Important Named Reactions
Named reactions that appear in CBSE Class 11 Chemistry annual exams — with reagents, conditions, products, and tips on what examiners look for.
Alkanes — Free Radical Reactions
Free Radical Halogenation
very high frequencyReagents / Substrate
Alkane (e.g., CH₄) + Cl₂ or Br₂
Conditions
UV light or high temperature (hν or Δ)
Product
Haloalkane + HX (e.g., CH₃Cl + HCl)
Substitution reaction of alkanes via free radical chain mechanism — initiation, propagation, termination steps
CH₄ + Cl₂ → CH₃Cl + HCl (light/heat). Mention all three steps: initiation, propagation, termination. Chlorination is faster but less selective than bromination.
Alkenes — Addition Reactions
Markovnikov's Rule
very high frequencyReagents / Substrate
Alkene + HX (HBr, HCl, HI)
Conditions
No peroxide (ionic addition)
Product
Haloalkane — H adds to carbon bearing more H atoms
Predicts regioselectivity of HX addition to unsymmetrical alkenes; H⁺ adds to less substituted carbon
CH₂=CH₂ + HBr → CH₃CH₂Br. For propene: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2-bromopropane). 'H goes to carbon with more H' — Markovnikov's rule.
Anti-Markovnikov Addition (Peroxide Effect / Kharash Effect)
very high frequencyReagents / Substrate
Alkene + HBr
Conditions
Presence of peroxides (ROOR) — free radical mechanism
Product
Haloalkane — Br adds to carbon bearing more H atoms (anti-Markovnikov)
Reversal of Markovnikov's rule only with HBr and peroxides; operates via free radical chain mechanism
CH₃CH=CH₂ + HBr (peroxide) → CH₃CH₂CH₂Br (1-bromopropane). Peroxide effect is observed ONLY with HBr, NOT HCl or HI.
Ozonolysis
high frequencyReagents / Substrate
Alkene + O₃ → ozonide
Conditions
O₃ in CCl₄ or CH₂Cl₂; then reductive work-up: Zn/H₂O (or oxidative: H₂O₂)
Product
Aldehydes and/or ketones (after Zn/H₂O work-up)
Cleaves C=C double bond — used to determine position of double bond in a molecule
Reductive ozonolysis (Zn/H₂O) gives aldehydes from terminal carbons and ketones from internal ones. Oxidative (H₂O₂) converts aldehydes to carboxylic acids.
Catalytic Hydrogenation
high frequencyReagents / Substrate
Alkene or alkyne + H₂
Conditions
Ni, Pt, or Pd catalyst; high pressure and temperature
Product
Alkane (complete reduction)
Syn addition of H₂ across double/triple bond; used to convert unsaturated to saturated hydrocarbons
R-CH=CH-R + H₂ → R-CH₂-CH₂-R. Both H atoms add to the same face (syn addition). Pt and Pd work at room temperature; Ni requires higher temperature.
Alkynes — Addition Reactions
Lindlar's Catalyst (Partial Hydrogenation of Alkynes)
high frequencyReagents / Substrate
Alkyne + H₂
Conditions
Lindlar's catalyst: Pd deposited on CaCO₃, poisoned with quinoline
Product
cis-Alkene (syn addition — same side)
Selective partial hydrogenation — stops at alkene stage giving cis (Z) product; used in stereospecific synthesis
Alkyne + H₂ (Lindlar's) → cis-alkene. For trans-alkene use Na/liq. NH₃ (Birch-type). Quinoline poisons the catalyst to prevent over-reduction.
Benzene — Electrophilic Substitution
Halogenation of Benzene
very high frequencyReagents / Substrate
Benzene (C₆H₆) + Cl₂ or Br₂
Conditions
Anhydrous Lewis acid catalyst: AlCl₃ or FeCl₃ (no light)
Product
Chlorobenzene (C₆H₅Cl) + HCl
Electrophilic aromatic substitution (EAS) — ring acts as nucleophile; aromaticity restored after substitution
C₆H₆ + Cl₂ → C₆H₅Cl + HCl (anhydrous AlCl₃). Distinguish from addition: EAS preserves the benzene ring. AlCl₃ activates Cl₂ to form Cl⁺ electrophile.
Nitration of Benzene
very high frequencyReagents / Substrate
Benzene + conc. HNO₃ (nitrating agent)
Conditions
Conc. H₂SO₄ (catalyst), 50–60°C
Product
Nitrobenzene (C₆H₅NO₂) + H₂O
EAS — introduces –NO₂ group; nitronium ion (NO₂⁺) is the electrophile generated by H₂SO₄
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (conc. H₂SO₄, 50–60°C). Temperature must be controlled — above 60°C gives dinitrobenzene. Electrophile = NO₂⁺ (nitronium ion).
Sulfonation of Benzene
high frequencyReagents / Substrate
Benzene + fuming H₂SO₄ (oleum, H₂S₂O₇)
Conditions
Fuming H₂SO₄ (SO₃ in H₂SO₄); reversible on heating with steam
Product
Benzenesulfonic acid (C₆H₅SO₃H) + H₂O
EAS — introduces –SO₃H group; reaction is reversible (desulfonation possible with steam)
Sulfonation is REVERSIBLE — heating benzenesulfonic acid with steam regenerates benzene. Electrophile = SO₃. Used in detergent manufacture.
Friedel-Crafts Alkylation
very high frequencyReagents / Substrate
Benzene + alkyl halide (R-Cl)
Conditions
Anhydrous AlCl₃ (Lewis acid catalyst)
Product
Alkylbenzene (C₆H₅-R) + HCl
EAS — introduces alkyl group onto ring; drawback is polyalkylation because alkyl group activates ring
C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl (anhydrous AlCl₃). Problem: polyalkylation. Carbocation rearrangement can also occur with primary halides.
Friedel-Crafts Acylation
very high frequencyReagents / Substrate
Benzene + acyl halide (RCOCl) or acid anhydride
Conditions
Anhydrous AlCl₃ (Lewis acid catalyst)
Product
Aryl ketone (C₆H₅COR) + HCl
EAS — introduces acyl group; no polyacylation because –COR is electron-withdrawing (deactivates ring)
C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (anhydrous AlCl₃). Preferred over alkylation in synthesis — no polysubstitution and no rearrangement. Gives ketone, not aldehyde.
Birch Reduction
moderate frequencyReagents / Substrate
Benzene + Na or Li metal + liquid NH₃ + alcohol (ROH as proton source)
Conditions
Liquid NH₃, sodium/lithium metal, t-BuOH or EtOH as proton source
Product
1,4-Cyclohexadiene (unconjugated diene)
Partial reduction of benzene ring to non-conjugated 1,4-diene; electrons attack unsubstituted positions
Birch reduction gives 1,4-cyclohexadiene (not 1,3). Electron-donating substituents: double bonds stay away from substituent. Electron-withdrawing substituents: double bonds stay near substituent.