Chapter 8: Carboxylic Acids
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1. (8.1) The carboxylic acid group (-COOH) consists of:
D. The sharing of one carbon between the two oxygens makes the group chemistry unique.
2. (8.1) Compared with a simple C=O, the carboxyl -COOH group:
A. This resonance is also the reason carboxylate (COO⁻) is so stable once deprotonated.
3. (8.2) Ethanoic acid (common name acetic acid) uses the IUPAC suffix:
B. COOH is the highest-priority functional group; no locant needed because it must sit at the end of the chain.
4. (8.2) The common name for HOOC-CH₂-CH₂-COOH is:
C. Malonic = C3 diacid; succinic = C4; glutaric = C5; adipic = C6.
5. (8.3) Carboxylic acids have unusually high boiling points because:
D. Two hydrogen bonds per dimer essentially double the species being volatilized, sharply raising bp.
6. (8.3) Short-chain carboxylic acids are:
A. Formic, acetic, propionic, butyric acids are fully miscible with water. Fatty acids (long tails) are not.
7. (8.4) Carboxylic acids have pKa values of about 4-5 because:
B. X-ray studies show the two C-O bonds are equal length in carboxylate, confirming full delocalization.
8. (8.4) Compared with alcohols (pKa ~16), carboxylic acids are:
C. The pKa drop of about 11 units corresponds to a factor of ~10¹¹ in acidity.
9. (8.5) An electronegative α-substituent (e.g., Cl) on a carboxylic acid:
D. Inductive effects fall off with distance; substituents on the α-carbon matter most.
10. (8.5) Which is the most acidic?
A. Three chlorines withdraw electron density strongly, making the conjugate base very stable. TCA is almost as strong as HCl.
11. (8.6) Dicarboxylic acids:
B. pKa values depend on how close the two COOH groups are, since the first anion inductively destabilizes the second.
12. (8.6) Oxalic acid (HOOC-COOH) is:
C. Each carboxyl pulls on the other inductively, making oxalic acid more acidic than typical monoacids.
13. (8.7) To reduce a carboxylic acid all the way to a primary alcohol:
D. NaBH₄ is too mild; it will not reduce carboxylic acids. LiAlH₄ can.
14. (8.7) DIBAL (diisobutylaluminum hydride) at low T can:
A. A single hydride equivalent at -78 °C, then aqueous workup, gives the aldehyde cleanly.
15. (8.8) Fischer esterification is:
B. Typical catalyst: sulfuric acid. Proceeds via protonation, nucleophilic addition, proton transfers, loss of water, and deprotonation.
16. (8.8) Fischer esterification equilibrium can be pushed toward the ester by:
C. Drying agents or Dean-Stark traps pull water out of the reaction to push the equilibrium forward.
17. (8.9) Carboxylic acid derivatives, in decreasing reactivity, rank:
A. Better leaving group + less electron donation into the carbonyl = more reactive. Amide N donation makes amides the least reactive derivative.
18. (8.9) Acid chlorides (R-COCl) are highly reactive because:
A. Acid chlorides react with water, alcohols, and amines almost instantly.
19. (8.10) Nucleophilic acyl substitution proceeds by:
B. Addition-elimination at an sp² carbon, not backside attack. The tetrahedral intermediate is the key species.
20. (8.10) A carboxylic acid can be efficiently turned into an amide by:
C. Direct mixing forms an acid-base salt (RCOO⁻ H₃N⁺R') that does not readily condense. Activation makes the COOH into a better electrophile.
21. (8.11) Decarboxylation of a carboxylic acid is:
D. Releasing CO₂ (a very stable molecule) provides thermodynamic drive; the cyclic TS provides kinetic accessibility.
22. (8.11) β-Keto carboxylic acids decarboxylate readily because:
A. A reason to make β-ketoesters in synthesis: they are easy to decarboxylate after alkylation.
23. (8.12) Biological carboxylic acids include:
B. Carboxylic acids underpin energy metabolism, protein structure, and membrane composition.
24. (8.12) A typical fatty-acid COOH has pKa about:
C. This is why fatty acids and amino acid side chains exist as carboxylates in cells.
Carboxylic acids are the family of -COOH compounds: the workhorse acidic functional group of biological and synthetic organic chemistry. Their pKa of about 4-5 makes them much more acidic than alcohols, thanks to the resonance stabilization of their conjugate base (the carboxylate, RCOO⁻). They also undergo a distinctive family of reactions called nucleophilic acyl substitution, which sets them apart from the addition-only aldehydes and ketones of Chapter 6.
The Central Analogy
In This Chapter
- 8.1 Structure of the -COOH Group
- 8.2 Nomenclature and Common Names
- 8.3 Physical Properties - Dimerization and Boiling Points
- 8.4 Acidity - Resonance Stabilization of the Carboxylate
- 8.5 Inductive Effects on Acidity
- 8.6 Dicarboxylic Acids and Their pKa Values
- 8.7 Reduction to Alcohols with LiAlH₄
- 8.8 Fischer Esterification
- 8.9 Formation of Amides, Anhydrides, and Acyl Halides
- 8.10 Nucleophilic Acyl Substitution - The Core Mechanism
- 8.11 Decarboxylation - Losing CO₂
- 8.12 Biological Carboxylic Acids - Fatty Acids, Amino Acids, Krebs Cycle
- Section Test