Keto-enol tautomerism

by James

Description: Ketones and other carbonyl compounds containing a hydrogen adjacent to the carbonyl are in equilibrium with a constitutional isomer called an enol.
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Notes: Generally speaking the “keto” form is more stable due to the strong C–O π bond. The rate of interconversion between keto and enol is accelerated by acid.

Examples:

Notes:

Several situations lead to the enol form being more stabilized than normal:

  1. aromaticity (example 3)
  2.  conjugation (example 2)
  3.  hydrogen bonding (example 3)
Other factors such as greater alkene substitution, etc. assist as well but these are the major factors.

Mechanism: Protonation of the ketone oxygen by an acid (Step 1, arrows A and B) leads to formation of an oxonium ion, which is then deprotonated at carbon to give the enol (Step 2, arrows C, D, and E).

Notes:

  • H–A here could be water (H2O) or an acid of some kind that is added such as H2SO4 or HCl.
  • The rate of tautomerism is greatly increased by the addition of strong acid, since formation of the oxonium ino makes the adjacent C–H bond considerably more acidic (and easier to remove).
  • It’s incorrect to draw the mechanism for tautomerism like this:

If you make a model you will see why this is impossible. If you compare the positions of Ha and Hb in the starting material and the product you should see why this does not work.

Video Walkthrough

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{ 5 comments… read them below or add one }

Annabelle

Hi! i read through your notes for keto-enol-tautomerism. I just have a questions, why is 2,4-pentandione is more favourable as an enol? is it because of the conjugation? could you please explain to me why? Thank you.

Reply

james

Two reasons! 1) if one ketone is present as an enol, then it can participate in a hydrogen bond with the other ketone carbonyl, which is a favorable interaction. And yes 2) it is also conjugated, so that has an impact as well.

Reply

rajasekhar

which conditions are suitable for amide tautomerism …………..for proton abstraction

Reply

Reid Osborn

The Video Walkthrough is down.

Reply

James Ashenhurst

Thanks, I need to fix this.

Reply

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