Amides, Esters and Anhydrides
High-Yield Summary
- All three derivatives (amides, esters, anhydrides) trace back to a carboxylic acid's carbonyl carbon with the hydroxyl group swapped out: amides → amino group, esters → alkoxy group, anhydrides → a second carbonyl bridged by one oxygen.
- Amides: -oic acid → -amide; N-alkyl substituents on the nitrogen get the "N-" prefix (e.g., N-methyl). Lactams (cyclic amides) named by ring size: β (4-membered, penicillin core), γ (5-membered), δ (6-membered).
- Esters form via dehydration synthesis (acid + alcohol → ester + water); named esterifying-group-first + "-oate" (ethyl ethanoate, isopropyl butanoate). Lab synthesis = Fischer esterification.
- Lactones (cyclic esters) named by ring size: α-acetolactone (3), β-propiolactone (4), γ-butyrolactone (5). Triacylglycerols = glycerol + 3 fatty acids via 3 ester bonds; saponification (strong base) reverses this into glycerol + soap.
- Anhydrides = condensation dimers of two carboxylic acids (lose water). Symmetrical: acid name, "acid"→"anhydride" (succinic anhydride). Asymmetrical: both acid names alphabetically + "anhydride" (ethanoic propanoic anhydride). Cyclic anhydrides form on heating from dicarboxylic acids with adjacent COOH groups.
Fischer Esterification Mechanism
- 1Acid catalyst (commonly H2SO4) protonates the carbonyl oxygen of the carboxylic acid, increasing the carbonyl carbon's electrophilicity.
- 2The alcohol attacks the carbonyl carbon, breaking the double bond and forming a tetrahedral intermediate.
- 3One of the intermediate's hydroxyl groups is protonated, converting it into water — a good leaving group.
- 4Water leaves, the carbonyl bond reforms, producing the ester. Overall products: ester + water.
Key Terms
- Amide
- R-C(=O)-NR₂ — carbonyl bonded directly to nitrogen; formed from an acid's -oic acid ending → -amide.
- Lactam
- Cyclic amide, named by Greek letter for ring size/nitrogen position: β (4-membered), γ (5-membered), δ (6-membered).
- Ester
- Formed by dehydration synthesis of a carboxylic acid + alcohol; named esterifying-group + "-oate."
- Lactone
- Cyclic ester, formed intramolecularly between a hydroxyl and a carboxylic acid group; named by Greek letter for ring size.
- Triacylglycerol
- Glycerol + 3 fatty acids joined by 3 ester bonds; the body's storage form of fat, a.k.a. triglyceride.
- Saponification
- Base-catalyzed hydrolysis of triacylglycerol ester bonds (strong base + heat) → glycerol + fatty acid salts (soap).
- Anhydride
- Condensation dimer of two carboxylic acids, losing one water molecule; two carbonyls bridged by a single oxygen.
Symmetrical vs. Asymmetrical Anhydride Naming
| Anhydride type | Naming rule |
|---|---|
| Symmetrical (both sides same acid) | Acid name with "acid" → "anhydride" (e.g., succinic acid → succinic anhydride) |
| Asymmetrical (two different acids) | Both acid names in alphabetical order + "anhydride" (e.g., ethanoic propanoic anhydride) |
Common MCAT Trap
- Ester naming order is reversed from what feels intuitive: the alcohol-derived group is named FIRST (as its own word), then the acid-derived "-oate" part — e.g. ethanol + ethanoic acid → "ethyl ethanoate," not "ethanoate ethyl."
- Don't confuse lactam (cyclic amide, N in ring) with lactone (cyclic ester, O in ring) — same Greek-letter ring-size naming convention applies to both, easy to mix up under time pressure.
- Cyclic anhydrides need no added reagent — just heat — when the two carboxyl groups are positioned close together (e.g., adjacent carbons), as in ortho-phthalic acid → phthalic anhydride.
Quick Recall
What is the general formula and naming pattern for an amide?
Why are penicillin-type β-lactams so reactive?
What does saponification of a triacylglycerol produce, and under what conditions?
How do you name an asymmetrical anhydride formed from ethanoic acid and propanoic acid?