Polar Protic? Polar Aprotic? Nonpolar? All About Solvents

by James

in Organic Chemistry 1, Organic Chemistry 2

A lot of students I talk to have questions about solvents, so I’ve decided to put together a reference post on them.

Solvents can cause considerable confusion in reactions, because they’re listed along with the reagents of a reaction but often don’t actually participate in the reaction itself. And to be honest, a lot of instructors (myself included) are less than consistent about when to include solvents and when not to. So the whole exercise can come across as somewhat arbitrary: when do you know when to include the solvent?

Let’s back up. What’s a solvent, anyway?

A solvent is a liquid that serves as the medium for a reaction. It can serve two major purposes:

  1. (Non-participatory) to dissolve the reactants. Remember “like dissolves like” ? Polar solvents are best for dissolving polar reactants (such as ions); nonpolar solvents are best for dissolving nonpolar reactants (such as hydrocarbons).
  2. Participatory:  as a source of acid (proton), base (removing protons), or as a nucleophile (donating a lone pair of electrons). The only class of solvents for which this is something you generally need to worry about are polar protic solvents (see below).

OK. So what does “polar” and “non-polar” mean?

  • Polar solvents have large dipole moments (aka “partial charges”); they contain bonds between atoms with very different electronegativities, such as oxygen and hydrogen.
  • Non polar solvents contain bonds between atoms with similar electronegativities, such as carbon and hydrogen (think hydrocarbons, such as gasoline).  Bonds between atoms with similar electronegativities will lack partial charges; it’s this absence of charge which makes these molecules “non-polar”.

There are two common ways of measuring this polarity. One is through measuring a constant called “dielectric constant” or permitivity. The greater the dielectric constant, the greater the polarity (water = high, gasoline = low).  A second comes from directly measuring the dipole moment.

There’s a final distinction to be made and this causes confusion. Some solvents are called “protic” and some are called “aprotic”.  What makes a solvent a “protic” solvent, anyway?

  • Protic solvents have O-H or N-H bonds. Why is this important? Because protic solvents can participate in hydrogen bonding, which is a powerful intermolecular force. Additionally, these O-H or N-H bonds can serve as a source of protons (H+).
  • Aprotic solvents may have hydrogens on them somewhere, but they lack O-H or N-H bonds, and therefore cannot hydrogen bond with themselves.
For the average first semester student, these distinctions come up the most in substitution reactions, where hydrogen bonding solvents tend to decrease the reactivity of nucleophiles; polar aprotic solvents, on the other hand, do not.
There are 3 types of solvents commonly encountered: nonpolar, polar aprotic, and polar protic. (There ain’t such a thing as a non-polar protic solvent).
OK, enough yammering. Here are some (hopefully useful) tables.

Nonpolar solvents: 

These solvents have low dielectric constants (<5) and are not good solvents for charged species such as anions. However diethyl ether (Et2O) is a common solvent for Grignard reactions; its lone pairs are Lewis basic and can help to solvate the Mg cation.

“Borderline” Polar aprotic solvents

These solvents have moderately higher dielectric constants than the nonpolar solvents (between 5 and 20). Since they have intermediate polarity they are good “general purpose” solvents for a wide range of reactions. They are “aprotic” because they lack O-H or N-H bonds. For our purposes they don’t participate in reactions: they serve only as the medium.

 

Polar aprotic solvents

These solvents all have large dielectric constants (>20) and large dipole moments, but they do not participate in hydrogen bonding (no O-H or N-H bonds). Their high polarity allows them to dissolve charged species such as various anions used as nucleophiles (e.g. CN(-), HO(-), etc.). The lack of hydrogen bonding in the solvent means that these nucleophiles are relatively “free” in solution, making them more reactive. For our purposes these solvents do not participate in the reaction.

Polar protic solvents

Polar protic solvents tend to have high dielectric constants and high dipole moments. Furthermore, since they possess O-H or N-H bonds, they can also participate in hydrogen bonding. These solvents can also serve as acids (sources of protons) and weak nucleophiles (forming bonds with strong electrophiles).

They are most commonly used as the solvent for their conjugate bases. (e.g. H2O is used as the solvent for HO(-); EtOH is used as the solvent for EtO(-). )

These types of solvents are by far the most likely to participate in reactions. There are many examples (too many to list) where a polar protic solvent such as water, methanol, or ethanol can serve as the nucleophile in a reaction, often when a strong electrophile (such as an acid) is present. So if you see this type of solvent, be on the lookout.

I’m sure I missed something or something wasn’t clear. If you’d like something expanded on, please leave a comment!

Source for data: Wikipedia

 

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{ 34 comments… read them below or add one }

mevans April 28, 2012 at 4:37 am

Something that eludes me: why is the dipole of chloroform smaller than that of DCM? Shouldn’t three Cl’s be more withdrawing than two? I feel like I’m missing something, but I can’t quite put my finger on it…

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james April 29, 2012 at 12:54 pm

I don’t understand that either.

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Darwin April 30, 2012 at 12:24 am

Since dipole moment is based on magnitude and separation of charges, I imagine the reason is that the 3 C-Cl bonds cancel each other out more than the 2 C-Cl bonds. If I’m not mistaken, I think chloromethane (CH3Cl) has a even higher dipole moment than either DCM or chloroform.

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james April 30, 2012 at 2:45 pm

You are correct – I’m finding it as 1.9 D. http://en.wikipedia.org/wiki/Chloromethane_(data_page)

I’m not the world’s best at algebra but I would have thought that the vector sum of the three C-Cl dipoles would lead to a greater overall dipole than just a single C-Cl bond. Perhaps the C-H bonds provide additional electron density that allows for greater overall polarization of the molecule.

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Darwin April 30, 2012 at 8:55 pm

The three C-Cl bonds are pointed in different directions though

Dipayan May 12, 2013 at 10:55 am

If you make respective carbocations of these three species by removing 1 chlorine atom and compare their stability, clearly ch3+ is most stable, that means chloromethane would most likely to be polarised.

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james May 12, 2013 at 2:06 pm

Actually, CH3+ would be least stable. Don’t forget that Cl can donate a pair of electrons to form a pi bond.

Dipayan May 12, 2013 at 4:25 pm

@James, But as Cl is 3rd periodic element where C is 2nd periodic, I don’t think their p-orbital will overlap very well due to size difference, in that case we have to prefer inductive effect over resonance.
For F though your point may be valid, it can be verified if the order of polarity of Fluoro-methanes are reverse of Cl substituted methanes.

Chet June 17, 2013 at 5:02 pm

I think it is because the inductive effect of the three Chlorines on chloroform cancel out much of the outward negative dipole while with DCM, there are only two chlorines to withdraw electrons, thus less cancelation of the inductive effect.

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mamid April 28, 2012 at 12:50 pm

Some observations:
- paragraph 5: “Use a polar solvent to dissolve a charged species (such as, say NaOH), but don’t use it to dissolve” (the sentence ends here) – dissolve what?
- paragraph under Polar aprotic solvents, last sentence: “For our purposes these solvents do not participate in the reaction” – dot missing at the end of the sentence
- the reaction above Polar protic solvents – shouldn’t the stereochemistry of the C atom in the product be reversed?

Otherwise, great work as usual.

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james April 29, 2012 at 2:41 pm

Thanks, I’m happy that you spotted the stereochemistry example. Important to make sure inversion is there!

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YASH July 29, 2012 at 4:03 pm

James, your work on this topic(solvents) very amazing. Many student confused about which solvent protic/aprotic as well as polar/non-polar. awesome work. thank you. Hats of you.

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Abinaya November 7, 2012 at 3:36 am

what happens when bromine is added to chloroform and also bromine added to dichloromethane ?

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Josh November 12, 2012 at 6:47 pm

The examples for the uses of each of the solvents were exactly what I needed help with and were indeed quite helpful. Those would be areas on interest to expand on. Thank you!

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Princess White November 18, 2012 at 7:10 pm

which is the most efficient running solvent between polar and non polar solvent and why?

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james November 19, 2012 at 11:15 am

that’s a hard question to answer without being more specific.

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LCM January 28, 2013 at 9:20 am

Can you clarify on what you want to run with the solvent

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LCM January 28, 2013 at 9:26 am

all I know is that it all depends on the polarity of the content you want with the solvent. If you want to run a polar content, run it with a polar solvent as the polarities are the ones that determine the interactions.. so polar to polar and non-polar to non-polar.

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Elon February 2, 2013 at 6:00 pm

I have a question about Acetic Acid’s low Dielectric Constant. Compared to all the other Polar protic solvents it’s dielectric charge is very low yet when doing the same comparison its dipole moment is exceedingly high. Why is this?

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Li April 9, 2013 at 1:15 pm

Hi

How is the electrical conductivity change with polarity? If the voltage is high, can polar aprotic solvent also electrolysis?

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santosh April 23, 2013 at 7:44 am

hi
please tell me why did wrote methyl shift bt not methanide. however CH3 is bearing a negative charg some others examples like phenyl .

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jay cho May 9, 2013 at 12:15 am

Hi,

As far as I know, Hexafluoroisopropanol (HFIP) is a good solvent for polymers, even though it is a polar protic solvent.

But intuitively I don’t get how it works.

Please explain this for me. Thank you.

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Reenu June 4, 2013 at 5:47 am

Chloroform is not a nonpolar solvent. It is polar.

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james June 4, 2013 at 4:08 pm

You seem pretty confident. Does chloroform mix well with water? : – )

Remember that polar/nonpolar is a continuum.

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urooba October 28, 2013 at 11:37 am

plz tell me `why sn1 reaction take place in poler solvent and sn2 reaction take place in non poler solvent

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Hmmmm October 29, 2013 at 2:57 pm

Using a polar protic solvent increases the rate of the reaction right? Well, how am I suppose to know whether to use a type 1 or type 2?

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david gozales November 14, 2013 at 5:25 am

Could anyone suggest materials in wich are resistance to wax (hydrocarbon)??

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Dean August 1, 2014 at 4:49 pm

Diethyl ether is polar ! ! !

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James August 4, 2014 at 4:23 pm

It is more polar than hydrocarbons, but is still not miscible at all with water. Therefore I’d classify it as a non-polar solvent.

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Angel August 6, 2014 at 12:18 am

Thank you sooooo much!! You rock :).

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Gujja.Naresh August 15, 2014 at 8:18 am

tetrahydrofuran also suitable solvent for grignard reaction

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Roger Beauregard September 27, 2014 at 6:49 pm

James, can you explain why the addition of HBr to alkenes WITHOUT the use of alkenes is favored by polar, protic solvents? Why protic?

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Roger Beauregard September 27, 2014 at 7:07 pm

Sorry, “WITHOUT THE USE OF PEROXIDES”.

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James September 30, 2014 at 12:11 pm

Hi Roger – the rate limiting step will be formation of the free carbocation. The rate will be increased if we use a solvent with a high dielectric constant (i.e. polar). Solvents with high dielectric constants also tend to be polar protic (e.g. methanol). However, polar protic solvents are not a requirement for HBr addition.

I would be wary of using a polar protic solvent because it can act as a nucleophile, trapping the carbocation.

Here is an example where dichloromethane is used as solvent:
http://www.orgsyn.org/Result.aspx?ga=na

My version of March’s advanced organic chemistry doesn’t mention typical solvents for HX addition.

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