Benzyne Pathway

Concept: Concept: General Mechanism

Video Transcript

At this point, we’ve learned about two different addition elimination mechanisms for benzene. One is electrophilic and one is nucleophilic. But it turns out that there's another form of nucleophilic substitution out there. But this one is an elimination then addition pathway. It’s weird. This is called the benzyne pathway.
It turns out that benzene can progress through an elimination addition pathway. But it's going to make a very unstable intermediate. That unstable intermediate is called an aryne or benzyne because it literally has a triple bond within the ring. Imagine how unstable it is to have a triple bond. Remember, triple bonds want to have a bond angle of what? 180 degrees. It's going to be forced into 120-degree bond angle on that cyclohexane. It’s going to suck. But it can happen and this is actually a way that we make aniline.
Let me show you the mechanisms that we’ve learned so far and how benzyne is similar and how it's different. It's mostly just different. I’m going to go through this really fast. We know that for EAS, I can add first and then it can eliminate. We get our addition and elimination through an electrophile. We call it EAS.
We know that for SNAr, I can also add first but the way we add is actually through a nucleophilic attack. Then we eliminate using the same anion. We also get addition elimination but this one is nucleophilic, so SNAr. The aryne pathway or the benzyne pathway is much, much different because what happens in the benzyne pathway is that we have to actually eliminate first.
How do you eliminate from a benzene? What you literally have to do in E2, you literally have to do a beta elimination on this benzene. You would grab one of the H’s with a strong base. You can just use a strong base and you would literally do the three arrows for an E2 attack. I would grab the H, make a triple bond and kick out the X. What this is going to make is a very unstable benzyne, again also called aryne because arene is a benzene ring, so a benzyne intermediate. Extremely unstable.
Now we have to add. The way we add is with the conjugate acid. The conjugate acid now can be used to add to the benzyne. The mechanism is a little weird but just bear with me. What we're going to do is we're going to take our conjugate acid and we’re going to attack one side of the triple bond and kick those electrons down to form an anion. What this is going to form is another intermediate that looks like this. I’ve got my benzene is back, now with a negative charge. Then I’ve got BH where now B has a positive change because that's one too many bonds.
To finish this up, this negative can do basically a proton exchange. It can finally grab the H and we can get our substitution. That substitution, it took a long time. It's kind of weird path. We’ve got a base to switch out for a halogen.
What I wanted to show you guys is how we can use this to make an aniline. How could we make an aniline using this mechanism? The base that we use, this is a very typical reaction. It is NH2-, a very strong base. We’re going to do an elimination first. We're going to take an H. We’re going to a beta elimination. This is going to give me my benzyne. There's my benzyne.
What happens next? Notice that now I actually have my base is NH3 and I still have a long pair. It’s still somewhat nucleophilic. I can use the conjugate to attack or add to one side and kick electrons down to that one, to that side of the double bond. What I'm now going to get is a molecule that looks like this. NH2, H+ and a negative charge. Then we do the proton exchange and we're finally going to get what? What you just finally get at the end of this is aniline. This is actually a way to make aniline. You can make aniline using an NH2 base on basically an aryl halide. We started off with an aryl halide plus NH2- can give us aniline.
Crazy, huh? Awesome. Let's go ahead and flip the page.

Concept: Example: Activating and deactivating groups


Problem: Which is NOT a possible product of the reaction?


Problem: Which is NOT a possible product of the reaction?