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Loading home…Methane, CH₄, has one carbon and four hydrogens — the simplest alkane.
It is the main component of natural gas.
Combustion of methane releases a large amount of heat energy.
It burns with a clean flame when oxygen supply is sufficient.
Pipelines deliver natural gas for domestic cooking and heating.
Complete combustion: CH₄ + 2O₂ → CO₂ + 2H₂O.
Open-chain alkanes have general formula CₙH₂ₙ₊₂.
Alkenes with one double bond have formula CₙH₂ₙ.
Alkynes with one triple bond have formula CₙH₂ₙ₋₂.
These formulae help classify an unknown hydrocarbon.
Ethene is C₂H₄ (not an alkane).
Ethyne is C₂H₂, the first member of the alkyne series.
In an addition reaction, atoms add across a multiple bond without losing a small molecule.
Alkenes and alkynes commonly undergo addition.
Example: ethene + Br₂ → 1,2-dibromoethane.
Hydrogenation adds H₂ across C=C in the presence of a metal catalyst.
Addition of HBr to ethene gives bromoethane.
Saturated alkanes do not typically show this addition behaviour.
Ethyne (acetylene) burns in oxygen to give a very hot flame.
The oxy-acetylene flame can melt steel for cutting and welding.
Combustion of ethyne is highly exothermic.
Ethyne is the first member of the alkyne series (HC≡CH).
It is generated when needed from calcium carbide and water in some setups.
Careful handling is required because acetylene mixtures can be explosive.
Alkanes are saturated and generally inert toward bromine water in the dark.
Alkenes decolourise bromine water by addition across C=C.
The orange-brown colour of Br₂ disappears with an alkene.
Alkanes may react with bromine only under UV light by substitution.
Baeyer’s test (cold dilute alkaline KMnO₄) also detects unsaturation.
Purple KMnO₄ is decolourised by alkenes (forming a diol).
Hydrogenation is the addition of hydrogen to a multiple bond.
An alkene reacts with H₂ in the presence of Ni, Pd, or Pt catalyst.
The product is the corresponding alkane.
Example: CH₂=CH₂ + H₂ → CH₃–CH₃.
Industrial hardening of oils uses hydrogenation of unsaturated fats.
The C=C bond is the site of addition.
Alkanes can be obtained from petroleum by fractional distillation.
Natural gas provides methane and related light alkanes.
In the laboratory, hydrogenation of alkenes/alkynes yields alkanes.
Wurtz reaction: 2R–X + 2Na → R–R + 2NaX in dry ether.
Wurtz is useful for symmetrical higher alkanes from alkyl halides.
Decarboxylation of sodium carboxylates with soda lime gives alkanes.
Example: CH₃COONa + NaOH (CaO) → CH₄ + Na₂CO₃.
Reduction of alkyl halides with Zn/HCl or LiAlH₄ can give alkanes.
Kolbe electrolysis of carboxylate salts yields hydrocarbons at the anode.
Cracking of large alkanes produces smaller alkanes and alkenes.
Industrial fuels rely mainly on refining rather than laboratory methods.
Conditions (heat, catalyst, dry ether) must be stated in equations.
Purity and isomer mixtures matter for fuel properties.
Methane from biogas is a renewable route in some communities.
School answers should give at least two distinct preparations with equations.
Mention that alkanes are less reactive, so many routes start from other families.
Safety: alkali metals and flammable solvents need careful handling.
Link preparation choice to the carbon number required.
Fractional distillation separates alkanes already present in crude oil.
Synthetic methods build carbon–carbon bonds or remove functional groups.
Always name the product and confirm the general formula CₙH₂ₙ₊₂.
Alkanes are relatively unreactive because C–C and C–H bonds are strong and nonpolar.
They burn in air/oxygen to give CO₂ and H₂O (complete combustion).
Incomplete combustion produces CO and soot, which are hazardous.
Halogenation occurs with Cl₂ or Br₂ under UV light by a free-radical mechanism.
Initiation produces halogen radicals; propagation substitutes H by X.
Termination combines radicals to stop the chain.
Multiple substitution can give mixtures of products.
Alkanes do not decolourise bromine water in the dark.
Cracking breaks large alkanes into smaller alkanes and alkenes on heating/catalyst.
Isomerisation rearranges straight chains to branched chains for better fuel.
Reforming produces aromatic compounds from alkanes in petrol refining.
Alkanes are used as fuels, solvents, and feedstocks.
Methane is a major greenhouse gas if released unburnt.
Reaction conditions distinguish substitution (UV) from addition (alkenes).
Write balanced combustion equations in exams.
State that reactivity increases slightly for tertiary C–H in some radical reactions.
Alkanes are insoluble in water but soluble in nonpolar solvents.
Physical properties (BP) increase with chain length.
Exam answers should cover combustion, halogenation, and cracking at minimum.
Emphasise free-radical halogenation steps by name.
Contrast alkane inertness with alkene reactivity.
Alkenes are produced industrially by cracking of petroleum fractions.
Dehydration of alcohols with concentrated H₂SO₄ or H₃PO₄ gives alkenes.
Ethanol at about 170 °C with excess conc. H₂SO₄ yields ethene.
Mechanism involves protonation and loss of water (E1/E2 depending on system).
Dehydrohalogenation of alkyl halides with alcoholic KOH gives alkenes.
Saytzeff (Zaitsev) rule: the more substituted alkene often predominates.
Partial hydrogenation of alkynes can give alkenes with suitable catalysts.
Heating quaternary ammonium hydroxides (Hofmann) can also yield alkenes.
Laboratory preparation of ethene from ethanol is a classic school experiment.
Conditions must avoid ether formation (lower temperature, excess alcohol → ether).
At about 140 °C with excess ethanol, diethyl ether forms instead of ethene.
Write equations and state temperature and reagent clearly.
Alkenes are feedstocks for polymers such as polythene.
Geometric isomerism may appear in suitably substituted alkenes.
Purification may use scrubbing to remove acidic gases from cracking streams.
School courses focus on dehydration and alcoholic KOH routes.
Confirm unsaturation with bromine water after preparation.
Safety: concentrated acids and flammable alkenes need care.
Choose the alcohol or halide that gives the desired carbon skeleton.
Industrial ethene is a bulk chemical for many derivatives.
Link preparation to the general formula CₙH₂ₙ.
The C=C bond is the reactive site in alkenes.
Electrophilic addition is the characteristic reaction type.
Hydrogenation with Ni/Pd/Pt gives alkanes.
Halogens (Br₂, Cl₂) add to give vicinal dihalides.
Decolourisation of bromine water is a test for unsaturation.
Hydrogen halides add to give halogenoalkanes (Markovnikov orientation often).
Hydration with H₂O/H₂SO₄ yields alcohols.
Cold dilute alkaline KMnO₄ hydroxylates to diols (Baeyer’s test).
Hot concentrated oxidants can cleave the double bond.
Polymerisation joins many ethene molecules into polythene.
Conditions for polymerisation include pressure, heat, and catalysts.
Addition polymers retain all atoms of the monomer in the chain.
Alkenes are more reactive than alkanes because of the π bond.
Write curly-arrow mechanisms when required for electrophilic addition.
Carbocation intermediates explain rearrangements in some additions.
Industrial chemistry uses ethene for ethanol, epoxyethane, and polymers.
Exam answers should list at least four addition reactions with equations.
Mention polymerisation as a major commercial reaction.
Contrast addition (alkenes) with substitution (alkanes under UV).
State Markovnikov’s rule for unsymmetrical alkenes when asked.
Safety: many alkenes are flammable gases or volatile liquids.
Alkynes contain a carbon–carbon triple bond (C≡C).
General formula for one triple bond is CₙH₂ₙ₋₂.
Ethyne is prepared from calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂.
Dehydrohalogenation of vicinal dihalides with strong base can give alkynes.
Industrial ethyne routes historically used coal/carbide chemistry.
Alkynes undergo addition reactions similar to alkenes but can add twice.
Hydrogenation: alkyne → alkene → alkane with excess H₂ and catalyst.
Lindlar’s catalyst stops hydrogenation at the cis-alkene stage.
Halogens and HX add stepwise to the triple bond.
Ethyne burns in oxygen with a very hot flame used in welding.
Terminal alkynes are weakly acidic; NaNH₂ can form acetylide salts.
Acetylides are useful in carbon–carbon bond forming reactions.
Polymerisation and special industrial processes use acetylene chemistry.
Test for terminal alkynes may use ammoniacal Cu(I) or Ag(I) precipitates.
Write balanced equations for carbide hydrolysis and combustion.
Compare reactivity: alkynes vs alkenes vs alkanes.
Ethyne is the first member of the alkyne series.
School practicals often generate small amounts of ethyne carefully.
Exam answers should cover preparation, addition, and welding use.
Note that incomplete combustion of ethyne can be sooty.
Triple-bond geometry is linear around the sp carbons.