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Aldehydes and Ketones II - Enolate Chemistry

Chapter 7: Aldehydes and Ketones II - Enolate Chemistry

Updated Apr 17, 2026
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1. (7.1) The α-carbon of a carbonyl is:
A. α-H acidity drives aldol, Claisen, and alkylation chemistry.
2. (7.1) Typical α-H pKa in a simple ketone or aldehyde is:
B. β-ketoesters (two flanking carbonyls) have pKa ~11, much more acidic still.
3. (7.2) Keto-enol tautomerism is:
C. Tautomers are structural (constitutional) isomers that rapidly interconvert.
4. (7.2) At equilibrium, simple ketones and aldehydes:
D. The C=O bond is stronger than C=C plus O-H combined. 1,3-diketones are an exception, where the enol can dominate.
5. (7.3) An enolate is formed by:
A. Enolates are ambident nucleophiles but usually react at the α-carbon to form new C-C bonds.
6. (7.3) The resonance structures of an enolate show the negative charge on:
B. The oxygen contributor (with C=C and O⁻) is lower energy than the carbon contributor (C⁻ with C=O).
7. (7.4) Kinetic vs thermodynamic enolates differ because:
C. Classic controlled deprotonation: LDA for kinetic; NaH or NaOEt for thermodynamic.
8. (7.4) LDA (lithium diisopropylamide) in THF at -78 °C favors:
D. LDA's steric bulk grabs the most accessible α-H, and the low T prevents equilibration.
9. (7.5) The aldol reaction:
A. A hallmark carbon-carbon bond-forming reaction in organic and biological chemistry.
10. (7.5) The aldol "condensation" step that follows the initial addition is:
B. Heat and acid drive elimination of water from the β-hydroxy aldol product.
11. (7.6) A crossed aldol reaction:
C. Without these tactics, a crossed aldol produces a mess of four possible products.
12. (7.6) A Claisen-Schmidt reaction is a crossed aldol between:
D. Benzaldehyde, with no α-H, cannot form an enolate, so it serves cleanly as the electrophile.
13. (7.7) A Claisen condensation occurs when:
A. The classic product ethyl acetoacetate (ethyl 3-oxobutanoate) comes from a Claisen condensation of ethyl acetate.
14. (7.7) A Claisen condensation requires:
B. Deprotonating the β-ketoester product removes it from the equilibrium, pulling the reaction forward.
15. (7.8) A Dieckmann cyclization is:
C. The two ester groups on a single chain react intramolecularly to form a ring.
16. (7.8) Intramolecular cyclizations generally favor:
D. Small rings are strained; very large rings are unlikely due to entropy. Baldwin's rules summarize which cyclizations are favorable.
17. (7.9) α-Alkylation of a ketone:
A. Effective alkyl halides are primary or methyl; secondary/tertiary tend to eliminate instead.
18. (7.9) Using LDA for enolate formation during alkylation favors:
B. Stoichiometric LDA generates the enolate quantitatively; any residual ketone is unavailable for second alkylation.
19. (7.10) A Michael addition is:
C. The nucleophile adds to the β-carbon; protonation at the α-carbon gives a saturated 1,5-dicarbonyl.
20. (7.10) Good Michael donors include:
D. Well-stabilized enolates favor reversible 1,4-addition rather than irreversible 1,2-addition.
21. (7.11) Biological analogues of aldol and Claisen chemistry include:
A. Enzymes stabilize enolate-like intermediates and position the two carbonyls for selective C-C formation.
22. (7.11) A common biological Michael-type addition:
B. The second step of β-oxidation hydrates the trans-Δ² double bond of an acyl-CoA, similar to Michael chemistry.
23. (7.11) Enolate chemistry is important in biology because:
C. Aldol/Claisen-like chemistry is ubiquitous in central metabolism.
24. (7.10) Michael addition typically gives:
D. Cuprate nucleophiles and stabilized anions strongly favor 1,4- over 1,2-addition.

Chapter 6 covered reactions WITH aldehydes and ketones - nucleophiles attacking the carbonyl. This chapter covers reactions where the aldehyde or ketone IS the nucleophile - specifically, the carbon alpha to the carbonyl becomes nucleophilic after a base removes the alpha-hydrogen. The resulting enolate (or its neutral tautomer, the enol) is the workhorse nucleophile of Chapter 7.

Enolate chemistry is what makes carbonyls so versatile in synthesis. An aldol condensation joins two carbonyls through their alpha-carbons. A Claisen does the same with esters. A Michael addition uses the enolate to add to an alpha-beta unsaturated carbonyl. Alpha-alkylation installs a new carbon substituent on the alpha-position. All of these reactions share the same fundamental step: base removes alpha-H, enolate attacks an electrophile.

The Central Analogy

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