Solvents

Solvents are the mostly inert compounds that provide a medium for the reaction to take place in. 

Concept: General format of reactions and how to interpret solvents.

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Video Transcript

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.
I'm here to tell you that that is usually going to be the solvent. 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 in this page. So let's go ahead and get started.
So solvents are basically inert compounds, that's the whole point, they can't react. The whole point of a solvent is that it's something that serves as a medium for you to run a reaction in. 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 it will 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.
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.
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 and then you're going to see an arrow and that arrow's 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.
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.
But really 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 really going to pay attention to this part. I'm just going to pay attention to what Johnny said was really important, which was the nucleophiles and the leaving groups.” 

Although extremely important in lab, they rarely affect the outcome of a written reaction in Orgo 1. In fact, for the purposes of this course, I will usually have you ignore solvents in questions requiring you to predict mechanisms.

There are exceptions to the above reaction format. If reagents are numbered, several may be both above and below the arrow. However, may simple reactions do follow this format. 

Concept: The difference between protic vs. aprotic solvents.

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Video Transcript

So as solvents go, there's basically three general categories that they fall under. The first would be polar solvents. And this has to do back with when we talked about dipoles, a really, really long time ago. You still have to remember that. Polar solvents are just the same way that we determine 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. So molecules with net dipoles would be polar solvents.
Then we have on two different types of solvents we have aprotic solvents and protic solvents. And this I have to define because we never really used that word before. Basically, the essence of a protic solvent is one that can hydrogen bond. If you're able to donate hydrogen because you are an oxygen, nitrogen or fluorine, that can be considered a protic solvent.
So you might already guess that an aprotic solvent would be one that can not display hydrogen bonding. Let's go ahead and write that down. 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. Be thinking about that.
There's another thing that they do, they also tend to slow down nucleophiles. So imagine 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, 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.
If we were to think about the type of solvents we would use for mechanisms, we've learned four 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. That 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 as I said before, up here.
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 make it bulky and to slow it down? No. I actually want an aprotic solvent, so it's not going to hinder it at all. I'm just going to zip right through the solvent. So does that kind of make sense, the distinction we're using? That's the kind of question that you could see – a conceptual question on your exam.
So now what I want to do for the rest of this page is just go through these eight 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 one. We're going to use these solvents for orgo one and for orgo two.
So we'll go ahead and we'll just have a different answer for every one. So guys just go ahead and take your time with number one and figure out is it apolar or is it polar protic or polar aprotic. So basically I have three options. You just have to check off one of the boxes. All right. And the way you're going to look at that is look at the dipole and then determine if it can hydrogen bond or not. So go ahead and try to figure it out. 

Polar solvents are solvents which contain a net dipole.

  • Aprotic Solvents are solvents that cannot display hydrogen-bonding.
  • Protic Solvents are solvents that display hydrogen-bonding (this stabilizes carbocations, but hinders nucleophiles)
  • Therefore, we will prefer to run SN1 & E1 in protic solvents, and SN2 & E2 in aprotic solvents.

Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Example: Identify the following solvents as apolar, polar aprotic or polar protic. 

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Solvents Additional Practice Problems

Rank the following in order of nucleophilicity in polar protic solvents. (1 – least nucleophilic, 3 – most nucleophilic)

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What are the names and the structures of LDA, DMSO, and DMF?

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Circle the only polar aprotic solvent:

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For the following pairs of reactions, mark an “X” in the box on the right indicating which will go faster. The compound on top of the arrow is the solvent.

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Which conditions favor an efficient nucleophilic substitution reaction on 2-chloro-2-methylpropane?

a. A weak nucleophile in a polar protic solvent

b. A weak nucleophile in a polar aprotic solvent

c. A strong nucleophile in a polar protic solvent

d. A strong nucleophile in a polar aprotic solvent

e. A strong nucleophile in a nonpolar solvent

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Number the following in each series according to the attribute listed.

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Write “non-polar”, “polar aprotic”, or “polar protic” to describe each of the following solvents: 

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Which one of the following is a polar aprotic solvent?

 

A. DMSO

B. DMF

C. Acetone

D. All the above

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Which solvent would favor and SN2 style mechanism?

A)  Methanol

B)  Dimethyl formamide

C)  Water

D)  Acetic acid

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Circle the strongest nucleophile in each category.

a) Cl -    vs    Br -    (in EtOH)

 

b) CH3OH   vs    CH3SeH

 

c) NH2-     vs       CH 3-

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Rank the species below in order of increasing nucleophilicity in hydroxylic solvents (weakest nucleophile first): 

CH3CO2-, CH3S-, HO-, H2O.

a) H2O  <  CH3S-  <  CH3CO2-  <  HO-

b) H2O  <  HO-  <  CH3CO2-  <  CH3S-

c) H2O  <  CH3CO2-  <  HO-  <  CH3S-

d) CH3CO2 - <  H2O  <  HO-  <  CH3S-

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Identify the correct potential energy diagram for a secondary alkyl bromide (2°RBr) undergoing an SN1 reaction in protic versus aprotic solvent. Provide rationale to support your choices. 

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In which of the solvents below would the reaction shown take place at the fastest rate?

CH3CH2CH2CH2Br   +   NaCN   →   CH 3CH2CH2CH2CN  + NaBr

a) ethanol 

b) acetic acid

c) dimethyl sulfoxide

d) water

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Circle “non-polar”, “polar aprotic”, or “polar protic” to describe each of the following solvents:

 

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