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Carbonyl Chemistry Chapter 14: Enols, Enolates, and Carbon–Carbon Bond Formation

Chapter 14: Enols, Enolates, and Carbon–Carbon Bond Formation

Building Larger Molecules

Estimated reading time: 2 min

In this chapter

Introduction

Many important organic reactions involve atoms adjacent to carbonyl groups.

These positions possess unusual acidity and can give rise to highly useful intermediates known as enols and enolates.

These species make possible the formation of new carbon-carbon bonds.

Keto-Enol Tautomerism

Carbonyl compounds may exist in equilibrium with enol forms.

Although keto forms are usually favored, both structures play important roles in reactivity.

Figure 14.1. Keto (1) and enol (2) tautomers of butan-2-one, in equilibrium — the keto form is normally favored, but the enol form is what actually reacts in enolate chemistry. ("Keto-enol tautomerism" by Pen1234567, CC BY 3.0, via Wikimedia Commons.)
Figure 14.1. Keto (1) and enol (2) tautomers of butan-2-one, in equilibrium — the keto form is normally favored, but the enol form is what actually reacts in enolate chemistry. ("Keto-enol tautomerism" by Pen1234567, CC BY 3.0, via Wikimedia Commons.)

Enolates

Removal of an α-hydrogen produces an enolate.

Enolates are stabilized by resonance and serve as powerful nucleophiles.

Carbon–Carbon Bond Formation

One of the most important goals of organic synthesis is constructing larger molecules.

Enolates enable this through reactions such as:

Aldol Reactions

Formation of β-hydroxy carbonyl compounds.

Figure 14.2. The aldol mechanism: base removes an α-hydrogen to form an enolate, which attacks the carbonyl carbon of a second aldehyde molecule to form the new C–C bond, giving a β-hydroxy carbonyl (aldol) product after protonation. ("Base-catalyzed aldol addition" by Pillsmarch, CC BY 4.0, via Wikimedia Commons.)
Figure 14.2. The aldol mechanism: base removes an α-hydrogen to form an enolate, which attacks the carbonyl carbon of a second aldehyde molecule to form the new C–C bond, giving a β-hydroxy carbonyl (aldol) product after protonation. ("Base-catalyzed aldol addition" by Pillsmarch, CC BY 4.0, via Wikimedia Commons.)

Claisen Condensations

Formation of β-keto esters.

These reactions represent important examples of carbon-carbon bond formation.

Themes That Reappear

Throughout carbonyl chemistry, familiar ideas continue to dominate:

  • resonance,
  • acids and bases,
  • nucleophiles,
  • electrophiles,
  • stability,
  • and electron flow.

Organic Chemistry II repeatedly revisits the principles established in earlier chapters.

Self-Assessment

I can:

☐ Explain keto-enol tautomerism.

☐ Identify the alpha position and explain why it is unusually acidic.

☐ Recognize an enolate as a resonance-stabilized nucleophile.

☐ Recognize aldol reactions and Claisen condensations as carbon–carbon bond-forming reactions.

Looking Ahead

Carbonyl chemistry demonstrates the remarkable power of resonance and electron flow.

The next part explores another extraordinary consequence of electron delocalization: aromaticity.

Benzene and related compounds possess unusual stability and exhibit their own distinctive patterns of reactivity.

Common Mistake — Viewing Reactions Independently

Better approach: Recognize recurring mechanistic themes.

Common Mistake — Overlooking Which Proton Is Most Acidic

Better approach: Not all C–H bonds in a molecule are equally acidic. Alpha protons — those on carbons directly adjacent to the carbonyl group — are unusually acidic because their removal produces a resonance-stabilized enolate. Identifying the alpha position correctly is the essential first step in enolate chemistry.