Solvents - Online Tutor, Practice Problems & Exam Prep

Solvents are inert compounds that provide a medium for chemical reactions, typically not affecting the reaction's outcome. They fall into three categories: polar solvents, which have a net dipole; protic solvents, which can hydrogen bond; and aprotic solvents, which cannot. Protic solvents stabilize carbocations, making them suitable for mechanisms like SN1 and E1, while aprotic solvents favor SN2 and E2 mechanisms by allowing nucleophiles to react more freely. Understanding these distinctions is crucial for predicting reaction behavior in organic chemistry.

General format of reactions and how to interpret solvents.

All right, guys. So we've been spending a lot of time talking about nucleophiles and leaving groups. But sometimes when you look at your reaction, you're going to see some weird letters at the bottom, like you might see DMF or DMSO or even just water, and you're wondering what kind of role does that compound play in this reaction. In fact, you might be getting freaked out like "I don't know what kind of reaction I have because I don't recognize this molecule." And I'm here to tell you that that is usually going to be the solvent. Solvent. Okay? If you see something at the bottom of your arrow, that's usually where the solvents go and that's what we're going to discuss on this page. So let's go ahead and get started.

Solvents are basically inert compounds. That's the whole point. They can't react. Okay? The whole point of a solvent is that it's something that serves as a medium for you to run a reaction in. And, even though they do have impacts on reactions to some degrees, what we're going to find in this course is that the impact of your nucleophile and leaving group is much, much more important than the impact of your solvent. So typically, when we see solvent questions, it's going to be in the form of a conceptual style question where we'll talk about what type of solvent is favored, but it's not going to be a mechanistic question where it will actually determine the type of reaction that you have. Okay? So I just wanted to let you know, just underline this part, that these solvents are rarely going to affect the outcome of a written reaction, so many times we're just going to neglect them. We're not going to look at them too much. Alright?

So overall, if we're looking at a chemical reaction because I know this is new to a lot of you guys, what we're going to do is we're going to see some kind of reagent, or some kind of starting product. Okay? And then you're going to see an arrow. And that arrow is going to have two compounds usually. It's usually going to have something at the top and that's usually the reagent. That's usually the active substance. Then at the bottom, typically, you'll see the solvent. The solvent is the thing that, like I said, does not react but just serves as a place for the reaction to occur. Okay? Now, what I'm saying right now isn't a written rule. So that means that there are going to be times where maybe if you have a two-step reaction, that you'll have on top of the arrow is one of the parts and the second part is at the bottom and both are reactive. Okay? But really what all I'm trying to say is that many times you're going to see reactions with solvents in them and really it's mostly your job to ignore them. It's mostly your job to say, hey, I'm not going to really pay attention to this part. Just going to pay attention to what Johnny said was really important, which was the nucleophiles and the leaving groups.

The difference between protic vs. aprotic solvents.

So as solvents go, there are basically 3 general categories that they fall under. The first would be polar solvents. Okay? And this has to do with when we talked about dipoles a really long time ago. You still have to remember that. Polar solvents are determined the same way we determined what a polar solvent was at the beginning when we talked about dipoles. It's the same thing here. It's a molecule that has a net dipole. So just go ahead and write that down. Okay? Molecules with net dipoles would be polar solvents. Then we have two different types of solvents. We have aprotic solvents and protic solvents. And this I have to define because we've never really used that word before. So, basically, the essence of a protic solvent is one that can hydrogen bond. Okay? So if you're able to donate hydrogen because you contain oxygen, nitrogen, or fluorine, that will be considered a protic solvent. So you might already guess that an aprotic solvent would be one that cannot display hydrogen bonding. So let's go ahead and write that down. And then that means that a protic solvent is going to be a solvent that displays H bonding. And that H bonding has a special property. It turns out that for different types of reactions it's going to have different effects. So, when you're making carbocations, if you can hydrogen bond, that's actually going to stabilize the carbocation and make it easier to generate a carbocation for that step. So be thinking about that. Okay? There's another thing that they do. They also tend to slow down nucleophiles. So imagine that you have this negative charge and you've got all these waters sticking to it. It's going to make that negative charge a little slower through the solution and a little bit more difficult to donate its electrons. So it kind of has the same effect for both the positive charge and the negative charge. The problem is that for the positive charge, it's a good thing to stabilize it. For the negative charge, it's a bad thing because it winds up hindering it and making it more bulky. Okay? So if we were to think about the type of solvents that we would use for mechanisms, we've learned 4 mechanisms so far: Sn1, Sn2, E1, and E2. What kind of mechanisms would I prefer to have a protic solvent in? Think about what protic means. It means it can hydrogen bond. So what would be a good mechanism to have with a protic solvent? Good. So it would be one that has carbocations, so that would be Sn1 and E1. Are you guys following the logic there? Because protic solvents are going to be able to stabilize the carbocation, I said before up here. Okay? And then that means that in an aprotic solvent, what would be a good mechanism to run? And that would be Sn2 and E2 because these are mechanisms that start off, remember, the very first step is a strong nucleophile attacks something. So do I want a protic solvent to bulk it up and to slow it down? No, I actually want an aprotic solvent so it's not going to hinder it at all. It's just going to zip right through the solvent. Alright? So does that kind of make sense, the distinction we're using? That's the kind of question that you could see in a conceptual question on your exam. Alright? So now what I want to do for the rest of this page is just go through these 8 solvents and this will kind of be a combination of identifying solvents and also just a teaching moment for you guys to learn a few common solvents that we use in all of organic chemistry. Not just in one, we're going to use these solvents for Orgo 1 and for Orgo 2. So we'll go ahead and we'll just have a different answer for everyone. So guys, just go ahead and take your time with number 1 and figure out if it is apolar or is it polar protic or polar aprotic. So basically, you have 3 options. You just have to check off one of the boxes. Alright? And the way you're going to look at that is to look at the dipole and then determine if it can hydrogen bond or not. Alright? So go ahead and try to figure it out.

Identification of polarity in solvents

All right, guys. So, did this molecule have a dipole? Absolutely. Remember that oxygens have lone pairs and those lone pairs have a dipole pulling up. On top of that, we had dipoles pulling up from the carbon, so we would have a net dipole pulling up. So this definitely couldn't be apolar. Right? How about being able to hydrogen bond? Okay. So we know it's one of the polar ones. Would it be able to hydrogen bond? The answer is no because it doesn't have a hydrogen directly attached to an O, N, or F, so this is what we would call polar aprotic. So as we discussed, this would be a solvent that would be really good to use for SN2 or E2 because it wouldn't hinder that nucleophile. And this is actually a really common solvent in general. It's called tetrahydrofuran. You don't have to memorize the full name, but you should note this is THF. So whenever you see THF, it's a very common solvent in organic chemistry. You should know that it's the cyclic ether basically. Okay? So and by the way, this is just for your own enrichment really. Like if on the test you forgot what THF looks like, you could probably still get a 100%. I just want you guys to get familiar with these guys because these are kind of like the usual suspects that we're going to see a lot in reactions. And it helps if you have some kind of context for what it is. Alright? So let's move on to the next one.

Identification of polarity in solvents

All right. So this one was really easy. It definitely wasn't apolar. This would be polar protic because it's just water. And water means it has a dipole and it's able to hydrogen bond. Alright? So, honestly, like, there's even there's nothing I can say about it. That's just a super easy one. Let's move on to the next one.

Identification of polarity in solvents

All right. So if you drew this one out, what you would find is that this is actually a CₙₘN with a lone pair and then a methyl group. Would this have a net dipole? Yes, it would. It would pull towards the lone pair. So that means that this isn't apolar. And is this able to hydrogen bond? No. So this would be another version of polar aprotic. Just so you guys know, this is a molecule called Acetonitrile. And it's not one of the most popular solvents ever, but it is something that I've seen before. So just so you guys know, it doesn't have any fancy acronyms, but that's called Acetonitrile. Alright. Let's move on to the next one.

Identification of polarity in solvents

This one was apolar because if you draw it out, what you would find is that even though we do have dipoles, we have no net dipole. Okay? So this isn't even the right way to draw it because remember this is a tetrahedral. So, really, the way I should draw it is with one of them facing towards the front, and then one of them facing towards the back, and then two on the plane. Okay? But what you would find is that they're all going to perfectly cancel out, meaning that overall this is apolar. And apolar solvents, you don't have to worry about hydrogen bonding or not because they're never really going to be able to hydrogen bond. Okay? So that's just apolar. It's in a category of its own. That's called carbon tetrachloride. You will see this later when we get to the addition chapter. We're going to use it for some reactions. Alright. So let's move on.

Identification of polarity in solvents

Dipole? Yes. Hydrogen bonding? No. This would be another polar aprotic. Okay. As you can see, there's a lot of polar aprotics. This one is called dimethylsulfoxide or just DMSO. And once again, I wouldn't be teaching you this unless I had seen it literally a hundred times in different textbooks. You're going to see DMSO as a solvent. And now you guys just know what the reaction, what the molecule looks like. Okay? I don't really care so much that you're able to reproduce it or draw it, but it's good just to be familiar that it's polar aprotic. Alright? Let's keep going.

Identification of polarity in solvents

Ethanol is easy. This one has a dipole and it's able to hydrogen bond because of the fact that it's an H attached to an O. This would be polar protic. Okay? In general, water, alcohol, and amines will be your polar protics. Okay? And that means it can hydrogen bond. Cool. Next compound.

Identification of polarity in solvents

All right. So this is actually another common polar aprotic compound and that is a solvent. It's called dimethylformamide or DMF. So, another one that has these weird letters. The reason that it's polar is because it has a lot of dipoles. The reason that it's aprotic is because there's no hydrogens directly attached to the nitrogen. If there had been just one hydrogen, that would make it protic. But these are actually methyl groups. Since they're methyl groups, they can't hydrogen bond at all. So that would be aprotic. And then finally, let's just do this last one.

Identification of polarity in solvents

All right, guys. I actually gave this one away earlier. This one is an amine. Okay? It's actually ammonia, so this would be polar protic. Why? Because it does have a dipole. You don't see it? That's because you haven't drawn it correctly. Remember that you have to draw nitrogen like a trigonal pyramidal and it needs to have a lone pair at the top. Okay? So there actually is a net dipole pulling up. Right? I forgot to draw the H's. And then all these H's are directly attached to N, so it can hydrogen bond. Okay? And what I said earlier was that basically if you're an alcohol, water, or you're an amine, you would be able to be polar protic. So I'm just going to move out of the way so you guys can see that I wrote "polar protic" there. Okay? Now my closing thoughts on the solvent area, I know it was kind of long because I gave you a lot of examples. The whole point is this, know that water, alcohol, and amine are your protics and know that everything with... Okay? I really haven't seen a version of an acronym that is a protic, so don't worry too much about that. Okay? So I hope that made sense to you guys. Let's move on to the next topic.

Here’s what students ask on this topic:

What is the role of solvents in chemical reactions?

Solvents are inert compounds that provide a medium for chemical reactions to occur. They do not react with the reactants or products but can influence the reaction environment. Solvents can affect the solubility of reactants, the rate of reaction, and the stability of intermediates. For example, polar protic solvents can stabilize carbocations, making them suitable for SN1 and E1 mechanisms, while polar aprotic solvents are better for SN2 and E2 mechanisms as they do not hinder nucleophiles. Understanding the role of solvents helps in predicting and controlling reaction outcomes.

What is the difference between protic and aprotic solvents?

Protic solvents are capable of hydrogen bonding because they contain hydrogen atoms bonded to electronegative atoms like oxygen, nitrogen, or fluorine. Examples include water (H₂O) and methanol (CH₃OH). Aprotic solvents, on the other hand, cannot form hydrogen bonds as they lack hydrogen atoms bonded to electronegative atoms. Examples include dimethyl sulfoxide (DMSO) and acetone (CH₃COCH₃). Protic solvents stabilize carbocations and are suitable for SN1 and E1 mechanisms, while aprotic solvents favor SN2 and E2 mechanisms by allowing nucleophiles to react more freely.

How do polar solvents differ from non-polar solvents?

Polar solvents have a net dipole moment due to the difference in electronegativity between atoms in the molecule, leading to partial positive and negative charges. Examples include water (H₂O) and dimethyl sulfoxide (DMSO). Non-polar solvents lack a significant dipole moment as the electronegativity difference between atoms is minimal, resulting in an even distribution of electron density. Examples include hexane (C₆H₁₄) and benzene (C₆H₆). Polar solvents are better at dissolving ionic and polar compounds, while non-polar solvents are more effective for dissolving non-polar substances.

Why are aprotic solvents preferred for SN2 reactions?

Aprotic solvents are preferred for SN2 reactions because they do not form hydrogen bonds with nucleophiles. This lack of hydrogen bonding allows nucleophiles to remain free and highly reactive, facilitating the nucleophilic attack on the substrate. In contrast, protic solvents can form hydrogen bonds with nucleophiles, hindering their reactivity by creating a solvation shell around them. Examples of aprotic solvents include dimethyl sulfoxide (DMSO) and acetone (CH₃COCH₃), which provide an environment where nucleophiles can react more efficiently in SN2 mechanisms.

What are some common solvents used in organic chemistry?

Common solvents used in organic chemistry include water (H₂O), methanol (CH₃OH), ethanol (C₂H₅OH), acetone (CH₃COCH₃), dimethyl sulfoxide (DMSO), dichloromethane (CH₂Cl₂), tetrahydrofuran (THF), and hexane (C₆H₁₄). These solvents vary in polarity and hydrogen bonding ability, making them suitable for different types of reactions. For example, DMSO is a polar aprotic solvent ideal for SN2 reactions, while ethanol is a polar protic solvent suitable for SN1 and E1 mechanisms.