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ChemistryIsomerismJEE · NEET
  1. 1. Isomer Basics
  2. 2. Functional
  3. 3. Metamerism
  4. 4. Keto-Enol
  5. 5. Other Tautomers
  6. 6. D.B.E. & Counting
  7. 7. Geometrical
  8. 8. Chirality
  9. 9. No Chiral Carbon
  10. 10. Counting Stereoisomers
  11. 11. Racemates & ee
  12. 12. Fischer & D/L
  13. 13. Alkane Conformations
  14. 14. Cyclohexane
Isomerism · Part 4 of 14

Tautomerism and Keto-Enol Equilibrium

Tautomers turn into each other and coexist in equilibrium. This part explains why the keto form usually wins, what makes an enol stable, how acid and base catalyse the change, and when tautomerism is impossible.

Builds on: Part 2 · Functional Isomerism.

Keto-enol

A 1,3-shift of hydrogen

The α-H moves to the carbonyl oxygen and the double bond shifts from C=O to C=C. The keto form is usually more stable by about 18 kcal/mol, because a C=O π bond (≈ 87 kcal/mol) is stronger than a C=C π bond (≈ 60).

Keto and enol forms of acetone.
Acetone and its enol.
Acetylacetone enol with its internal hydrogen bond.
The dashed line is the intramolecular H-bond.
Enol helpers

Chelation and conjugation

In acetylacetone's enol, the O–H bonds to the other C=O and closes a six-membered ring, and the C=C is conjugated with that C=O. Together they make it about 76 % enol.

The book's table

Enol content (%)

From acetone (0.00025 %) to dibenzoylmethane (100 %). The values are older measurements; exams test the order: 1,3-diketones beat keto-esters, which beat diesters, and phenyl groups add conjugation.

Bar chart of enol content.
Values as given in the book, on a log scale.
Cyclohexadienone and phenol.
The dienone is negligible.
Aromaticity

Phenol is all enol

Phenol is the enol of cyclohexa-2,4-dienone. Because the enol is aromatic, phenol exists totally in the enol form.

Summary

Key formulas

Enol order: CH₃COCH₃ < CH₃COCH₂COOC₂H₅ < C₆H₅COCH₂COOC₂H₅ < CH₃COCH₂CHO < CH₃COCH₂COCH₃ < C₆H₅COCH₂COCH₃ < cyclohexane-1,3-dione < C₆H₅COCH₂COC₆H₅ Factors that stabilise the enol
Worked examples

From the video

1. Enol content is maximum in acetone, acetophenone, acetic acid or acetylacetone?

Acetylacetone: chelated and conjugated, ≈ 76 % enol.

JEE-style question

Your turn

Enol content is maximum in:

(a)acetone
(b)acetophenone
(c)acetic acid
(d)acetylacetone
Show the answer and the traps

Acetylacetone: its enol is chelated by an internal H-bond and conjugated. About 76% enol: option (d).

Acetone has a single C=O and no help. Acetophenone gets some conjugation but no chelation. In acetic acid, the OH donates into the C=O, so it hardly enolises.

Watch out

Common mistakes

Assuming a more polar solvent gives more enol.Polar protic solvents H-bond to the keto C=O; non-polar solvents or the gas phase raise the enol content.
Treating a bridgehead α-H as enolisable.The C=C would sit at a bridgehead, which Bredt's rule forbids.
Practice

Try these

1. Which of the following is a dynamic isomerism? (a) Metamerism (b) Geometrical isomerism (c) Tautomerism (d) Optical isomerism

(c) Tautomerism (GRB Ch 4, Illustration Q1).

2. Which shows no keto-enol tautomerism: acetone, benzaldehyde, acetophenone, butan-2-one?

Benzaldehyde: it has no α-hydrogen.

3. Arrange by increasing enol content: acetylacetone, acetone, ethyl acetoacetate.

Acetone < ethyl acetoacetate < acetylacetone.

4. Why is ethyl acetoacetate 46 % enol in hexane but 0.4 % in water?

Water H-bonds to the keto C=O and competes with the enol's internal H-bond; hexane leaves the chelate intact.

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