond attacks the H, kicks out the X, so I wind up getting a carbocation that looks like this with a positive charge here and an H here. Then that positive charge would grab – or I'm sorry, the X would grab the positive charge and I would get my intermediate, my first product.
But if I expose it to more than one equivalent, for example, let's say that I exposed it to two equivalents total, then it could react again. So then I would get this double bond attacking the H, kicking out the X. What I would wind up getting is a carbocation that now looks like this, where my X is there, my H is there. And once again, I would get my X – and now, by the way, there's two H's there. And now my X would attack there again.
So what I would wind up getting is a product that looks like this, where I have a five-membered ring with two X's in the Markovnikov position. That's the geminal dihalide part and that's a Markovnikov part. Then I would have two H's that came from my addition reaction.
And you might be wondering well, what about that carbon? Shouldn't it have 3 H's? Shouldn't it be CH3? Yeah. Well, that H was always there, though. That kind of looks messy. I'm just going to redraw that. That H was always there because that's the H that was originally on that triple bond anyway. I'm just saying that through this reaction, you wind up adding H twice and X twice. Does that make sense?
I hope so. I hope that that flows. It's really not a complicated reaction as long as you know what hydrohalogenation is. 

General Reaction:

  • Product: gem-dihalides