Orgo Reactions Cheat Sheet: Every Named Reaction You Need
There are over 200 named reactions in Organic Chemistry, but only about 32 appear consistently on orgo exams and the MCAT. This guide organizes them by mechanism type with memory hooks — so instead of memorizing 32 separate facts, you learn 5 mechanism families that unlock all of them.
AI-generated content. This guide was written by MedAI's AI and is intended as a study aid. Always cross-reference with your official course materials, textbooks, and instructor guidance before your exam.
Why Organization by Mechanism Beats Memorization
Reactions grouped by name seem unrelated. Reactions grouped by mechanism reveal a pattern: the same electron-pushing logic repeats across different functional groups. Learn the family, and the members follow logically.
Family 1: Nucleophilic Substitution (SN1 & SN2)
| Reaction | Substrate | Reagent | Key Feature |
|---|---|---|---|
| SN2 | Primary alkyl halide | Strong nucleophile (OH⁻, CN⁻, RS⁻) | Inversion of configuration (Walden inversion), bimolecular |
| SN1 | Tertiary alkyl halide | Weak nucleophile in polar protic solvent | Racemization, carbocation intermediate, unimolecular |
| Williamson Ether Synthesis | Primary alkyl halide + alkoxide | RO⁻ (strong nucleophile) | Makes ethers via SN2 |
| Gabriel Synthesis | Primary alkyl halide | Phthalimide anion | Clean route to primary amines without over-alkylation |
Family 2: Elimination (E1 & E2)
| Reaction | Conditions | Key Feature |
|---|---|---|
| E2 | Alkyl halide + strong base (KOH, t-BuO⁻) | Anti-periplanar H and leaving group required; Zaitsev product (more substituted alkene) unless bulky base → Hofmann |
| E1 | Tertiary substrate + weak base in polar protic | Carbocation intermediate, Zaitsev product |
| Hofmann Elimination | Quaternary ammonium + strong base | Always gives LESS substituted alkene (anti-Zaitsev) |
| Cope Elimination | Tertiary amine oxide, heat | Syn elimination; concerted mechanism |
Family 3: Addition to Alkenes
- Hydrohalogenation (HX adds): Markovnikov regioselectivity — H adds to the less substituted carbon, X adds to the more substituted. With peroxides: anti-Markovnikov.
- Halogenation (Br₂/Cl₂): anti addition via cyclic halonium ion intermediate. Trans product.
- Hydration (H₂O/H⁺ catalyst): Markovnikov. Proceeds via carbocation. Rearrangements possible.
- Oxymercuration-Demercuration: Markovnikov addition of water. No carbocation → no rearrangement. Syn-selective.
- Hydroboration-Oxidation: anti-Markovnikov, syn addition. OH ends up on less substituted carbon. Stereospecific.
- Ozonolysis: cleaves C=C double bond. With Zn/H₂O: gives aldehydes/ketones. With H₂O₂: gives carboxylic acids.
Markovnikov vs Anti-Markovnikov
Markovnikov = 'the rich get richer' — H adds to the carbon that already has more H's (less substituted), creating the more stabilized carbocation on the other carbon. Anti-Markovnikov (radical or hydroboration) = opposite.
Family 4: Carbonyl Reactions
| Reaction | Reagent | Product |
|---|---|---|
| Nucleophilic addition (aldehyde/ketone) | RMgBr (Grignard) | Alcohol (primary from HCHO, secondary from RCHO, tertiary from ketone) |
| Aldol condensation | Base (NaOH) + two carbonyl compounds | β-hydroxy carbonyl (aldol product) → α,β-unsaturated carbonyl (after dehydration) |
| Claisen condensation | Strong base + two esters | β-ketoester |
| Reductive amination | Aldehyde/ketone + amine + NaBH₃CN | Amine product (N replaces =O) |
| Wolff-Kishner reduction | Carbonyl + NH₂NH₂ + KOH/heat | Methylene (C=O → CH₂) |
| Cannizzaro reaction | Non-enolizable aldehyde + strong base | Disproportionation: one oxidized to acid, one reduced to alcohol |
Family 5: Aromatic Reactions (EAS)
Electrophilic Aromatic Substitution (EAS) is the defining reaction of benzene and its derivatives. A Lewis acid catalyst generates a strong electrophile that attacks the π system.
- Halogenation: Br₂ + FeBr₃ catalyst → bromobenzene. Cl₂ + FeCl₃ for chlorination.
- Nitration: HNO₃ + H₂SO₄ → generates NO₂⁺ (nitronium ion) → nitrobenzene.
- Sulfonation: H₂SO₄/SO₃ → sulfonic acid. Reversible (unlike other EAS reactions).
- Friedel-Crafts Alkylation: RCl + AlCl₃ → R⁺ attacks ring. Limitation: polyalkylation and carbocation rearrangements.
- Friedel-Crafts Acylation: acyl chloride + AlCl₃ → acyl group added. No rearrangement. Reducible to alkyl with Clemmensen.
- Directing effects: activators (OH, NH₂, OR, alkyl) → ortho/para directors. Deactivators (NO₂, CF₃, COOH) → meta directors. Halogens = ortho/para directors but deactivators.
MCAT Orgo Reaction Priority
The MCAT does not require memorizing every reaction. The highest-yield reactions are: SN1/SN2/E1/E2, Grignard, aldol, Diels-Alder, EAS basics, hydroboration, and ozonolysis. If you know these cold you can answer 85% of MCAT organic questions.
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