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.
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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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{ 13 comments… read them below or add one }
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
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.
Thank you for putting together this valuable resource. It is very helpful.
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!
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.
http://www.masterorganicchemistry.com/2010/09/22/five-key-factors-that-influence-acidity/
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
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!
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.
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!
Hi, is the pKa for H2 or H-H 42 or 35? Because in some sites, they say the pKa is 35.
Corrected. pKa should be about 36 (in DMSO) . Thank you
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.
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