Functional groups
Functional groups are the “vocabulary” of organic chemistry: they predict polarity, acidity/basicity, and typical reactions. Learn them as electrophiles, nucleophiles, and spectroscopic signatures.
Core set (structures, reactivity, IR clue)
| Group | Key motif | Typical role | IR clue (cm⁻¹) |
|---|---|---|---|
| Alkene | C=C | π nucleophile; electrophilic addition | ~1640–1680 (C=C) |
| Alkyne | C≡C | π nucleophile; terminal alkynes mildly acidic | ~2100–2260 (C≡C) |
| Arene | benzene | EAS (electrophilic aromatic substitution) | ~1450–1600 (C=C aromatic) |
| Alcohol | R–OH | weak nucleophile/base; can be activated | ~3200–3600 broad (O–H) |
| Alkyl halide | R–X | electrophile (SN1/SN2/E1/E2) | ~500–800 (C–X, weak) |
| Carbonyl | C=O | electrophile at carbon; nucleophile at oxygen | ~1650–1800 strong (C=O) |
| Carboxylic acid | R–C(=O)OH | acidic proton; acyl substitution | ~1700 (C=O) + 2500–3300 broad (O–H) |
| Ester | R–C(=O)OR | acyl substitution (slower); carbonyl electrophile | ~1735 (C=O) |
| Amide | R–C(=O)NR₂ | very resonance-stabilized; least reactive acyl derivative | ~1650–1690 (C=O) |
| Amine | R–NH₂/R₂NH/R₃N | base; nucleophile | ~3300 (N–H, if present) |
| Nitrile | C≡N | electrophile at carbon (hard); polar handle | ~2210–2260 sharp (C≡N) |
Carbonyl family: who’s most reactive?
Acyl derivatives compete between resonance stabilization (reduces electrophilicity) and leaving group ability (controls substitution). A useful trend in typical reactivity toward nucleophilic acyl substitution:
acyl chloride > anhydride > ester ≈ carboxylic acid > amide
Why? Better leaving groups (Cl⁻, carboxylate) and less resonance donation into the carbonyl increase reactivity.
⚠️ Common Pitfalls
- Alcohol vs. Carboxylic Acid: Both have -OH, but -COOH is acidic (pKa ~5), while alcohols are not (pKa ~16). Don't confuse them in IR spectra either.
- Amide vs. Amine: An amide nitrogen is not basic because its lone pair is tied up in resonance with the carbonyl. An amine nitrogen is basic.
- Esters vs. Ethers: Esters react with nucleophiles at the carbonyl carbon. Ethers are generally inert (except epoxides).
Quick classification
| Usually nucleophilic | Usually electrophilic |
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