Alkanes: Conformations & Halogenation Meet Alkanes: The simplest hydrocarbons Alkanes are saturated hydrocarbons — only single C–C and C–H bonds. Every carbon is sp 3 -hybridised and tetrahedral (bond angle about 109.5°). Their general formula fits straight and branched chains. Think of them as the "fuel family" powering stoves, cylinders, cars, and candles. General formula Methane, ethane, propane, butane: all sp 3 -tetrahedral, following the general formula C nH 2n+2 . Single bonds allow free rotation (source of conformations). Alkane Saturated hydrocarbon (C and H only; all single bonds), formula C nH 2n+2 . paraffin (older term) Paraffin Older term for alkanes; high-carbon alkanes form paraffin wax. Different 3D arrangements of atoms due to rotation about single bonds (no bonds broken). Conformation Newman projection View looking down a C–C bond; front carbon as a dot with three bonds, back carbon as a circle with three bonds. Staggered Conformation with 60° dihedral angle between adjacent bonds; lowest energy. Eclipsed Bonds align (0° dihedral); highest energy; passes over during rotation. Staggered conformation in butane where CH3 groups are 60° apart; less stable than anti. Gauche Anti Staggered in butane where CH3 groups are 180° apart; most stable. 2 alkyl halides + 2 Na (dry ether) → coupled alkane; clean for symmetric products. Wurtz reaction Wurtz–Fittig reaction Alkyl halide + aryl halide + 2 Na (dry ether) → alkylbenzene. Decarboxylation Soda-lime (NaOH/CaO) removes CO2 from RCOO− to give alkane with one less C. Light/heat-initiated radical process with initiation, propagation, and termination steps. Free-radical chain mechanism Initiation Homolysis generates radicals (e.g., Cl 2 h 2Cl ). Propagation Radicals react to form product and regenerate radicals (chain carrying). Termination Radicals combine to end the chain (no radical left). Thermal or catalytic breakdown of long alkanes to smaller alkanes + alkenes. Pyrolysis / Cracking Catalytic conversion of alkanes/cycloalkanes to aromatic hydrocarbons (BTX feed). Aromatization (Reforming) Quick glossary Nomenclature — quick recap with common names IUPAC naming: identify the longest carbon chain (root), number to give substituents the lowest set of locants, name substituents as prefixes, and end with the suffix “-ane”. Some helpful examples: - Methane (IUPAC: methane), SMILES: C - Ethane (ethane), SMILES: CC - Propane (propane), SMILES: CCC - n-Butane (butane), SMILES: CCCC; branched isomer 2-methylpropane (isobutane), SMILES: CC(C)C Branched isomers often have higher octane ratings (better engine performance), a point used in refining. Physical properties — predictable from weak intermolecular forces Non-polar molecules: only London dispersion (van der Waals) forces. Boiling/melting points rise with chain length; branching lowers boiling point. State at room conditions: C1–C4 gases; C5–C17 liquids; C18+ waxy solids (paraffins). Insoluble in water, soluble in non-polar solvents (hexane, benzene). Low density; lighter than water. remember Daily life connection: Natural gas (mostly methane, CH4) cooks your food; LPG (propane + butane) runs stoves and vehicles; petrol/diesel mixtures power engines; higher alkanes form paraffin wax used in candles and coatings. Where you see alkanes: natural gas flame (CH4), LPG cylinder (propane + butane), petrol at pumps (mixed alkanes), and paraffin candles (high-carbon alkanes). Preparation of alkanes — high-yield routes Make alkanes from several functional groups. Learn what each method is best for and its limitation — a frequent NEET-trap. Method Principle (key reagents/conditions) Suitable for Limitation Alkane preparation methods Route Catalytic hydrogenation Add H2 across C=C/C≡C over Ni/Pt/Pd (heated). Alkenes/alkynes → alkanes Needs unsaturation; catalysts/heat required. Wurtz reaction 2 R–X + 2 Na, dry ether → R–R + 2 NaX Primary alkyl halides; symmetric alkanes (R–R) Mixed halides give a mixture; poor for unsymmetrical targets. Wurtz–Fittig reaction R–X + Ar–X + 2 Na, dry ether → Ar–R + 2 NaX Alkylbenzenes (e.g., ethylbenzene) Not a route to saturated acyclic alkanes; mixture on mixing substrates. Kolbe electrolysis Electrolyse RCOO− (Na/K carboxylate) → R–R + CO2 Dimerising primary radicals to alkanes with even C Gives mainly symmetric dimers; mixtures if mixed salts present. Decarboxylation (soda-lime) RCOONa + NaOH/CaO (heat) → RH + Na2CO3 Alkane with one less C than the acid Only one-carbon loss; not for chain extension. Reduction of alkyl halides R–X + Zn/HCl → RH + ZnX2 Converts RX to RH Elimination/side reactions with some substrates; works best for primary RX. Predict symmetric product; mixed halides → statistical mixture. Dry ether, Na couples two identical alkyl halides to a higher alkane. Couples an aryl and an alkyl halide using Na in dry ether to give alkylbenzene. Aryl–aryl coupling (Ar–X + Ar–X + 2 Na → Ar–Ar) — context to contrast with Wurtz–Fittig. Carboxylate electrolysis dimerises radicals to R–R with loss of CO2. Soda-lime removes CO2, giving alkane with one carbon fewer. Conformations of ethane — staggered vs eclipsed Single bonds rotate. In ethane (CH3–CH3; SMILES: CC), rotation about C–C interconverts conformations seen best in Newman projections: staggered (lowest energy) and eclipsed (highest). The dihedral angle is the H–C–C–H angle when viewed down the C–C bond. - Staggered: dihedral 60°, 180°, 300° (H atoms are maximally apart; minimal torsional strain). - Eclipsed: dihedral 0°, 120°, 240° (H atoms align; torsional strain peaks). Rotation barrier (staggered → eclipsed) for ethane is about 12 kJ mol⁻¹. That’s small enough for rapid rotation at room temperature, but big enough to prefer staggered at any instant. 2026-05-26T17:05:22.641Z Ethane Newman projections: left — staggered (lowest energy); right — eclipsed (highest). Mark dihedral angles. gpt-image-2 Side-by-side Newman projections of ethane. Left: staggered with 60° dihedral; right: eclipsed 0°. Front carbon as dot, back as circle. Label 'staggered' and 'eclipsed', show dihedral angle arc. Clean 2D vector, red curved arrows for rotation, neutral palette. Ethane and n-butane conformations Molecule Conformation Dihedral angle(s) Energy/Note Ethane Staggered 60°, 180°, 300° Lowest energy; torsional strain minimal Ethane Eclipsed 0°, 120°, 240° Highest energy; barrier 12 kJ mol -1 n-Butane Anti-staggered 180° (CH3 groups opposite) Most stable n-Butane Gauche-staggered 60° (CH3 groups close) Less stable than anti (gauche interactions) n-Butane Eclipsed (H–H) 120°, 240° High energy n-Butane Eclipsed (CH3–CH3) 0° Highest energy (strong repulsion) 2026-05-26T17:05:22.344Z Energy profile for n-butane rotation: anti (lowest) → eclipsed (highest) → gauche → eclipsed → anti. gpt-image-2 Energy vs dihedral angle curve for n-butane from 0° to 360°. Mark minima at 180° (anti, global min) and 60°/300° (gauche, higher). Maxima at 0° (eclipsed CH3–CH3, highest) and 120°/240° (eclipsed H–H). Label conformers clearly. Clean textbook vector style. Anti is Away (180°); Gauche is Getting Close (60°). Free-radical halogenation — mechanism and selectivity Alkanes undergo substitution with halogens (Cl2 or Br2) in presence of UV light or heat via a free-radical chain mechanism. Chlorination is fast but poorly selective (mixtures). Bromination is slower but highly selective for tertiary > secondary > primary positions because forming a more stable radical in the H-abstraction step lowers the energy barrier. Initiation Propagation Propagation Termination Termination Termination Mechanism map: UV light splits Cl2 (initiation), radicals abstract H and form CH3· (propagation), and radicals finally combine (termination). UV light or heat Initiation: homolytic cleavage of Cl2 to 2 Cl· by UV light (hν). homolysis radical H-abstraction Propagation 1: Cl· abstracts H from CH4 → HCl + CH3·. radical substitution Propagation 2: CH3· attacks Cl2 → CH3Cl + Cl· (radical regenerates). Termination: radical–radical recombinations (Cl· + Cl·, CH3· + CH3·, CH3· + Cl·) remove radicals. coupling Substitution of H by X (Cl/Br) via a radical chain under UV light or heat. Free-radical halogenation of alkanes (methane example) methane substrate (RH) methane dichlorine chlorine halogen source (X2) chain carrier methyl radical methyl hydrogen chloride by-product hydrogen chloride Halogenation selectivity and use Feature Chlorination (Cl2, hν) Bromination (Br2, hν) Industrial note Reactivity High (fast) Lower (slower) Chlorination widely used in bulk Selectivity (3° > 2° > 1°) Low (gives mixtures at different positions) High (strong preference for 3° C–H) Bromination used when regioselectivity is needed Typical outcome on higher alkanes Mixture of isomeric R–Cl Major product at the most substituted site Br2 costlier; specialty processes Reactivity follows 3° > 2° > 1° due to radical stability. Bromination is highly selective, chlorination is less selective and often gives mixtures. All hydrogen atoms in a higher alkane are equally likely to be substituted during free-radical halogenation. Combustion — clean vs sooty flames In excess oxygen, alkanes burn completely to CO2 and H2O (blue, clean flame). With limited oxygen, incomplete combustion forms CO (toxic) and soot (C). That’s why a yellow, smoky flame indicates unburnt carbon (pollution and waste of fuel). Balance equations carefully in stoichiometry problems. Combustion of alkanes (balanced in n) Complete combustion gives CO2 and H2O; incomplete can give CO/soot. Blue (complete combustion) vs yellow sooty (incomplete) flames and their products. 2026-05-26T17:05:23.648Z Split-panel diagram: Left blue flame with arrows to CO2 + H2O; Right yellow sooty flame with arrows to CO + C (soot). Include O2 excess vs limited labels. Clean vector schematic, no text inside molecules. gpt-image-2 Industrial: cracking and aromatization (reforming) Cracking breaks long-chain alkanes to smaller alkanes and alkenes used as fuels and petrochemical feedstocks (e.g., ethene, propene for plastics). Aromatization (catalytic reforming) improves fuel quality and makes aromatic feedstocks (BTX: benzene, toluene, xylenes). Example: n-hexane to benzene with hydrogen at about 770 K over Pt/Al2O3. Cracking schematic: long C-chain → mixture of shorter alkanes + alkenes; Aromatization: n-hexane → benzene + H2 (Pt/Al2O3, ~770 K). 2026-05-26T17:05:23.725Z Flow diagram: heavy alkane feed to cracking unit → outputs (C2–C4 alkanes/alkenes). Separate panel: n-hexane to benzene + 4 H2 over Pt/Al2O3 at 770 K. Label catalysts/conditions. Vector refinery schematic style. gpt-image-2 neet-alert Aromatization/reforming: n-hexane Pt/Al 2O 3, 770 K benzene + 4H2 (BTX source). Cracking makes smaller alkanes + alkenes — key feed for polymer industry. High-yield checkpoints and traps Wurtz gives symmetric R–R cleanly; mixing different RX gives statistical mixture. Newman mastery: ethane (staggered lowest, eclipsed highest); n-butane anti < gauche < eclipsed. Free-radical halogenation: bromination is more selective than chlorination (3° > 2° > 1°). Combustion stoichiometry uses C nH 2n+2 — balance O2 as (3n+1)/2. Remember They are less reactive, but not inert: they burn (combustion) and undergo halogenation under light/heat. Industrially they are cracked and reformed under catalysts. Alkanes are inert and do not react under normal conditions. Wurtz reaction can be used to cross-couple two different alkyl halides cleanly. Mixing different RX in Wurtz usually gives a mixture of products. Use it to make symmetric alkanes selectively. Octane rating and isomerism: Branched alkanes ignite smoothly and resist knocking. 2,2,4-Trimethylpentane (iso-octane) is a high-octane reference; n-heptane defines the low end. Refining increases branching via isomerization and reforming to improve petrol quality. Acid-catalyzed Hydration ( H 2O, H + ) Markovnikov (Rearrangements possible via Carbocation) Non-stereospecific (Mixture of enantiomers) Hydroboration-Oxidation ( BH 3 THF / H 2O 2, OH - ) Anti-Markovnikov ( OH on less substituted C ) Syn-addition ( H and OH on same side) Halogenation ( Br 2/CCl 4 ) Not applicable (Symmetrical addition) Anti-addition (via Cyclic Bromonium Ion) Oxymercuration-Demercuration ( Hg(OAc) 2, H 2O / NaBH 4 ) Markovnikov (No Rearrangements) Anti-addition (Net hydration is mixed) Hydrohalogenation ( HX ) Markovnikov (Rearrangements possible) Non-stereospecific Hydrohalogenation with Peroxide ( HBr / R 2O 2 ) Anti-Markovnikov (Kharasch Effect) Radical Mechanism (Non-stereospecific) Syn-Dihydroxylation (Cold dilute KMnO 4 or OsO 4 ) Not applicable Syn-addition (Forms cis -glycols) Halohydrin Formation ( X 2 / H 2O ) Markovnikov ( OH adds to more substituted C ) Anti-addition Ozonolysis (Reductive: O 3 / Zn, H 2O or DMS ) Cleavage of C=C Bond Formation of Aldehydes/Ketones Epoxidation ( mCPBA or RCO 3H ) Not applicable Syn-addition (Retention of Alkene geometry) Catalytic Hydrogenation ( H 2 / Pt, Pd, Ni ) Not applicable Syn-addition (Surface mediated) Alkene Reaction Variants Organic Chemistry Alkenes Reaction Mechanism NEET High Yield Stereochemistry Hydro-B-Syn is Anti-Mark, while Acid-Mark rearranges the park; Bromine goes Anti in the dark. Distinguishing between Syn/Anti addition and Markovnikov/Anti-Markovnikov products. REACTION VARIANTS Reagent Reaction Name/Type Regioselectivity Stereochemistry Directing Effect Activation/Deactivation Examples Organic Chemistry Benzene Electrophilic Aromatic Substitution Directing Groups Activators and Halogens love the neighbors (Ortho/Para), while Strong Deactivators prefer the Meta distance. Predicting ortho, para, or meta products in synthesis. Electrophilic Aromatic Substitution Directors Group Type TREND ortho, para -directing Strongly Activating ( +M > -I ) -NH 2, -NHR, -NR 2, -OH, -O - ortho, para -directing Moderately Activating ( +M ) -NHCOCH 3, -NHCOR, -OCH 3, -OR ortho, para -directing Weakly Activating ( +I, Hyperconjugation ) -CH 3, -C 2H 5, -R, -C 6H 5 ortho, para -directing Weakly Deactivating ( -I > +M ) -F, -Cl, -Br, -I meta -directing Moderately Deactivating ( -M, -I ) -CHO, -COR, -COOR, -COOH, -SO 3H, -CN meta -directing Strongly Deactivating ( -M, -I ) -NO 2, -NR 3 +, -CF 3, -CCl 3