Alkenes & Alkynes: Addition Reactions, Markovnikov, Ozonolysis Why this matters for NEET and real life Alkenes and alkynes are the workhorses of petrochemicals. Predicting where and how molecules add across C=C and C≡C is a perennial NEET favourite. Once you understand who attacks whom (electrophile vs nucleophile), Markovnikov vs anti-Markovnikov becomes almost automatic. Ethene (trigonal planar, sp2) vs Ethyne (linear, sp). See how a π bond (above–below) makes C=C reactive; two π bonds make C≡C linear and more acidic but less eager for electrophiles. Glossary — learn the words you’ll meet in questions alk-ene Unsaturated hydrocarbon with at least one C=C double bond; general formula C n H 2n for acyclic mono-alkenes. Alkene olefin acetylene (for ethyne) alk-yne Alkyne Unsaturated hydrocarbon with at least one C≡C triple bond; general formula C n H 2n-2 for acyclic mono-alkynes. π bond Sideways overlap of p orbitals. Present in C=C (one π) and C≡C (two π). Weak to electrophilic attack compared to σ. Electrophilic addition Reaction where an electrophile first adds to the π bond, followed by nucleophile. Typical of alkenes/alkynes. Markovnikov's rule In addition of HX to an unsymmetrical alkene, H attaches to the carbon with more H (less substituted), X to the more substituted carbon, via the more stable carbocation. With HBr and peroxides, Br• adds first to give the less substituted radical, overall Br ends at the less substituted carbon. Anti-Markovnikov (peroxide effect) Another name for the peroxide effect (anti-Markovnikov HBr addition). Kharasch effect Lindlar's catalyst Pd on CaCO3 poisoned with Pb (sometimes quinoline): partially hydrogenates alkynes to cis-alkenes. Cleavage of C=C (or C≡C) by ozone to carbonyls after reductive workup; maps the double-bond position. Ozonolysis Cold, dilute, alkaline KMnO4 oxidizes C=C to vicinal diol; purple decolorizes to brown MnO2. Baeyer test BH3 (or B2H6) adds syn across C=C, then H2O2/NaOH gives anti-Markovnikov alcohol. Hydroboration–oxidation Oxymercuration–demercuration Hg(OAc)2/H2O adds Markovnikov OH across C=C without carbocation rearrangement; NaBH4 removes Hg. Alkyne with RC≡CH; more acidic (pKa ~25); forms acetylide anions with strong base. Terminal alkyne Anion RC≡C− (as Na+, Ag+, or Cu+ salts) from deprotonation of terminal alkynes. Acetylide Saytzeff (Zaitsev) product In elimination giving an alkene, the more substituted (more stable) alkene predominates. General formula for alkenes General formula for alkynes Structures, hybridization, and geometrical isomerism Alkene carbons are sp2-hybridized, trigonal planar, approximately 120°. A π bond locks rotation, so alkenes can show geometrical isomerism: cis/trans (or E/Z by Cahn–Ingold–Prelog). Alkynes have sp-hybridized carbons, linear (180°), two π bonds perpendicular to each other. Terminal alkynes (RC≡CH) are significantly more acidic than alkenes/alkanes due to greater s-character in the C–H bond. Geometrical (cis/trans; E/Z) quick-check Must have restricted rotation (C=C) and each double-bond carbon must carry two different groups. cis/trans works when each side has a comparable "A vs B" distinction. When substituents differ beyond simple H vs Me, use E/Z priority rules. E = higher priority groups on opposite sides; Z = zusammen (together) on the same side. Preparations of alkenes you must know Alcohols → Alkenes (acid-catalysed elimination) 1) Dehydration of alcohols: Conc. H2SO4 at about 443 K (or H3PO4) eliminates water to give alkenes. Major product follows Saytzeff’s rule (more substituted alkene). 2) Dehydrohalogenation: Alkyl halides with alcoholic KOH eliminate HX to give alkenes; Saytzeff major product. 3) Dehalogenation of vicinal dihalides: Zn dust in alcohol removes halogens to give an alkene. 4) Partial hydrogenation of alkynes: Lindlar’s catalyst gives cis-alkene; Na/liquid NH3 (Birch-type) gives trans-alkene. Alkyl halide + alcoholic KOH → Alkene + KX + H2O Elimination favours the more substituted alkene 2026-05-26T17:05:24.206Z How catalysts control stereochemistry: Lindlar gives cis-alkene; Na/NH3 gives trans-alkene from the same alkyne. gpt-image-2 Two-panel mechanism sketch on white background. Left: Lindlar hydrogenation of RC≡CR' on Pd/CaCO3/Pb showing syn addition to give cis-alkene (label 'cis'). Right: Na/liquid NH3 electron–proton–electron sequence on RC≡CR' giving trans-alkene (label 'trans'). Red curved arrows, black atoms, clean vector style. Electrophilic addition to alkenes: Markovnikov vs anti-Markovnikov HBr addition to propene via a more stable secondary carbocation gives 2-bromopropane (Markovnikov). Curved arrows show electron flow. Markovnikov addition (as given in v1) H goes to carbon with more H; X goes to the more substituted carbon Why Markovnikov? The first step is attack of the electrophile (H+) on the C=C to form a carbocation. The pathway that forms the more stable carbocation (3° > 2° > 1°) dominates. Then the nucleophile (X−) attacks the carbocation. Electrophilic addition of HX to prop-1-ene (propene) prop-1-ene substrate Propene Hydrogen bromide electrophile/nucleophile source bromane (aqueous hydrobromic acid as reagent) 2-Bromopropane product 2-bromopropane π electrons attack H+ Protonation of the less substituted carbon of C=C to give the more stable secondary carbocation on C-2 (Markovnikov). Nucleophilic attack by Br− on the carbocation to give 2-bromopropane. H+ adds to the terminal carbon to generate the secondary carbocation; Br− captures it. Anti-Markovnikov addition of HBr (peroxide effect) HBr adds anti-Markovnikov in the presence of peroxides (radical mechanism). Only HBr. 2026-05-26T17:05:24.103Z Side-by-side: Without peroxide (Markovnikov) vs with peroxide (anti-Markovnikov) addition of HBr to propene. Track where Br lands. gpt-image-2 Split diagram. Left: HBr addition to propene via carbocation → 2-bromopropane (Markovnikov). Right: ROOR-initiated radical chain showing Br• adds first to terminal carbon to give 1-bromopropane. Red arrows for radical steps, clean vector style, labels for 'with peroxide'/'without peroxide'. Initiation by RO–O• cleavage; Br• adds first to form the more stable radical, followed by H• transfer from HBr. Initiation: ROOR heat/light 2 RO•; RO• + HBr → ROH + Br• Propagation 1: Br• adds to the alkene to form the more substituted carbon radical (stabilized). Propagation 2: Radical abstracts H from HBr to give anti-Markovnikov bromoalkane + Br• (chain). Termination: Radical–radical combination (minor). Radical addition of HBr to alkenes (Kharasch effect) neet-alert Peroxide effect works only with HBr. HF, HCl, HI do not show reliable anti-Markovnikov radical addition at NEET level. Other hallmark additions to alkenes π bond opens to add two groups across C=C Catalytic hydrogenation: H2/Pt, Pd, or Ni converts C=C to C–C (alkane). Syn addition on surface. Cold, dilute, alkaline KMnO4 (Baeyer test): syn-diol formation; purple → brown MnO2. Hydroboration–oxidation: BH3 then H2O2/NaOH → anti-Markovnikov alcohol (overall), syn addition. Oxymercuration–demercuration: Hg(OAc)2/H2O, then NaBH4 → Markovnikov alcohol, no carbocation rearrangement. Addition of conc. H2SO4: forms alkyl hydrogen sulfate (Markovnikov), hydrolysis gives alcohol. Must-know reagent outcomes Anti-Markovnikov alcohol via syn addition; no rearrangements Markovnikov hydration without rearrangement Cold alkaline KMnO4 → syn-1,2-diol; purple to brown Alkene (example) Reagent/conditions Major product Regioselectivity Stereochemistry Electrophilic addition to alkenes — what forms? Case Prop-1-ene (C=CC) HBr (no ROOR) 2-Bromopropane Markovnikov Mixed (via carbocation) Prop-1-ene HBr + ROOR 1-Bromopropane Anti-Markovnikov Radical (no fixed stereo) Cyclohexene Cold dilute KMnO4 (alkaline) cis-1,2-Cyclohexanediol Syn Prop-1-ene BH3 then H2O2/NaOH Propan-1-ol Anti-Markovnikov Syn Prop-1-ene Hg(OAc)2/H2O then NaBH4 Propan-2-ol Markovnikov No rearrangement Oxidative cleavage: Hot KMnO4 and Ozonolysis Oxidative cleavage breaks the double bond, revealing which carbonyls come from each C of the alkene. With hot, concentrated KMnO4 (acidic or alkaline), alkenes are cleaved; terminal =COOH can further oxidize to CO2 + H2O. Ozonolysis (O3) followed by reductive workup (Zn/H2O) gives aldehydes/ketones directly and is used to deduce the double-bond position. O3 forms ozonide; reductive workup gives carbonyl compounds gpt-image-2 Four-panel vector diagram showing 1,3-dipolar cycloaddition of O3 to an alkene (molozonide), rearrangement to ozonide, and Zn/H2O reductive workup to aldehyde/ketone pairs. Use propene and cyclohexene examples. Red arrows; labels for each intermediate; clean textbook style. 2026-05-26T17:05:24.958Z Ozonolysis map: each alkene carbon becomes a carbonyl carbon. Follow the break to predict products. Ozonolysis product patterns (reductive workup Zn/H2O) Alkene Products Ethene (C=C) 2 × Methanal (formaldehyde) Prop-1-ene (C=CC) Ethanal (acetaldehyde) + Methanal But-2-ene (CH3-CH=CH-CH3) 2 × Ethanal (if cis/trans, both give same carbonyl set) 1-Methylcyclohexene Cyclohexanone + Methanal Alkynes: preparation, additions, acidity, and tests Hydrogenation of an alkyne (as given in v1) General formula for a mono-alkyne is C n H 2n-2 . Common lab preparation: double dehydrohalogenation of vicinal or geminal dihalides (strong base, e.g., alcoholic KOH followed by NaNH2/liquid NH3) to form C≡C. Industrially, acetylene (ethyne, C C) is made by adding water to calcium carbide: CaC2 + 2 H2O → C2H2 + Ca(OH)2. Calcium carbide itself is prepared by heating quicklime with coke in an electric arc furnace: CaO + 3 C → CaC2 + CO. Hydrogenation: Lindlar’s catalyst → cis-alkene; Na/NH3(l) → trans-alkene; full H2/Pt or Pd → alkane. Addition of HX: Markovnikov; second molecule adds to give gem-dihalide (e.g., CH3–CBr2–CH3 from prop-yne + 2 HBr). Hydration (HgSO4/H2SO4): enol forms then tautomerizes to ketone; ethyne gives ethanal (acetaldehyde). Terminal alkynes give methyl ketones (Markovnikov) except ethyne case. Acidity: terminal alkynes (RC≡CH, pKa ≈ 25) form acetylides with Na/NaNH2; with AgNO3/NH3 → white precipitate (Ag acetylide), with CuCl/NH3 → red precipitate (Cu acetylide). Polymerization: 3 C2H2 873 ,K, Cu~tube C6H6 (benzene); also forms linear polyacetylene under specific catalysts. Characteristic reactions of alkynes RC≡CH + NaNH2 → RC≡C− Na+ + NH3 Reagent system Observation Species formed Terminal alkyne tests (qualitative) AgNO3 in NH3 (ammoniacal Ag+) White precipitate RC≡C–Ag (silver acetylide) CuCl in NH3 (ammoniacal Cu+) Red precipitate RC≡C–Cu (copper(I) acetylide) Na or NaNH2 Gas evolution (H2) or NH3 retained; formation of sodium acetylide RC≡C− Na+ Quick comparisons and trap-busters Classic HX addition to alkene HX (no ROOR) H to less substituted C; X to more substituted C Via carbocation (3° > 2° > 1°) Peroxide effect HBr + ROOR, heat/light Br to less substituted C Radical chain; only reliable with HBr Hydroboration–oxidation BH3 then H2O2/NaOH OH at less substituted C Syn addition overall Oxymercuration–demercuration Hg(OAc)2/H2O then NaBH4 OH at more substituted C No rearrangements Scenario Reagents Where the electrophile/nucleophile ends up Notes Markovnikov vs anti-Markovnikov at a glance Tests for unsaturation Test Reagent/conditions Positive observation What it indicates Baeyer test Cold, dilute, alkaline KMnO4 Purple decolorizes; brown MnO2 forms C=C/C≡C present Bromine water test Br2 in water or CCl4 (no light) Reddish-brown color disappears Electrophilic addition across unsaturation Ozonolysis O3 then Zn/H2O Gives two carbonyls Locates double-bond position Product mapping trick: In ozonolysis or hot KMnO4 cleavage, mentally cut the double bond and cap each cut with O to form carbonyls. For terminal alkenes, the terminal carbon can over-oxidize to CO2/H2O under strong conditions. tip Catalytic hydrogenation (correct form for clarity) Hydroboration–oxidation (overall stoichiometry) Oxymercuration–demercuration (overall) Terminal alkyne acidity (formation of sodium acetylide) From ethene to plastics and ethyne to welding: the same functional groups power both materials and metalwork. Ethene (ethylene) feeds polyethylene (LDPE/HDPE), ethylene glycol (antifreeze), styrene; propene → polypropylene and propylene oxide; buta-1,3-diene → synthetic rubber. Ethyne (acetylene) burns in O2 for welding (≈3300 ,° C flame) and was once a PVC precursor route. remember Anti-Markovnikov outcomes exist: HBr with peroxides (radical) and hydroboration–oxidation give the opposite regiochemistry on alkenes. All additions to unsymmetrical alkenes/alkynes follow Markovnikov's rule. They are for acyclic mono-alkenes/mono-alkynes. Rings or multiple double/triple bonds change hydrogen count; use degree of unsaturation. The formulas C n H 2n and C n H 2n-2 always apply directly for any hydrocarbon. Exactly the opposite: H adds to the carbon that already has more H (less substituted); the other atom/group (X or OH) goes to the more substituted carbon. Markovnikov means 'more H goes to more substituted carbon'. Alkynes hold π electrons more tightly (sp), so they are generally less reactive toward electrophilic addition, though they are more acidic at the terminal C–H. Alkynes are always more reactive than alkenes in electrophilic addition. Solve-and-check mini set General formula for Alkenes (v1 preserved) General formula for Alkynes (v1 preserved) Markovnikov's addition of HX to an alkene (v1 preserved) Hydrogenation of an alkyne (v1 preserved)