1. Selective vs Vigorous Oxidizing Agents
Core oxidants in organic synthesis: (1) Pyridinium Chlorochromate (PCC in anhydrous CH2Cl2): Mild and selective. Oxidizes primary alcohols to ALDEHYDES (stops oxidation without producing carboxylic acids) and secondary alcohols to KETONES. (2) Alkaline KMnO4 or Acidified K2Cr2O7: Vigorous oxidants. Oxidizes primary alcohols and aldehydes directly to CARBOXYLIC ACIDS; oxidizes alkylbenzenes with at least one benzylic hydrogen (toluene, ethylbenzene, isopropylbenzene) directly to Benzoic Acid (tert-butylbenzene does not react due to lack of benzylic H). (3) Jones Reagent (CrO3 in aqueous H2SO4/acetone): Oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones. (4) Tollens' Reagent ([Ag(NH3)2]+ OH-): Ammoniacal silver nitrate; selectively oxidizes aliphatic and aromatic aldehydes to carboxylates with distinctive Silver Mirror precipitate (does NOT oxidize ketones). (5) Fehling's Solution (CuSO4 + alkaline sodium potassium tartrate / Rochelle salt): Oxidizes aliphatic aldehydes producing red precipitate of Cu2O (benzaldehyde does NOT reduce Fehling's).
2. Reducing Agents: Strength & Functional Group Selectivity
Targeted reductions: (1) Lithium Aluminium Hydride (LiAlH4 in dry ether): Extremely powerful reducing agent. Reduces aldehydes and ketones to alcohols; reduces carboxylic acids, esters, acid anhydrides, and acyl chlorides to primary alcohols; reduces amides and nitriles to primary amines. C=C double bonds are NOT reduced (except when conjugated with phenyl ring in cinnamaldehyde). (2) Sodium Borohydride (NaBH4 in ethanol or water): Mild and highly selective. Reduces ONLY aldehydes, ketones, and acyl chlorides to corresponding alcohols. Does NOT reduce esters, amides, or carboxylic acids. (3) DIBAL-H (Diisobutylaluminium hydride, [(CH3)2CHCH2]2AlH at -78 °C): Selectively reduces esters and nitriles (cyanides) directly to ALDEHYDES without over-reduction to alcohols. (4) Catalytic Hydrogenation (H2 with Ni, Pt, or Pd): Readily reduces alkenes, alkynes, nitro groups (-NO2 -> -NH2), and nitriles.
3. Organometallic Reactions: The Grignard Reagent (R-Mg-X)
Preparation: Alkyl/aryl halide reacted with Magnesium metal in anhydrous dry ether: R-X + Mg -> R-Mg-X (carbon carries strong partial negative charge δ-, acting as powerful nucleophile and strong base). (1) Nucleophilic Addition to Carbonyl Compounds (followed by H3O+): Formaldehyde (HCHO) + RMgX -> PRIMARY Alcohol (R-CH2OH); Any other Aldehyde (R'CHO) + RMgX -> SECONDARY Alcohol (R-CH(OH)-R'); Ketone (R'COR'') + RMgX -> TERTIARY Alcohol (R-C(OH)(R')(R''); Solid CO2 (Dry Ice) + RMgX -> Carboxylic Acid (R-COOH); (2) Reaction with Active Hydrogen: Any compound with acidic hydrogen (H2O, ROH, RNH2, terminal alkynes R-C≡CH) destroys Grignard reagent instantly, producing corresponding alkane: RMgX + H2O -> R-H + Mg(OH)X.
4. Diagnostic Chemical Tests for Functional Groups
Distinguishing functional groups in lab analysis: (1) Lucas Test (conc. HCl + anhydrous ZnCl2): Distinguishes 1°, 2°, and 3° alcohols based on alkyl chloride cloudiness: Tertiary alcohols produce immediate turbidity; Secondary alcohols form turbidity within 5 minutes; Primary alcohols remain clear at room temperature and form turbidity only upon heating; (2) Iodoform Test (I2 + aqueous NaOH): Yellow crystalline precipitate of Iodoform (CHI3, mp 119 °C) forms with compounds containing CH3-C=O group (Acetaldehyde, all methyl ketones like acetone, acetophenone) or CH3-CH(OH)- group (Ethanol, propan-2-ol, butan-2-ol); (3) Hinsberg Test (Benzenesulphonyl chloride, C6H5SO2Cl): Distinguishes amines: 1° amine forms N-alkylbenzenesulphonamide which is SOLUBLE in alkali (possesses acidic N-H); 2° amine forms N,N-dialkyl derivative which is INSOLUBLE in alkali; 3° amine does not react; (4) Carbylamine Test: Primary amines (aliphatic or aromatic) heated with chloroform (CHCl3) and alcoholic KOH produce extremely foul-smelling isocyanides (R-NC).
5. Dehydration & Halogenation Reagents
Specialized transformation agents: (1) Thionyl Chloride (SOCl2 in pyridine): Converts alcohols to alkyl chlorides (Darzens process): R-OH + SOCl2 -> R-Cl + SO2(g) + HCl(g). Preferred because gaseous byproducts bubble out, leaving exceptionally pure alkyl chloride; (2) Concentrated H2SO4 at 443 K: Dehydrates ethanol to ethene (CH3CH2OH -> CH2=CH2 + H2O); at 413 K with excess alcohol: intermolecular dehydration produces diethyl ether (CH3CH2-O-CH2CH3); (3) Phosphorus tribromide (PBr3) and PCl5: Converts -OH groups into corresponding halides.
6. Teacher's Exam Hall Hack: The 2-Step Carbon Ladder Rule
In organic conversion questions, always determine first if the carbon count must INCREASE (step-up) or DECREASE (step-down): To STEP-UP by 1 carbon: react alkyl halide with KCN (R-X + KCN -> R-CN), which can then be hydrolyzed with H3O+ to carboxylic acid (R-COOH) or reduced with LiAlH4 to amine (R-CH2NH2); to step-up by multiple carbons: use Grignard addition. To STEP-DOWN by 1 carbon: convert carboxylic acid to amide (R-CONH2) and apply Hoffmann Bromamide degradation (Br2 + NaOH), dropping one carbon atom as Na2CO3 to yield pure R-NH2.