Questions: Enols, Enolates, and the Aldol Reaction

Question 1

A chemist deprotonates the alpha carbon of acetone with a strong base to form an enolate. The enolate then attacks the carbonyl of benzaldehyde. Through which atom does the enolate carbon attack, and what new bond is formed?

• A. The oxygen atom attacks; a new C–O bond forms, giving a vinyl ether product
• B. The alpha carbon attacks; a new C–C bond forms, giving a beta-hydroxy carbonyl (aldol product)
• C. The carbonyl carbon attacks; a new C–C bond forms at the wrong end of the enolate
• D. The oxygen atom attacks; a new O–H bond forms, regenerating the enol

Answer: (B)

Enolates are ambident nucleophiles — the negative charge is delocalized over both carbon and oxygen — but attack occurs through carbon under normal thermodynamic and kinetic conditions, forming the new C–C bond of the aldol product. Oxygen attack would give a vinyl ether (an O-alkylation product), which requires special conditions to obtain. The aldol reaction's synthetic power is precisely that it creates carbon–carbon bonds.

Question 2

A student draws keto and enol tautomers of acetaldehyde and labels them as 'resonance structures.' What is wrong with this?

• A. Nothing — resonance and tautomerism are different names for the same phenomenon
• B. The keto form has lower energy, so they cannot be in equilibrium
• C. They are distinct chemical species with different atom connectivity, not different depictions of the same molecule
• D. Acetaldehyde does not have an enol form because it lacks an alpha carbon

Answer: (C)

Resonance structures are different electron arrangements for the same atom connectivity — atoms do not move. Tautomers are constitutional isomers: different molecules with different atom connectivity (the alpha hydrogen has migrated from carbon to oxygen, or vice versa). Keto-enol tautomers interconvert rapidly through proton transfer, but they are genuinely distinct species. Calling them resonance structures is a category error that obscures the actual chemistry.

Question 3

Keto-enol tautomers are resonance structures of a carbonyl compound.

• T. True
• F. False

Answer: False

This is one of the most common misconceptions in organic chemistry. Resonance structures differ only in electron distribution — they share the same atom connectivity and are not separate species. Tautomers have different atom connectivity (the position of the alpha hydrogen changes, altering a C–H to an O–H bond). Keto and enol forms can even be isolated separately under the right conditions, proving they are distinct molecules, not electron-pushing conventions for the same molecule.

Question 4

The reason alpha C–H bonds are unusually acidic (pKa ≈ 20) compared to typical C–H bonds (pKa ≈ 50) is that removal of the proton produces a carbanion stabilized by resonance delocalization onto the carbonyl oxygen.

• T. True
• F. False

Answer: True

The approximately 30-unit drop in pKa corresponds to an enormous increase in acidity. Deprotonation at the alpha carbon gives an enolate anion in which the negative charge is delocalized across the C–O system: the lone pair on carbon is conjugated with the carbonyl π* orbital, spreading electron density to the electronegative oxygen. Without this resonance stabilization, the alpha C–H would be as difficult to remove as any ordinary sp³ C–H bond.

Question 5

Why does the aldol reaction form a C–C bond at the alpha carbon specifically, rather than at the carbonyl carbon?

Model answer: The carbonyl group activates the alpha carbon by making its attached hydrogens acidic. Base removes an alpha proton, generating an enolate in which negative charge is delocalized onto the carbonyl oxygen. This enolate is a carbon nucleophile — it attacks the electrophilic carbonyl carbon of a second molecule through its alpha carbon, forming a new C–C bond. The carbonyl carbon of the enolate is already electron-deficient (electrophilic) and would not attack another electrophile.

The key is the dual reactivity of carbonyl compounds: the carbonyl carbon is electrophilic (attacked by nucleophiles in addition reactions), while the alpha carbon becomes nucleophilic when deprotonated. The aldol reaction pairs these two sites from two separate molecules — or from the same molecule in intramolecular aldol reactions. Recognizing which carbon is the nucleophile and which is the electrophile is the essential mechanistic insight.