# Ace Your Next OrganicChemistry Exam.

## Comment section

### 21 thoughts on “The pKa Table Is Your Friend”

1. David V Flores says:

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. Connor says:

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. Jose says:

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

3. mflores says:

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!

4. maria says:

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!

5. Petr Menzel says:

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.

6. Audrey says:

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. Mazen says:

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

2. Paolo says:

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

7. David Lo says:

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

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

1. David Lo says:

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.

8. Jennifer says:

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

9. Marcus says:

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!

10. Jonathan says:

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.