Comments on: Reactions of Enols – Acid-Catalyzed Aldol, Halogenation, and Mannich Reactions

By: James Ashenhurst

James Ashenhurst — Mon, 08 May 2023 01:16:07 +0000

In reply to Wojciech Domagala. Fixed! Thanks for the spot!

By: Wojciech Domagala

Wojciech Domagala — Mon, 08 May 2023 00:01:54 +0000

Please double check the “Acid-Catalysis In The Aldol Reaction, Part 1: Acid catalyzes keto-enol tautomerism” reaction scheme. The centre structure (protonated acetone) is uncharged and features a ten-electron oxygen atom. Replacing one electron pair at the oxygen atom with a positive charge will sort the problem out.

By: James Ashenhurst

James Ashenhurst — Mon, 18 Jul 2022 19:54:10 +0000

In reply to Meet. yes, keto-enol tautomerism can be assisted through the use of either acid or base.

By: James Ashenhurst

James Ashenhurst — Mon, 18 Jul 2022 19:53:50 +0000

In reply to Andrews George.

In the acid-catalyzed aldol addition, the slow step is attack of the enol on the aldehyde/ketone. But for more information than you probably want, I recommend: https://pubs.acs.org/doi/10.1021/ja01553a056

By: James Ashenhurst

James Ashenhurst — Mon, 18 Jul 2022 19:52:31 +0000

In reply to Giovanni Poli.

Thanks for writing!
Noyce studied this: https://pubs.acs.org/doi/10.1021/ja01553a056
and concluded that while the E1 can certainly occur, especially where a relatively stable carbocation that can be formed (e.g. p-MeOC6CH4).the elimination generally goes through the enol. This is also the case for the self-condensation of acetaldehyde. see: https://pubs.acs.org/doi/10.1021/ja01553a056

By: Giovanni Poli

Giovanni Poli — Sat, 05 Dec 2020 13:20:16 +0000

James,
I liked very much this post! Thank you for your work.
In the Clayden book 2012 (pg 616), and in the Karty book (pg 912) the acid catalyzed dehydration is claimed as an E1, from the carbonyl, and not from the enol.
My feeling is that this mechanism involves an E1, but from the enol. Indeed, in this case, when water leaves, the carbenium ion formed benefits of three resonance contributors: one with the positive charge on the beta carbon, another one with the positive charge on the carbonyl carbon (the double bond has shifted), plus the oxycarbenium ion. So, this carbocation has many reasons to be a stable intermediate, and if you formally remove H+ from the oxycarbenium ion you have the final product.

By: James Ashenhurst

James Ashenhurst — Tue, 25 Jun 2019 04:40:21 +0000

In reply to Vikrant Kumbhar. It might happen to some extent but the product is a hemiacetal, and hemiacetals easily revert to starting materials. So it's usually a cul-de-sac.

By: James Ashenhurst

James Ashenhurst — Tue, 25 Jun 2019 03:51:14 +0000

In reply to Binkey Acharjee. I would double check your starting materials.

By: Binkey Acharjee

Binkey Acharjee — Thu, 13 Jun 2019 13:01:33 +0000

Dear Sir,I am a research scholar,my area organic synthesis,I am doing aldehyde ketone reaction with acid catalyst,I used 2 acetyl benzofuran and 2, amino 5,chloro benzaldehyde,but my expected product is not come, but product came 2,amino,5, chloro accetophenone, single crystal xrd confirmed the structure,but how it is formed in which mechanism I want to try find mechanism please help any suggestions,

By: Daniel

Daniel — Tue, 07 Nov 2017 03:03:04 +0000

In a crossed aldol reaction between 2-butanone and 2,2-dimethylbutanal, 2-butanone would be the carbanion/nucleophile considering 2,2-dimethylbutanal lacks acidic hydrogens. But aren’t there technically two different possible nucleophiles that 2-butanone could form, and wouldn’t that lead to two separate products? I understand that the more acidic hydrogen on 2-butanone is on carbon 3, but is hydrogen on carbon 1 of 2-butanone acidic at all? Can it not become the nucleophile, or is it just so unlikely that we don’t consider it?