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

What happens when we add X2 instead of HX to an alkyne? We end up with four halogens in our product. Let's see how that works:

Concept #1: Double halogenation of alkynes.

Transcript

Now let's go to halogenation. Halogenation of alkynes is going to be a very similar ordeal, where usually what would we expect to get for a halogenation of an alkyne? You would expect to get vicinal dihalides. But instead of getting just two halogens, I'm going to react this twice, so I would actually expect to get a tetrahalide product. Obviously, that's going to be a whole lot of halogens. Let's see how that works.
Basically, remember that the way this mechanism looks is that the triple bond or the double bond grabs the X, the X grabs the double bond or the triple bond, the pi bond, kicks out the X. So what we wind up getting is we wind up getting a basically, a bridge. What that bridge looks like is like this. So I'd have X, like that and like that. Then another X comes around and goes to the Markovnikov position and kicks out the X that's on the bridge.
So what I'd wind up getting is this original product where I have vicinal dihalides. These are the vicinal dihalides I was talking about. And that would be the product if I only reacted it with one equivalent. If I specifically said one equivalent, this is your product. You're done.
But what if I said two equivalents, one plus one. Or how abut if I just said excess, meaning more than one. Then I would react it again because I still have a double bond, so what I would wind up getting is the same thing all over again, where I'd react this again with X, X. I would form my bridge. And eventually what you wind up getting is a tetrahalide where it looks like this, where I have an X and an X from the first addition and then I would get another X and another X from the second addition. I'd wind up getting four X's next to each other.
Then finally, what's at the end here? This would still be an H, but that H came from the H that's here. The important part is that now I'm getting four halogens just from the addition of one reagent because it's reacting twice.
None of this tough. None of this is really hard, it's just something that you should know how to do. Let's go ahead and move on. 

General Reaction:

  • Product: tetrahalides