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Acid Base Reactions

By James Ashenhurst

The pKa Table Is Your Friend

Last updated: March 21st, 2019 |

The importance of pKas in organic chemistry can’t be overestimated, in my opinion.  Not knowing pKa’s in organic chemistry is like not knowing the value of the hands in poker. In this scheme, alkyl anions are the equivalent of the royal flush – they win the proton from everything underneath them in the table.

Why are pKas so important? Because every nucleophile is potentially a base, and vice versa. If you have a reaction where it looks like you might get SN2 or E2, look closely first – is there any chance of a simple acid-base reaction? For instance, take NaOH plus an alkyl thiol, R–SH. Is it an SN2? Or possibly an E2? Both are incorrect. The reaction that happens is the simplest one – deprotonation of SH, to provide water and the deprotonated thiol.

Also, the pKa table tells you about leaving group ability. Good leaving groups are weak bases!

If you don’t know the relative values of the pKas of the major functional groups, you’ll be flying blind in the course. Expect to hit a tree.

PDF VERSION NOW AVAILABLE (click here)

For more complete lists, be sure to check out Evans, Reich, and Stoltz. (check out the resources on Reich’s page by the way – fantastic!) Blessed are the OCD, for they produce the most beautiful and complete web resources.

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21 thoughts on “The pKa Table Is Your Friend

  1. 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

    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.

    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.

  2. 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!

  3. 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!

  4. 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.

  5. 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!

    1. 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

      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.

  6. 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

  7. 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!

  8. 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!

    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,

  9. 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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