Organic chemistry

Mechanisms, functional groups, test-tube analysis

The mechanisms to know cold

• Free-radical substitution (alkanes + Cl₂, UV): initiation → propagation → termination.
• Electrophilic addition (alkenes + HBr/Br₂): Markovnikov — H adds to the carbon with more H; the more stable carbocation forms the major product.
• Nucleophilic substitution (haloalkanes + OH⁻/CN⁻/NH₃) and elimination (ethanolic KOH → alkene).
• Nucleophilic addition (carbonyls + HCN/NaBH₄), electrophilic substitution (benzene, year 2).

Identifying functional groups (RP6 & RP10)

• Alkene: bromine water orange → colourless.
• Aldehyde: Tollens' reagent → silver mirror; Fehling's → brick-red precipitate. Ketones: no change with either.
• Any carbonyl (C=O): 2,4-DNPH → orange precipitate; its melting point identifies the compound.
• Primary/secondary alcohol or aldehyde: acidified potassium dichromate orange → green. Tertiary alcohols: stays orange.
• Carboxylic acid: sodium carbonate → effervescence (CO₂ turns limewater milky).

Distinguish an aldehyde from a ketone: both give orange ppt with 2,4-DNPH, but only the aldehyde gives a silver mirror with Tollens'.

3.3.1Naming and isomerism rules

• Longest chain = stem; number from the end giving the lowest locants; substituents alphabetical (di/tri ignored when ordering).
• Structural isomers: chain, position, functional group. Stereoisomers: E/Z (restricted C=C rotation, two different groups per carbon — use CIP priority: higher atomic number wins) and optical (year 2).
• Optical isomerism: a carbon with four different groups is chiral; the two mirror-image enantiomers rotate plane-polarised light in opposite directions. A 50:50 racemic mixture shows no rotation.

3.3.4The mechanisms, step by step

• Free-radical substitution (CH₄ + Cl₂, UV): initiation Cl₂ → 2Cl• (homolysis); propagation Cl• + CH₄ → HCl + •CH₃ then •CH₃ + Cl₂ → CH₃Cl + Cl•; termination any two radicals combine.
• Electrophilic addition (propene + HBr): π-electrons attack Hδ+; H adds to the CH₂ end giving the secondary carbocation; Br⁻ attacks it → 2-bromopropane (major).
• Nucleophilic substitution (haloalkane + OH⁻/CN⁻/NH₃): the nucleophile's lone pair attacks Cδ+; C–X breaks heterolytically. Rate: C–I fastest (weakest bond) — bond enthalpy beats polarity.
• Elimination (haloalkane + ethanolic KOH, hot): OH⁻ acts as a base, removing an H from the carbon adjacent to C–X → alkene + water + halide.

3.3.3Reaction condition pairs that decide the product