Comments on: The pKa Table Is Your Friend

By: Ayush Sharma

Ayush Sharma — Thu, 24 Aug 2023 13:43:41 +0000

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?

By: James Ashenhurst

James Ashenhurst — Thu, 21 Apr 2022 16:54:20 +0000

In reply to jess ward. Hi Jess - I am so glad you have found this useful to you in the UK! James

By: jess ward

jess ward — Wed, 20 Apr 2022 09:59:30 +0000

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!

By: Pratibha

Pratibha — Sat, 16 Apr 2022 05:45:59 +0000

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

By: Heidi

Heidi — Sat, 30 May 2020 10:32:05 +0000

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!

By: Heather Race

Heather Race — Fri, 30 Nov 2018 06:13:21 +0000

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.

By: James

James — Wed, 26 Sep 2018 20:19:12 +0000

In reply to Jonathan.

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,

By: Jonathan

Jonathan — Wed, 26 Sep 2018 19:55:43 +0000

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!

By: Paolo

Paolo — Sun, 08 Apr 2018 17:11:28 +0000

In reply to Audrey. 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

By: Mazen

Mazen — Wed, 15 Nov 2017 19:19:17 +0000

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