Chapter 6: Aldehydes and Ketones I
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1. (6.1) The carbonyl group consists of:
B. The polarity of C=O is the source of its reactivity: nucleophiles target carbon, electrophiles target oxygen.
2. (6.1) The geometry at a carbonyl carbon is:
A. The unhybridized p orbital on carbon overlaps with a p orbital on oxygen to form the π bond.
3. (6.2) An aldehyde has the IUPAC suffix:
C. Ketone suffix is -one with locant for carbonyl. Aldehyde carbonyl is always terminal, so no locant is needed.
4. (6.2) The IUPAC name for CH₃COCH₃ is:
D. Acetone is the simplest ketone; its carbonyl sits at C2.
5. (6.3) Carbonyl compounds have higher boiling points than alkanes of similar mass because:
B. No O-H on the carbonyl means no self H-bonding (alcohols still boil higher than comparable ketones/aldehydes).
6. (6.3) Small aldehydes and ketones are water-soluble because:
A. Acetone is fully miscible with water. Larger ketones/aldehydes lose solubility as the alkyl part dominates.
7. (6.4) Nucleophilic addition to a carbonyl involves:
C. The tetrahedral intermediate is then protonated to the neutral addition product.
8. (6.4) Acid catalysis of carbonyl addition works by:
D. This is why acid catalysis is useful for weakly nucleophilic partners like alcohols and water.
9. (6.5) Hydration of a carbonyl gives:
B. Electron-withdrawing groups shift the equilibrium toward the hydrate (e.g., chloral hydrate).
10. (6.5) Formaldehyde (H₂C=O) in water exists primarily as:
A. Formaldehyde lacks any alkyl destabilization, so its hydrate is heavily favored in aqueous solution.
11. (6.6) A hemiacetal forms when:
C. The cyclic hemiacetals of sugars (e.g., glucose pyranose) are relatively stable because of the five- or six-membered ring.
12. (6.6) A full acetal (R-CH(OR')₂) forms when:
D. Under basic or neutral conditions, acetals are unreactive; they can be removed later by aqueous acid.
13. (6.7) A cyanohydrin forms when:
B. Cyanohydrins are useful because the nitrile can later be converted to carboxylic acids, amines, or aldehydes.
14. (6.7) Cyanohydrins are valuable because:
A. LiAlH₄ takes nitrile to primary amine; aqueous acid hydrolysis takes it to carboxylic acid.
15. (6.8) An imine forms when:
C. Imines are C=N double bonds. They are used by enzymes (e.g., Schiff bases in retinal vision).
16. (6.8) An enamine forms when:
D. Without an N-H to lose, the nitrogen cannot form a stable C=N. Instead the α-H leaves, giving the enamine.
17. (6.9) Hydride reducing agents (NaBH₄, LiAlH₄):
B. After acid workup, the alkoxide becomes the neutral alcohol.
18. (6.9) LiAlH₄ is a stronger reducing agent than NaBH₄ because:
A. The Al-H bond is more polarized than B-H, delivering "harder" hydride to less electrophilic carbons.
19. (6.10) Adding a Grignard or organolithium to a carbonyl gives:
C. Adding to formaldehyde gives primary alcohols. This is a key way to build up carbon skeletons.
20. (6.10) Grignard reagents cannot be used with substrates containing:
D. Always use anhydrous solvents (diethyl ether or THF) and dry starting materials.
21. (6.11) Aldehydes (but not ketones) can be oxidized to carboxylic acids because:
B. Oxidation of an aldehyde involves only removing that H plus adding an O-H, a net two-electron oxidation.
22. (6.11) Tollens' reagent (Ag(NH₃)₂⁺) oxidizes aldehydes to carboxylates and:
A. The shiny Ag coating is the classic positive test for aldehydes (and reducing sugars).
23. (6.12) The Wolff-Kishner reaction:
C. Complements Clemmensen for substrates that tolerate base but not acid.
24. (6.12) The Clemmensen reduction is:
D. Useful for substrates that are stable under acid but would not survive basic Wolff-Kishner conditions.
The C=O carbonyl is the most important functional group in organic chemistry. Aldehydes and ketones both center on it, differing only in whether a hydrogen or a carbon sits next to the carbonyl. Despite that minor structural variation, the carbonyl’s polarized C=O bond drives a whole family of reactions - nucleophiles attack the electrophilic carbon, electrons push onto the oxygen, and a new single bond forms. Master this one reaction pattern and you will see it reappear in every chapter from 6 through 10.
This chapter is “Aldehydes and Ketones I” - addition reactions where a nucleophile adds to the C=O. Chapter 7 covers “Aldehydes and Ketones II” - enolate chemistry, where the C-H bond alpha to the carbonyl becomes acidic and generates a new nucleophile of its own.
The Central Analogy
In This Chapter
- 6.1 Structure of the Carbonyl Group - Polarization and Electrophilicity
- 6.2 Nomenclature of Aldehydes and Ketones
- 6.3 Physical Properties - Boiling Points, Solubility, Dipoles
- 6.4 Nucleophilic Addition - The Core Reaction
- 6.5 Addition of Water - Hydration to Gem-Diols
- 6.6 Addition of Alcohols - Hemiacetals and Acetals
- 6.7 Addition of HCN - Cyanohydrin Formation
- 6.8 Addition of Amines - Imines and Enamines
- 6.9 Addition of Hydride - NaBH₄ and LiAlH₄
- 6.10 Addition of Organometallic Reagents - Grignard and Gilman
- 6.11 Oxidation of Aldehydes - Tollens, Jones, Permanganate
- 6.12 Wolff-Kishner and Clemmensen Reductions
- Section Test