Chapter 25 — Key Takeaways

What you should leave Chapter 25 with

  1. Aldehyde/ketone + nucleophile → tetrahedral alkoxide → alcohol. This is the universal pattern for Family I reactivity. The nucleophile attacks the C=O carbon at the Bürgi-Dunitz angle (107°). The π electrons collapse onto oxygen, generating a tetrahedral alkoxide. Protonation gives the alcohol.

  2. Aldehydes and ketones cannot undergo acyl substitution because they have no leaving group on C. The only thing they can do is add. Hence Family I.

  3. The Bürgi-Dunitz angle (~107°) is the geometric trajectory for nucleophilic attack. It controls stereochemistry of attack on prochiral substrates. Combined with steric arguments (Felkin-Anh model, π-face selectivity in cyclic ketones), it predicts which diastereomer or enantiomer forms.

  4. Hydration (water + carbonyl) gives a gem-diol. Equilibrium favors carbonyl for most aldehydes/ketones, but the gem-diol predominates for very electrophilic carbonyls (formaldehyde, chloral, hexafluoroacetone).

  5. Hemiacetal formation (alcohol + carbonyl) is reversible. Glucose's pyranose form is the canonical biological example — intramolecular cyclization gives a 6-membered cyclic hemiacetal that is ~99.98% of the equilibrium population.

  6. Acetal formation (2 alcohols + carbonyl + acid) gives a stable, base-tolerant protecting group. Cyclic acetals (1,3-dioxolane, 1,3-dioxane) are the workhorses. Hydrolyze back with aqueous acid.

  7. Imines (Schiff bases) form from primary amines + carbonyls + loss of water. Optimal at pH ~5. Reversible. Biologically: vision (retinal-opsin), PLP-dependent enzymes (transamination), hemoglobin glycation (HbA1c diabetes marker).

  8. Enamines (β-amino-α,β-unsaturated) form from secondary amines + carbonyls + loss of water from the α-position. Used in α-carbon alkylation chemistry (Ch 27, Stork enamine synthesis).

  9. Cyanohydrin formation (HCN + carbonyl) extends the carbon chain by one. Useful for amino acid synthesis (Strecker), for α-hydroxy nitrile pharmaceuticals.

  10. Grignard reagents ($R{-}MgX$) are carbon nucleophiles that add irreversibly to aldehydes/ketones. Aldehyde → 2° alcohol; ketone → 3° alcohol; ester → 3° alcohol (after 2 equiv). One of the most important C-C bond-forming reactions in synthesis.

  11. Hydride reduction: NaBH₄ for aldehydes/ketones; LiAlH₄ for everything reducible. The selectivity reflects the Ch 24 reactivity ranking — esters/amides need a more aggressive hydride. Stereochemistry of reduction depends on whether the hydride is bulky (L-Selectride: axial attack on cyclohexanones) or small (NaBH₄: equatorial attack).

  12. Reductive amination (carbonyl + amine + NaBH₃CN at pH 5–6) is the standard method for installing an amine on a carbonyl carbon. Used in countless drug syntheses.

  13. Wittig reaction (ylide + carbonyl) gives an alkene + Ph₃P=O. Stabilized ylides give E (trans); unstabilized give Z (cis). Geometry-controlled alkene synthesis.

  14. Acetals are the canonical protecting group for aldehydes and ketones. Stable in base; hydrolyze in acid. Use cyclic acetals (dioxolane, dioxane) for maximum stability.

  15. Reversibility differs across the family. Hydration, hemiacetal, imine: reversible (and biology exploits this). Grignard, hydride, Wittig: irreversible (and synthesis exploits this).

  16. The reaction in biology mimics the in-vitro chemistry. NADH ↔ NaBH₄. Schiff base chemistry ↔ in-vitro imine. Glucose's pyranose ↔ a synthetic hemiacetal. Mechanism-first thinking lets you predict biological reactions from the in-vitro principles.

  17. The Strecker synthesis (aldehyde + NH₃ + HCN) makes α-amino nitriles, which hydrolyze to α-amino acids. First reported in 1850; still used industrially.

  18. The reverse of an aldol is a retro-aldol (Ch 28). Glycolysis step 4 (aldolase) is essentially a retro-aldol catalyzed by an enzyme.

  19. DIBAL-H at low temperature can stop an ester reduction at the aldehyde. This is a partial reduction strategy useful for making aldehydes from carboxylic acid derivatives.

  20. Mastery of Chapter 25 is the foundation for Chapters 26–29. Once you understand addition, acyl substitution (Ch 26) is "addition followed by leaving-group departure," and aldol/Claisen (Ch 28) is "addition with the nucleophile being an enolate."

Cross-references

  • Chapter 24 — The carbonyl group; reactivity ordering. Foundation for this chapter.
  • Chapter 26 — Nucleophilic acyl substitution (Family II). Builds on Family I addition.
  • Chapter 27 — Enols, enolates, α-carbon chemistry (Family III). Uses concepts from Family I.
  • Chapter 28 — Aldol and Claisen condensations. Aldol = enolate adding to a carbonyl (this chapter's mechanism, with an enolate as nucleophile).
  • Chapter 30 — Amines. Imine and reductive amination context.
  • Chapter 32 — Carbohydrate chemistry. Glucose, glycosides, and reducing sugars.
  • Chapter 33 — Proteins. PLP-dependent enzymes; α,β-elimination from imine intermediates.
  • Appendix B — pKa table.
  • Appendix C — Reaction summary (all carbonyl additions).
  • Appendix F — Named reactions: Grignard, Wittig, Strecker, Mannich, etc.

Study tip

For each reaction in this chapter, draw the starting material, the tetrahedral intermediate, and the product. Practice with real molecules: cyclohexanone + various Nu's, glucose + reductive amination, an α-keto aldehyde + Wittig. If you can fluently draw a half-dozen such mechanisms, you have Chapter 25 in your bones — and Chapter 26 will follow effortlessly.