FAD1018 Organic — Reactions & Mechanisms
[!note] Caveman edition. No fluff. Only reactions. ⭐ = exam priority. Memorize mechanisms cold — Sir Farhan gives no hints.
Quick index:
| Section | Weight |
|---|---|
| Alcohol Reactions | ⭐⭐⭐ |
| Phenol Reactions | ⭐⭐⭐ |
| Carbonyl Reactions | ⭐⭐⭐ |
| Carboxylic Acid Reactions | ⭐⭐ |
| Amine Reactions | ⭐⭐ |
| Polymerization Mechanisms | ⭐ |
Alcohol Reactions ⭐⭐⭐
1. Active Metal (Na/K)
2 ROH + 2 Na → 2 RONa + H₂ ↑
Reactivity: methanol > ethanol > 2° > 3°. 3° very slow. Use K or NaH/THF instead.
2. Substitution → Haloalkane
| Reagent | Product | Notes |
|---|---|---|
| HX (HCl/HBr/HI) | R-X | Ease: 3° > 2° > 1° > phenol. HCl < HBr < HI |
| HCl + ZnCl₂ (Lucas) | R-Cl | Test: cloudiness rate = 3° fastest, 1° slowest |
| PX₃ (PCl₃/PBr₃) | R-X | Good 1°/2°. No rearrangement. Bad for 3°. |
| SOCl₂ + pyridine | R-Cl | Gaseous by-products. Retention of configuration. |
Lucas mechanism: Zn²⁺ coordinates O lone pair → weakens C-O bond → carbocation forms (3° fast, 2° slower, 1° no).
3. Dehydration (Elimination)
| Conditions | Product |
|---|---|
| conc. H₂SO₄, 180 °C | Alkene |
| 85% H₃PO₄, 350 °C | Alkene |
| Al₂O₃, 350 °C | Alkene |
| conc. H₂SO₄, ~140 °C | Symmetrical ether |
Mechanism: E1 (3°, 2°) or E2 (1°).
- Zaitsev's rule: most substituted alkene = major product.
- Ease: 3° > 2° > 1° (carbocation stability).
- Rearrangement possible (hydride/alkyl shift).
4. Oxidation
1° alcohol --[O]--> aldehyde --[O]--> carboxylic acid
2° alcohol --[O]--> ketone
3° alcohol --[O]--> resistant (need C-C cleavage)
| Substrate | Reagent | Stops At |
|---|---|---|
| 1° | PCC, CH₂Cl₂ | Aldehyde |
| 1° | K₂Cr₂O₇/H⁺ or KMnO₄/H⁺ | Carboxylic acid (over-oxidizes) |
| 2° | Any (PCC, K₂Cr₂O₇, KMnO₄) | Ketone (can't over-oxidize) |
Chromic acid test: orange → green/blue = 1°/2° present. No change = 3°, ketone, alkane.
5. Esterification
Alcohol + Carboxylic acid --H⁺--> Ester + H₂O
Equilibrium. Use excess alcohol or remove water to push.
Alcohol + Acyl chloride --> Ester + HCl
Irreversible. Exothermic. Faster method.
6. Iodoform Test (CH₃CH(OH)-)
Positive for structure CH₃-CH(OH)-R (ethanol only 1° alcohol). Also positive for CH₃-CO- (methyl ketones, ethanal). Yellow CHI₃ precipitate.
Phenol Reactions ⭐⭐⭐
1. O-H Bond: Salt Formation
C₆H₅OH + NaOH → C₆H₅ONa + H₂O
Phenol more acidic than water → reacts with NaOH. Does NOT react with Na₂CO₃/NaHCO₃ (carboxylic acid test).
2. Ester Formation
C₆H₅ONa + RCOCl → C₆H₅OCOR + NaCl
Phenol itself poor nucleophile. Phenoxide = good nucleophile.
3. Electrophilic Aromatic Substitution
-OH strongly activating, ortho/para directing. No Lewis acid catalyst needed (unlike benzene).
Halogenation:
- Low T, non-polar: o- + p-monohalophenol.
- Aqueous, higher T: 2,4,6-trihalophenol (white ppt with Br₂).
Nitration:
- Dilute HNO₃, RT: o- + p-nitrophenol.
- Conc. HNO₃: 2,4,6-trinitrophenol (picric acid).
4. What Phenol Does NOT Do
- No acid-catalyzed elimination (no H on carbinol C).
- No SN2 (C-O too strong).
- Not easily oxidized.
Identification Tests
Bromine water: decolorizes + white 2,4,6-tribromophenol ppt. FeCl₃ (aq): light purple complex.
Carbonyl Reactions ⭐⭐⭐
1. Nucleophilic Addition (General)
Nu⁻ + C=O → Nu-C-O⁻ --H⁺--> Nu-C-OH
Electrophilic C (partial +) attacked by nucleophile. Mechanism: Nu attack → tetrahedral intermediate → protonation.
With HCN (Cyanohydrin)
R-CHO + HCN --OH⁻--> R-CH(OH)-CN
Base-catalyzed. One C extension. α-hydroxynitrile product.
With Grignard (RMgX)
Formaldehyde + RMgX → 1° alcohol
Aldehyde + RMgX → 2° alcohol
Ketone + RMgX → 3° alcohol
Anhydrous conditions essential. Water destroys Grignard.
With Alcohols (Acetal/Ketal)
Aldehyde + ROH --H⁺--> Hemiacetal + ROH --H⁺--> Acetal + H₂O
Ketone + ROH --H⁺--> Hemiketal + ROH --H⁺--> Ketal + H₂O
Acetals/ketals = protecting groups for carbonyls. Reversible under acidic aqueous conditions.
With NaHSO₃
>CO + NaHSO₃ → >C(OH)SO₃Na (crystalline solid)
Purification technique. Aldehydes > methyl ketones > other ketones.
2. Addition-Elimination (N Nucleophiles)
Same first step (Nu attacks C=O). Then elimination of H₂O.
| Reagent | Product | Test Use |
|---|---|---|
| NH₂OH (hydroxylamine) | Oxime (>C=N-OH) | — |
| NH₂NH₂ (hydrazine) | Hydrazone (>C=N-NH₂) | — |
| 2,4-DNPH (Brady's) | 2,4-DNP derivative | Orange/red ppt = carbonyl present |
| RNH₂ (1° amine) | Imine (Schiff base) (>C=N-R) | — |
[!warning] Brady's reagent = 2,4-DNP. Know this name for exam.
3. Oxidation (Aldehydes Only)
Ketones resistant to oxidation.
| Test | Reagent | Positive Result |
|---|---|---|
| Tollens' | [Ag(NH₃)₂]⁺ | Silver mirror |
| Fehling's | Cu²⁺ (tartrate complex) | Brick-red Cu₂O |
| Benedict's | Cu²⁺ (citrate complex) | Brick-red Cu₂O |
| Schiff's | Schiff reagent | Pink/magenta |
4. Reduction
Aldehyde + NaBH₄/LiAlH₄ → 1° alcohol
Ketone + NaBH₄/LiAlH₄ → 2° alcohol
NaBH₄ = mild. LiAlH₄ = strong (also reduces acids/esters).
5. Special Reactions
Aldol condensation:
- Two carbonyl molecules with α-H.
- Base-catalyzed. Forms β-hydroxy carbonyl → heat → α,β-unsaturated carbonyl.
- C-C bond forming reaction.
Cannizzaro reaction:
- Aldehyde with no α-H (e.g. formaldehyde, benzaldehyde).
- Disproportionation: one oxidized to acid, one reduced to alcohol.
- Concentrated base required.
Haloform reaction:
- Methyl ketones (CH₃-CO-R) + X₂/OH⁻.
- Iodoform test: CHI₃ yellow precipitate.
Carboxylic Acid Reactions ⭐⭐
1. Salt Formation (Acid-Base)
R-COOH + NaOH → R-COONa + H₂O
R-COOH + NaHCO₃ → R-COONa + CO₂ + H₂O ← Effervescence = carboxylic acid test
Distinguishes from phenol (phenol no CO₂ with carbonate).
2. Esterification (Fischer)
R-COOH + R'-OH --H⁺--> R-COOR' + H₂O
Equilibrium. Acid catalyst. Heat. Le Châtelier: remove H₂O or use excess alcohol.
3. Reduction
R-COOH --LiAlH₄--> R-CH₂OH (1° alcohol)
LiAlH₄ required. NaBH₄ cannot reduce carboxylic acids.
4. Derivative Formation
R-COOH + SOCl₂ → R-COCl (acyl chloride) + SO₂ + HCl
R-COOH + PCl₅ → R-COCl + POCl₃ + HCl
Acyl chlorides most reactive derivative → entry point to all other derivatives.
5. Decarboxylation
R-COOH --heat--> R-H + CO₂
Special case: β-keto acids lose CO₂ easily. General: requires specific conditions.
Derivative Reactions
Acyl chloride →:
- Alcohol → Ester + HCl
- Amine → Amide + HCl
- Water → Carboxylic acid + HCl
- Gilman reagent → Ketone
Ester →:
- Hydrolysis (H⁺ or OH⁻/saponification) → Acid + Alcohol
- Reduction (LiAlH₄) → Two alcohols
- Transesterification → Different ester
- Grignard (excess) → 3° alcohol
Amide →:
- Hydrolysis → Acid + Amine/Ammonia
- Dehydration (P₂O₅) → Nitrile
- Hoffmann rearrangement (Br₂/NaOH) → 1° amine (C count −1)
Reactivity Chain
Acyl chloride > Anhydride > Ester > Acid > Amide
(Leaving group quality: Cl⁻ > RCOO⁻ > RO⁻ > HO⁻ > NH₂⁻)
Amine Reactions ⭐⭐
1. Alkylation (Nucleophilic Substitution)
RNH₂ + R'X → R-NH-R' + HX
Can over-alkylate → mixture of 2°, 3°, quaternary. Poor synthetic method for pure products.
2. Acylation (Amide Formation)
RNH₂ + RCOCl → RCONHR + HCl
RNH₂ + (RCO)₂O → RCONHR + RCOOH
RNH₂ + RCOOR' → RCONHR + R'OH
1° and 2° amines react. 3° amines DO NOT (no N-H proton to transfer to Cl).
3. Reaction with Nitrous Acid (HNO₂) — Distinguishing Test
HNO₂ prepared in situ: HCl/NaNO₂ or H₂SO₄/NaNO₂.
| Amine | Product | Observation |
|---|---|---|
| 1° aliphatic | Unstable diazonium → N₂ + carbocation → alkene/alcohol/haloalkane | N₂ bubbles |
| 1° aromatic | Stable diazonium salt (<5 °C) | No gas at low T |
| 2° (aliphatic + aromatic) | N-nitrosamine | Yellow oil |
| 3° aliphatic | Trialkylammonium salt + nitrosoammonium chloride | Clear solution |
| 3° aromatic | C-nitrosation at para | Solid precipitate |
Diazonium salt (1° aromatic) reactions:
| Reagent | Product |
|---|---|
| H₃O⁺, heat | Phenol |
| CuCl | Chlorobenzene |
| CuBr | Bromobenzene |
| CuCN | Benzonitrile |
| KI | Iodobenzene |
| Phenol (azo coupling) | Orange azo dye |
4. Bromine Water (Aniline)
Aniline + 3 Br₂(aq) → 2,4,6-tribromoaniline (white ppt) + 3 HBr
Brown bromine decolorizes + white precipitate = positive test. Reason: -NH₂ strongly activating, ortho/para directing.
5. Hinsberg Test
Distinguishes 1°, 2°, 3° amines with benzenesulfonyl chloride.
- 1°: forms soluble sulfonamide (deprotonated in base).
- 2°: forms insoluble sulfonamide.
- 3°: no reaction.
Amino Acid Reactions
Carboxyl group reactions:
- With NaOH → salt + H₂O (neutralization).
- With alcohol + H⁺ → ester + H₂O (esterification).
Amino group reactions:
- With HCl → aminium salt.
- With acid chloride → acyl derivative.
- With HNO₂ at 0 °C → α-hydroxy carboxylic acid.
Zwitterion: NH₃⁺-CHR-COO⁻ at neutral pH. Net charge = 0. High melting point (ionic solid).
Peptide bond: -CO-NH- formed by condensation (H₂O eliminated). Between carboxyl of one amino acid + amino of another.
Polymerization Mechanisms ⭐
Addition (Chain-Growth)
Unsaturated monomers (C=C). Peroxide initiator (CH₃OOCH₃). No small molecule eliminated. Polymer formula = monomer formula.
Stages:
- Initiation: Radical forms (peroxide → 2 RO•).
- Propagation: Radical attacks C=C → new radical → chain grows.
- Termination: Two radicals combine or disproportionate.
Examples: PE, PVC, PS, PTFE, PP, neoprene, SBR.
Condensation (Step-Growth)
Monomers need ≥2 functional groups each. Small molecule eliminated (H₂O, CH₃OH, HCl). Molecular weight builds slowly.
Two major classes:
Polyamide: Diacid + diamine → amide links + H₂O.
- Nylon 6,6: hexane-1,6-diamine + adipic acid.
- Kevlar: 1,4-diaminobenzene + terephthalic acid.
Polyester: Diacid + diol → ester links + H₂O.
- PET/Dacron: terephthalic acid + ethylene glycol.
- Terylene: dimethyl terephthalate + ethylene glycol.
Key difference:
- Addition: no by-product. Double bond breaks, chain forms.
- Condensation: by-product (H₂O, etc.) eliminated. Functional groups react.