Ch. 8 - Elimination 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

Alkyl halides aren't the only type of leaving groups out there. Let's explore some of the other types that exist. 

Concept #1: The 3 important leaving groups to know. 

Transcript

At the beginning of this topic, I made some generalizations about nucleophiles and about leaving groups. If you remember about leaving groups, what I said was that there's a lot of different types, but the most common is alkyl halides. And that's what we've been using this entire topic. Another generalization I made was made was with nucleophiles. When I said that just think that a negatively charged nucleophile is strong and a neutral one is weak. I just said, let's just say that for now.
What I want to do for this topic is go more in-depth on leaving groups and nucleophiles so that we can really understand all the different types of strengths instead of just making generalizations. Let's get started with the leaving groups first.
As I said before, alkyl halides are the most common leaving groups of organic chemistry, so 90% of the time you're just going to see alkyl halides and that's why we've been dealing with them so much because they literally are so ubiquitous. They're everywhere. But it turns out that there's other types of leaving groups as well.
Another really common one being sulfonate esters. Now these don't show up as often as alkyl halides, but they do show up a good amount. What a sulfonate ester is it's a molecule with the general formula SO3R. Now typically we wouldn't – and this is actually the way it looks by the way. The sulfonate ester general structure is that you have your chains. Say this is my chain here. And let's say that this is attached. And it's going to be an O then an S and two O's and then an R.
Now typically would we expect O to be a good leaving group? No. Remember that O is not as electronegative as an alkyl halide. It's actually further this way. So we would expect that O would not be a good leaving group and O negative would suck. But, this molecule is special because it can resonate so much. And I did talk about this earlier when we were talking about leaving groups that we could – once I get a negative charge there, this would be able to resonate and make double bonds and distribute that negative charge everywhere.
So sulfonate esters turn out to be really, really good leaving groups, even better than alkyl halides in some cases because they have so much resonance available, so it's going to stabilize the leaving group just like a conjugate base would be stabilized by the resonance effect. Remember that parallel that I drew between conjugate bases and leaving groups? It's the same thing.
So it turns out that the sulfonate ester, you might just see it drawn as SO3R in which case you need to know what that is. You also might see it drawn OSO2R, same thing. That's just the way it's actually drawn out. There's an O first. You need to be able to recognize that that's a leaving group.
But on top of that, you could see a special type of sulfonate ester. Sulfonate ester is the general category, but there's actually three unique types of sulfonate esters. Those are tosylates, mesylates, and triflates. Now the difference between those three actually just has to do with the R group. So everything else is the same. The S is the same. The O's are the same, everything. The only thing that changes is the identity of the R group. So let's go really quickly into that.
If that R group is just a methyl group, which is this middle situation, that's going to be called a mesylate. A mesyl group or a mesylate once it has the negative charge. And it's abbreviated Ms. So if you see that, you know it's a sulfonate ester. How about if it's a benzene ring with a methyl group on it? If it's a benzene ring with a methyl group that 's going to be called a tosylate and that's abbreviated Ts. And then if it's a C with three F's instead of 3 H's, then that's called a trifyl or triflate once it has the negative charge.
Now I know you guys might be wondering when do I use the word tosyl, when do I use tosylate. The ending -ate just means that there's a negative charge. That's common throughout lots of chemistry. We say instead of – I don't know. In orgo two we use it a lot more, that naming system. It just means that if you have an -ate at the end, it just means you have a negative anion.
So anyway, the whole point here is I don't need you to memorize exactly each sulfonate ester. I don't want you to be able to draw it if I give it to you. But what I do want you to be able to do is recognize that if you see these weird letters like OTs or OMs or whatever, that you're going to know that this is a sulfonate ester, so it's a really good leaving group.
Now I did notice one little error here. This should have been OMs, not OTs, so I'm going to change that. But anyway, you guys get the whole point that basically if you see one of these things, consider it the same as an alkyl halide. If there's an RX, it's the same thing as an OMs or whatever.
So that's the first leaving group that I want to tell you guys about. It's important. Don't pay too much attention to it. Just treat it the same as you would an alkyl halide. So if I see a secondary mesylate, that's the same thing as a secondary iodine or whatever.
The next one that I want to talk about is water. Water is actually a pretty common leaving group that we're going to use in a little bit. We haven't used it yet. But the way that we do it, the way we get water as a leaving group is to protonate alcohol with a strong acid.
So what you'll notice here is I have alcohol. Is alcohol typically a good leaving group? No. Typically, it sucks because once I kick off that O, what I'm going to get is OH- and that's very unstable. That's actually a really strong base. So that's not a good leaving group. But if I can protonate the alcohol first with a strong acid like, for example, sulfuric acid, which is a really common one that's used, then it's going to leave as water.
Let me show you. If the first step is let's say – actually let's use an easier one. Let's just use HCl. If my first step is to expose the alcohol to my strong acid, guess what's going to happen? My alcohol is going to grab the H and it's going to become protonated. Once it's protonated, it looks like this, OH2+. That's not very happy the way it is because it has a formal charge now.
Now guess what can happen in the next step. It can leave all on its own just like an alkyl halide would in a mechanism. And then what you're going to get is you're going to get let's say a carbocation if this is an SN1 reaction or an E1 plus water. Is water a good leaving group? Is it stable? Yeah, it's super stable because it's just neutral.
So see that by protonating my alcohol first, I could turn it from a bad leaving group to actually a really good leaving group.
So now you guys know, overall general idea here. Alkyl halides are the most important. You're going to see them all the time. If you see a sulfonate ester, it's going to be one of these weird letters. Don't worry about it too much, just treat it the same as an alkyl halide. If you see water with a strong acid, treat it the same way as an alkyl halide. The whole point is just that it leaks. That's the end goal. I don't want you guys to get mixed up by all this.
These are really the three main ones that we see. We don't see a whole lot of others. So don't worry about anything else as a leaving group except these three categories. 

1. Alkyl Halides

We’ve been dealing with these the whole lesson, formula –RX. You should be cool with these

 

2. Sulfonate Esters

These are molecules with the general structure –OSO2R or –SO3R. These are the ultimate leaving groups of organic chemistry. They might look a little weird, but in the end of the day, remember they just leave. NBD. 

3. Water

Also an awesome leaving group, formed after alcohol is protonated with a strong acid.