Hofmann Elimination

Concept: Concept: General Reaction

Video Transcript

Now we’re going to talk about a specific reaction called the Hofmann elimination. The name Hofmann elimination can be a little confusing because we use both of these terms before. We've talked plenty about elimination reactions in Organic Chemistry 1. We learned that they could either follow Zaitsev’s rule or Hofmann’s rule. But when I mention Hofmann elimination here, I'm not referring to that. I’m just referring to a specific reaction called Hofmann elimination, not Hofmann's rule regarding elimination. This is also referred you as exhaustive methylation or Hofmann degradation. If you hear of those words or your professors call it any of those other terms, just know that this is the same reaction. The name Hofmann elimination actually is helpful for us because it’s going to serve to remind us that we're going to produce a Hofmann elimination product, meaning that it is going to follow Hofmann’s rule but specifically it’s going to be with an amine. In Organic Chemistry 1, we learned that alcohols can be eliminated, dehydrated to form double bonds. That was actually a reaction called dehydration. In the same way, amines can’t do but they need to be turned into a good leaving group because amines as they are, if you were just to kick off an NH2 negative, terrible leaving group. That’s like the strongest base ever. The point of the first step of this reaction is going to be to try and make the nitrogen a good leaving group so then we can kick it out and do an elimination reaction with a base. That's exactly what the first step is. The first step is going to be some kind of alkyl halide, usually an alkyl iodide. You may see it written as in excess, or you may not. In the absence data telling you excess or number of equivalence, always assume that there’s excess. In this case, I don’t have to write excess but your professor may be nice and write excess.
What that's going to do it’s going to react to my nitrogen multiple times. It’s going to make it a great leaving group. Then what we’re going to do in my second step is after the nitrogen is a great leaving group, it’s a quaternary ammonium. Then what we're going to do is we're going to react it with silver oxide, which is going to serve as my elimination reagent. It's going to be a base. It’s going to generate a base which can then eliminate. Notice that my nitrogen had two options of where to eliminate. I could have either eliminated along the blue line here or along the red line here. One of them was more substituted. One of them was less substituted. I went with the least substituted product. It’s not going to be a major or minor products in there. You’re pretty much always going to get 100% of the less substituted product. We'll talk more about that when we get into the mechanism.
Just for right now, I just want you guys to memorize these reagents. We’ve got some kind of amine reacting with an excess of alkyl halide and some mixture of silver oxide and base or water. That's going to generate the less substituted alkene. In next video, I’m going to show you guys the full mechanism for this reaction.