Alcohols, Epoxides and Ethers
Alcohols – Acidity and Basicity
Last updated: November 4th, 2022 |
Acid-Base Reactions Of Alcohols
Alcohols are mild acids. Typical aliphatic (i.e. “alkyl”) alcohols such as ethanol, isopropanol, and t-butanol have a pKa of about 16-18, making them slightly more acidic than water.
- Alcohols that are in conjugation with a pi bond or aromatic ring will be more acidic since the conjugate base is resonance-stabilized. One key example is phenol (C6H5OH). (pKa = 10).
- Nearby electron-withdrawing groups will stabilize the negative charge of the conjugate base through inductive effects. For example, 2,2,2-trifluoroethanol (pKa = 12) is considerably more acidic than ethanol (pKa = 16).
Alcohols are also weak bases. They can react with strong acids to give oxonium ions which have a pKa of about -2.
One of the keys to the reactions of alcohols as we go forward is that the conjugate acid is a better leaving group and the conjugate base is a better nucleophile.
Table of Contents
- Four Key Points To Review About Acid-Base Reactions
- Favorable and Unfavorable Acid-Base Reactions of Alcohols (2 Examples)
- Reviewing The Key Factors Which Determine Acidity
- Applying These Factors To The Acidity of Alcohols
- A Practice Question
- Summary: Acidity and Basicity of Alcohols
- Quiz Yourself!
1. Four Key Points To Review About Acid Base Reactions
- Every acid-base reaction has 4 components: an acid, a base, a conjugate acid, and a conjugate base.When an acid loses a proton, it becomes its conjugate base. When a base gains a proton, it becomes its conjugate acid. As mentioned in the previous post, the conjugate bas of an alcohol is called an alkoxide. The conjugate acid of an alcohol is called an oxonium ion. (See post: Acid-Base Reactions in Organic Chemistry)
- We usually describe acid-base reactions as an equilibrium. In acid-base reactions, the equilibrium will favor the direction where a stronger acid and stronger base produces a weaker acid and a weaker base.When you add HCl to NaOH, a violent acid-base reaction occurs, which leads to the formation of H2O (a weaker acid than HCl) and NaCl (a weaker base than NaOH). As you’ve no doubt discovered when adding table salt (NaCl) to water, this reaction doesn’t proceed to any significant extent in the reverse direction.
- We measure acidity using a term called pKa. This is a measure of the equilibrium constant for a species giving up a proton to form its conjugate base. pKa is on a scale of about -10 to 50. Sixty orders of magnitude! The higher the pKa the less acidic it is. Lower pKa (more negative ) = more acidic.
Water (pKa of 14.0) is a weaker acid than HCl (pKa of -8). (See article: How To Use a pKa Table)
- The stronger the acid, the weaker the conjugate base. The weaker the acid, the stronger the conjugate base. The conjugate base of the strong acid HCl (pKa -8) is the innocuous chloride ion (Cl-), a very weak base. The conjugate base of the weak acid H2O (pKa 14) is the strongly basic hydroxide ion (HO-).
2. Examples of Acid-Base Reactions Of Alcohols
Here’s an example of a favorable acid-base reaction of alcohols. Note how we’re going from a stronger acid and stronger base to a weaker acid and weaker base [pKa values tell us for sure]
Here, deprotonation is very favourable. Note that the conjugate base of an alcohol is called an alkoxide.
Here’s an example of a (very) unfavorable acid-base reaction of alcohols: protonation of an alcohol by NH3. The most important reason why this is unfavourable is because we’re going from a weaker acid (pKa 38) and weaker base to a stronger acid (pKa -2) and stronger base. The equilibrium constant is about 40 orders of magnitude in the wrong direction!
3. Reviewing The Key Factors That Determine Acidity
What determines how acidic a molecule is, anyway?
The key factor in determining acidity is the stability of the conjugate base. Any factor which makes the conjugate base more stable will increase the acidity of the acid. (See post: 5 Key Factors Which Influence Acidity)
What does that mean, exactly? Usually, it means stabilizing negative charge since the conjugate base will always be one unit of charge more “negative” than the acid. (See post: 7 Factors That Stabilize Negative Charge)
How is negative charge stabilized? Two ways.
- First, by bringing the charge closer to the positively charged nucleus [“opposite charges attract”, remember]. Across a row of the periodic table, for example, basicity decreases as we go from H3C– to H2N– to HO– to F– because the electronegativity of the atom is increasing. That negative charge is being held closer to the nucleus, and therefore is more stable. A good rule of thumb is, “the more stable a lone pair, the less basic it is. This is also why certain species are made acidic by adjacent electron-withdrawing groups.
- Second, by spreading charge out over a larger volume. Diffuse charge is more stable than concentrated charge. Down a row of the periodic table, for example, basicity decreases as we go from F– to Cl– to Br– to I– because that negative charge is being spread out over a larger volume (larger atoms). The larger atoms are said to be more “polarizable”. [Note that this effect dominates rather than electronegativity in this case.] This is also why resonance serves to stabilize charges; the charge is being spread across multiple atoms, therefore reducing individual charge density. (See post: In Summary – Resonance)
4. Applying These Principles To The Acidity Of Alcohols
How do these principles relate to alcohols? It’s quite simple, actually. Since we’ll always be comparing the same atom (oxygen) we don’t need to worry about periodic trends, and we just need to focus on resonance and adjacent electron-withdrawing groups.
Alcohols where the conjugate base is resonance stabilized will be more acidic. The classic example is cyclohexanol and phenol.
Cyclohexanol has the pKa of a typical alcohol (about 16). The pKa of phenol, however, is about 10. Let’s look:
See how that negative charge on the oxygen of phenol can be “delocalized” back into the ring? That means the charge can be spread out throughout the molecule, which is stabilizing. Any factor which stabilizes the conjugate base will increase acidity.
Here’s another example. Compare ethanol (pKa 16) to 2,2,2-trifluoroethanol (pKa about 12). Why do you think trifluoroethanol is more acidic?
Compare their conjugate bases. What is fluorine doing here to make the conjugate base more stable?
This is an example of an inductive effect. Fluorine, being highly electronegative, pulls electron density away from the neighbouring carbon. That carbon, now being electron poor, pulls electron density away from the carbon next door. And that carbon, being slightly electron poor, can pull some electron density away from the oxygen.
The net result is that the oxygen has lower electron density, which is stabilizing. Again, stabilize the conjugate base –> increase acidity.
(See post: The Stronger The Acid, The Weaker The Conjugate Base)
This also works if we compare alcohol variations where we change the distance between the OH and the fluorine atom.
That’s because the inductive effect decreases in magnitude the farther away we go from the electronegative atom.
We can also use electronegativity trends to determine the order of acidity in these molecules. Since fluorine is more electronegative than chlorine which is more electronegative than bromine which is more electronegative than iodine, the inductive effect will be highest for CF3 and lowest for CI3.
5. A Practice Question
Finally, one last example. We can even think of examples where these two effects are combined:
Which do you think might be most acidic here?
6. Summary: Acidity Of Alcohols
Now that we’ve covered the key factors governing the acidity of alcohols, we’re more prepared to get into the nitty gritty of their different reactions. In the next post we’ll start discussing how acidity and basicity affects the reaction conditions we can use.
For alcohols, since we’re always dealing with oxygen, the only relevant factors here are resonance and electron withdrawing groups.
Next Post – The Williamson Ether Synthesis
(Advanced) References and Further Reading
1. Collected Acidity-Basicity Data
This website from the University of Estonia has a large curated list of studies on the acidity and basicity of various organic compounds.
Here is a leading reference. These pKa values refer to acetonitrile as solvent, so will be substantially different from those measured in aqueous solution, although the overall trends will be the same.
2. Strengths of Acids in Acetonitrile
A. Kütt, S. Tshepelevitsh, J. Saame, M. Lõkov, I. Kaljurand, S. Selberg, I. Leito
Eur. J. Org. Chem. 2021, 2021, 1407.
30 thoughts on “Alcohols – Acidity and Basicity”
I apologize if I’m wrong, but isn’t the pKa of ethanol closer to 16 (15.9 is the most cited value)?
Also, “the electronegativity of the nucleus is increasing” and “the negatively charged oxygen on trifluoroethanol will have some of its charge redistributed to fluorine” might be somewhat difficult to understand to undergraduate students. Maybe a little bit of elaboration and/or rephrasing might be in order.
Thank you once more. Fixed. I am in your debt for your excellent suggestions.
thank you mamid…..I am an undergraduate and honestly i could not understand some of the phrases being used to explain
what is effect of the positive effects on acidity of alcohol(OH)
Thank you for sharing the article. It’s very interesting. I’d love to hear more from you.
Just one word for this-awesome! really helpful! u saved me man :-)
thanku so much! it is really helping me get through my MCAT :)
This is just an excellent write up. My organic chemistry textbook wasn’t even as detailed as this is.
Glad you found it useful Clarence!
I’m sorry but I have a question why primary >secondary >tertiary > alcohols in acidity order
Think about how alkyl groups affect the electron-richness of the alcohol, relative to hydrogen. Are they electron donors or electron acceptors? Do they make it more electron rich or less electron rich? One of the key factors affecting acidity is the stability of the conjugate base, which in this case will be an alkoxide (O-). The oxygen already has a negative charge. What will electron donors do to that negative charge? Make it bigger or smaller? And what do you think is more stable, a larger charge, or a smaller charge? If the charge is less stable, that means that the conjugate base will be more basic, which means that the parent alcohol will be less acidic.
Regarding the phenol vs cyclohexanol example, since most resonance forms break the aromaticity of the ring, is the charge delocalization still significant enough as to make it a more stable conjugate base?
Amazing. Reopen my mind of organic chem.
I have a question. Do you think the acidity/basicity is correlated to its 1H NMR chemical shift? Specifically for protonated amino acids.
what is the effect of the positive effect type of inductive effects on acidity of alcohol and basicity amine
the order on your second to last rank-the-acidity is incorrect. it would be correct if electronegativity were the only player in ranking acidity, but it is not. the size of the halogen also factors in here…spreading out the charge increases acidity…i.e. why HF is a weak acid…<HCl< HBr < HI …consequently, your ranking of 2,2,2trifolorethanol should be reversed…2,2,2trifolorethanol (weakest) < 2,2,2tricholor etc… pka for triflorethanol is ~12.46..pka for tricholorethanol is ~12.02
OK, so I am curious here.. I posted a question regarding electronegativity vs polarizability. I can understand polarizability of the larger halogens generating the stronger acid through the creation of the larger dipole moment or the fact of the small halogen and it’s higher electronegativity (closeness of opposite charges) withdrawing electrons more strongly, but what conditions (possibly such as solvent) will determine reactivity of the species in concern ?? I see it said that polarizability takes presidence yet as you have stated he shows opposing acidity trends in the example as you have shown. Are there environmental conditions I am overlooking that aren’t stated to justify this ?? Or maybe the characteristics oh the double bonded oxygen contributing ??
The person you replied to is correct on one point and wrong on everything else. 2,2,2-trichloroethanol is indeed more acidic than 2,2,2-trifluoroethanol.
However, the difference is very small: 12.24 vs 12.46 pka, respectively.
Furthermore, aside from 2,2,2-trichloroethanol, the pattern is in fact correct. It should not be wholly reversed like the person you replied to stated. 2,2,2-tribromoethanol clocks in at 12.7 pka, in accordance with the pattern, and 2,2,2-triiodoethanol hasn’t been measured as far as I could find.
Finally, the mechanism at play here has nothing to do with the polarizability in the context you’re used to or anything related to the halogenic acids, HF, HCl, HBr, and HI. The polarizability of larger molecules is relevant for you in explaining intermolecular forces (induced dipole attractions). The stability of halogen anions (the conjugate base of a halogenic acid) has to do with the charge density. Fluorine is very small, so carrying a negative charge by itself concentrates the negative charge in a very small space alongside other electrons, which leads to a repulsive and destabilizing interaction. In iodine, the single extra electron is spread out over a much larger volume which minimizes destabilizing interactions. This along with orbital overlap (HSAB theory – traditionally covered in your first inorganic chemistry course) should more or less account for the differences in halogenic acid pka.
Honestly, just follow this page’s advice and ignore the technicality on 2,2,2-trichloroethanol. The difference of 0.3 pka is so trivially small that it would involve a complex analysis of a multitude of variables to pinpoint why, so there’s no way it pops up on an exam with the expectation you know about the exception of 2,2,2-trichloroethanol.
Thank you, Blaise!
Question.. besides solvent influence how exactly does electronegativity suddenly take precedence over polarizability in stabilizing negative charge and increasing acidity of the hydroxyl function ?? I am confused here. I understand polarizability as taking precedence due stability of the distributed charge of the larger atom and can further justify polarizability and it’s effect of charge stabilization here through the fact of the longer time required for electrons to travel around the larger atom creating a larger dipole moment and therefore a greater partial charge. Yet starting a new sentence, the higher electronegativity (due higher attraction generated through closness of opposing charges) of the smaller halides mainly in question here can b justified to be electron withdrawing to carbon and oxygen there fore stabilizing the anion from both points being electronegativity, and polarizability.. which i guess answers my question but raises then where does polarizability actually take precedence ??
Any meaningful way to spread out the charge density (ex. resonance or “polarizability”) takes precedence.
For example, phenol (the right-most molecule in question before the conclusion above), has a pka of 10 due to resonance structures. That’s several hundred times more acidic than 2,2,2-trifluoroethanol, the alcohol with 3 fluorines used as an example for the inductive effect above. CF3 is a lot of inductive power, but resonance is still more important.
Another example for polarizability: primary alcohols (OH group) have a pka around 16 while primary thiols (SH group) have a pka of around 10 — a million times more acidic.
Just be careful if your “polarizability of an electron cloud” is referring to stabilization by e.g. alkyl chains (I’m pretty sure this is actually just hyperconjugation), as opposed to the “polarizability” stabilizing an iodide or sulfide conjugate base (pretty sure this is just a shell or size effect). The former effect is weak (+-1 pka) and its trend reverses in water vs gas phase (e.g. H2O > methanol > ethanol), because solvent effects overpower it. Trends from the latter effect (e.g. halogenic acid strength or H2O vs H2S) are not reversed between water and gas phase.
You say that, in acidity of alcohols; more stabilization of the alkoxide ion formed (by inductive effects, etc), means more acidic is the alcohol.
Sure, but the trend you would get is not analogous to the data found in the gas phase oppose the data found from the solutions.
The acidity of alcohols is mainly due to polarizibility and solvation.
Organic Chemistry (Sixth Edition) by Robert Thornton Morrison & Robert Neilson Boyd [Pg. 228]
Hi – I make the assumption that we are dealing with solution-phase chemistry, not gas-phase. Any quoted pKa values are quoted in water and/or DMSO as solvent.
What is more acidic alkene or alcohol?
What do you think, based on a pKa table?
I am not convinced that we should learn:
Stronger Acid + Stronger Base –> Weaker Acid + Weaker Base
It depends on other factors.
Hydrolysis of an ester. You start with water (pKa = 16) and an ester and end up with a carboxylic acid (pKa = 4) and an alcohol.
Or you leave a nice glass of wine on Friday night to find vinegar on Saturday morning.
Ester hydrolysis (and its opposite, Fischer esterification) are not acid-base equilibria. This post is about simple acid-base reactions where the only bonds forming and breaking are to hydrogen.
Esterification-hydrolysis is definitely an equilibrium. If you do not drive the reaction you end up in the middle.
I agree that it is not a purely acid base reaction, but there is definitely an acid base component in it, and this is why you need an acid or a base to at least start it up.
But I see your point, If you have a simple acid base reaction in a close container it is correct.
I have a more serious question ( I am not a chemist ).
In the Fischer esterification reaction, the first step is a protonation at the oxygen of the carbonyl group. I think this step is highly unfavorable. It amount to move the equilibrium to an enol, or to consider the pKa of RCO2H2+ which would be a very very strong acid.
My first question is:
Why don’t we do the nucleophile attack FIRST, and the protonation AFTER on the side of the O-. It looks more favorable.
My second question is:
Chemists seems to have no problem making an alkoxy anion leave the tetrahedral intermediate, but it seems to be a big mistake to make an hydroxy anion leave. Why?
The pKa of their conjugate acid is about the same.
Note: Your web site is wonderful.
This is the great conceptual website which give proper guidness and understanding about the reactions.greatly thanks sir .excellent work🤗🤩😍😍😍
While some may think the difference in acidity of trichloro v trifluoroethanol is trivial, I think it is the norm. Can anyone give me one example in which a fluoro substituent is a better electron withdrawer than a chlorine, except for haloacetic acids. If you wish to refer to the haloacetic acids, then please explain the acidity of the substituents of substituted acetic acids as a class.
Correct me if I have the wrong person, but I believe you have been commenting on this very topic for over 10 years. Do you have an article that you might recommend I could direct my readers to so that they could be more fully informed on this point. Thank you.