The pKa Table Is Your Friend

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26 thoughts on “The pKa Table Is Your Friend”

1. All strong acids have -1 charge while h30+ have H20 neutral conjugate base , shouldn’t H20 be called strongest acid with most stable neutral conjugate base
   How will u explain it?

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2. Hi – I’m studying chemistry in the UK and whilst I know these resources are based at US students, they have become a central part of my study. Explanations like these have helped me tackle complex synthesis questions and break down otherwise terrifying mechanisms – thank you so much!

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   1. Hi Jess – I am so glad you have found this useful to you in the UK! James

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3. Can you please tell the exact pka values of 1 butyne, 1 pentyne and 1 hexyne ..

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4. Hi!
   When comparing sulfoxides and carbonyls using the trend (acidity increases from left to right across a period and as you go down a group). Why are carbonyls more acidic than their sulfoxide counterparts? Shouldn’t it be the other way around?

   I’d appreciate your help!
   Thanks and I hope you are safe in these crazy times!

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5. Your pdf pka table goes from high pka values to low pka values when everywhere else, including your own video, does the opposite. It makes this even more confusing and I only say this because I learn different and this inconsistency makes me struggle. Also you pdf doesn’t include carboxylic acid, could you add this please? Thank you for the rest of the information, very helpful.

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6. Why are ketone hydrogens more acidic than ester hydrogens? By trends in acidity, shouldn’t the ester hydrogens be more acidic due to the presence of additional induction from the extra O? Thanks!

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   1. Hi Jonathan – thanks for asking.

      Actually esters are less acidic. The reason takes some getting used to , but will crop up all through your organic chemistry course.

      Oxygen has a dual nature. On one hand, it’s highly electronegative (3.4), and it can remove electron density through inductive effects.

      On the other hand, when it’s adjacent to a carbonyl, you can draw a resonance form where oxygen donates a lone pair to the carbonyl carbon (forming a new double bond) breaking the C=O bond in the process (giving O – ).

      This property is called “pi donation”.

      It’s hard to know from first principles which of these two – induction or pi-donation – is more important.

      Based on experiment (i.e. measuring pKa), pi-donation wins out as being more important.

      What that means is that the resonance form where oxygen donates a lone pair to the carbonyl is quite significant.

      Now, when you form the conjugate base of the ester, you break the C-H bond, forming a carbanion.

      To be better stabilized by resonance, that carbanion should form a double bond with the carbonyl carbon.

      However the lone pair form the carbanion has to “compete” with the lone pair from the oxygen – in other words, the carbonyl is already fairly electron-rich due to the oxygen.

      There’s less resonance stabilization available than if the carbonyl were adjacent to a group (like an alkyl) that wasn’t capable of pi-donation.

      I hope that answers your question!

      James

      PS Amides are even less acidic because the nitrogen lone pair is even more prone to pi-donation,

      Reply
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8. Hi James,

   I am doing a PhD in polymer chemistry and I wanted to understand the reactions of my acid chlorides a bit better. Your pages did help me, thanks a lot!

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9. Do you have a pdf file of the pKa table? I would like to print it out but it doesn’t fit on one sheet.

   Thanks

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10. Hi, is the pKa for H2 or H-H 42 or 35? Because in some sites, they say the pKa is 35.

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    1. Corrected. pKa should be about 36 (in DMSO) . Thank you

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       1. You’re welcome. Don’t forget to delete the old picture of the pKa table with the error. Right now you appear have two pictures of the pKa table.

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11. Hi… so the pKa of water is 16… if we use pKa +pkb =14, the pKb of water would be -2…? Does that mean water is very basic? I’ve seen on Yahoo answers people do something like Ka = [H+][OH-] / [H2O] with 1000g water but I remember in gen chem we were saying H2O is in liquid phase and we didn’t include anything in liquid phase when calculating equilibrium constant… Please help!

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    1. It is a common mistake to consider the pKa of water to be different from 14 at 25 degrees C!

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    2. Hi Audrey,

       Quoting the pKa of water to be 15.7 is actually incorrect. This is an artefact of a wrong calculation by Bordwell in the 1970’s. The true pKa of water is indeed 14, as explained in Silverstein, T. P.; Heller, S. T. pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water? Journal of Chemical Education 2017, 94 (6), 690. doi: 10.1021/acs.jchemed.6b00623

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12. Hi, can you help me, how to tell what direction of the reaction will be? When I don not have pKa table. A) CH3CH2CH2-OH + cyclopentanylmagnesium iodide CH3CH2CH2-O-MgI + cyclopentane B) 2-methylpropan-1-ol + NH3 (CH3)2CHCH2-O(-) + NH4(+) C) 3-methylcyclopentane-1-ol + NH3 natrium 3-methylcyclopentanolate + NH4(+) D) ethanol + kalium phenolate kalium ethanolate + phenol. Please for same general solution how to solve this. :-) Thx, P.

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13. thank you for making this website. you don’t know how this mean to me and my grades…. hope your life is getting better everyday!

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14. This inspired me to make study games for learning pka values. I hope everyone can learn from these:

    http://www.sporcle.com/games/woomaybe/orgo_pkas

    http://www.purposegames.com/list/organic-chemistry-pka-values

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15. Hello,

    How can we tell amongst different compounds, for example in a ranking situation, which are the most acidic, basic and least acidic, basic? It would be great to let me know how you determine whether something is a strong, weak (acid, base). This is a little confusing. Thanks!

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    1. Usually in a ranking situation there is one variable of interest. Knowing the 5 key factors that influence acidity can help. The pKa table is what you use when there are multiple variables in play.

       https://www.masterorganicchemistry.com/2010/09/22/five-key-factors-that-influence-acidity/

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16. Thank you for putting together this valuable resource. It is very helpful.

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17. I’m working on a translation from Spanish to English about isoelectric points of amino acids. I’m not a chemist nor wannabe but need to understand this in layman’s terms. How to calculate an isoelectric pH, what an isoelectrico point is vs. a point of zero charge, and what does the K stand for in pKas?
    If you could respond asap I would appreciate it or just tell me where to look.
    Thanks,
    David

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    1. K is the letter used in chemistry to denote the equilibrium constant. The K in pKa stands for the acid dissociation equilibrium constant Ka, and the p denotes that it’s a logarithmic funcdtion.

       As far as calculating isoelectric pH and Pzc, beyond the obvious (Wikipedia) I might suggest crowdsourcing your answer by asking Reddit chemistry or chemical forums; biochem, I’m sorry to say, is not my strength.

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    2. To calculate the Isoelectric point of an amino acid or peptide (or protein, if you are daring), you take the pKa of the carboxylic acid (between 2 and 4 usually) and the amine (9-11), add them, and divide by 2 for the case of an amino acid with no side chain that has a pKa value. If there is a side chain pKa involved, you take the 2 pKa values that are closest together, add them, and then divide by 2.

       An isolectric pH is the pH at which a given amino acid has a net zero charge. In solution, amino acids have various states to which they are charged (protonated). For simplicity, if Compound A has a pKa of 4 and is in a solution with a pH less than 4, it will be protonated (charged); above a pH of 4, it will be unprotonated (neutral). Amino groups are positively charged when protonated; carboxylix acid groups are neutral when protonated and negative when deprotonated. There is a specific pH somewhere in the middle of those 2 pKa’s that the acid will be protonated and the amine deprotonated (net zero charge) or the amine is protonated and the carboxylic acid is deprotonated (net zero charge). That is why we add the 2 pKa’s together and divide by 2.

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