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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{ 49 comments… read them below or add one }

mevans

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

I don’t understand that either.

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Darwin

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

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

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

Anonymous

Think through vectors

Dipayan

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

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

Dipayan

@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

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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Frank

Back to vector sums, folks. If you place CH2Cl2 and CHCl3 on a cartesian diagram so that the overall dipole would point to a value of -Y (straight down, traditionally), then the C-Cl bonds have larger -Y values for CH2Cl2 than for CHCl3. Simple geometry puts the angles from the x-axis as 35º below X for CH2Cl2 and only 15º below the X-axis for CHCl3. Hard to describe, but if you build a molecule and imagine it on a cartesian coordinate system, you’ll be fine.

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rakshit

If you actually try the same by law of vector sum it’ll be clear

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mamid

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

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

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BADA KEHINDE

if you understand why HI is more acidic than HF despite the strong electronegativity of flourine then you will not have issue with dichloromethane, being more polar than chloroform, despite the higher number of the electron withdrawing group attached to the carbon, the bond- strenght is a key, and the faster the bondbreaking the beter the polarity of the solvent to pertake in reaction. note this is limited to polarity.

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YASH

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

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

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Josh

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

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

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james

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

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LCM

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

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LCM

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

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

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

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

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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Kewalin

The best property of HFIP as I know is it can dissolve either polar in non-polar molecule or non-polar in polar molecule. I used it for making the polymer blending between polyester and protein. But I don’t know how it works since I am a biologist, not chemist. If someone know about that, please give me some explanations.
Cheers

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Reenu

Chloroform is not a nonpolar solvent. It is polar.

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james

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

Remember that polar/nonpolar is a continuum.

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urooba

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

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

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

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Dean

Diethyl ether is polar ! ! !

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James

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

Thank you sooooo much!! You rock :).

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Gujja.Naresh

tetrahydrofuran also suitable solvent for grignard reaction

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Roger Beauregard

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

Sorry, “WITHOUT THE USE OF PEROXIDES”.

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James

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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Roy

A refined product was once described to me to be a “solvent-solvent”.
Dr. James, could you enlighten me as to what that might mean in terms of its ability to dissolve various types of chemical blockages?
Thanks,
Roy

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LYRA

when methyl iodide is treated with sodium metal, ethane is formed .The reaction takes place when ether is used as a solvent !!!!!!! why is that so????

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James

Well, this could be happening several different ways, but one pathway is reductive formation of the methyl anion followed by substitution. Another is formation of methyl radical and subsequent coupling of two radicals (termination). Wurtz coupling . Not a very high yielding reaction

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hiren merja

Which method use remove moister in dimethayl formamide ?

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James
ronnie
Chelsea Armbrister

Is Azobisisobutyronitrile highly soluble with dimethylformamide? If you combine these and then add polystyrene, will the compound dissolve into the polystyrene?

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James

AIBN is soluble in DMF, yes. It’s a free radical initiator. I think the reaction you’re describing is free radical polymerization of styrene to form polystyrene.

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ram sitharpatty

hi
how to easily find out polar protic and polar aprotic?

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James

Does it have an OH or NH group anywhere? If so, it’s polar protic.

Reply

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