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14.1 Alkanes

Syllabus
9701–2028–2029
Topic
14.1
Level
AS

Learning objectives

14.1.1Preparation of alkanes• Recall preparation of alkanes: (a) addition of hydrogen to an alkene in a hydrogenation reaction, H2(g) and Pt/Ni catalyst and heat; (b) cracking of a longer chain alkane, heat with Al2O314.1.2Describe• Describe:: (a) the complete and incomplete combustion of alkanes; (b) the free-radical substitution of alkanes by Cl 2 or Br2 with ultraviolet light, as exemplified by the reactions of ethane14.1.3Mechanism: free-radical substitution• Describe the mechanism of free-radical substitution using the initiation, propagation and termination steps14.1.4Suggest how cracking can be used to obtain• Suggest how cracking can be used to obtain more useful alkanes and alkenes of lower M r from heavier crude oil fractions14.1.5The general unreactivity of alkanes• Understand the general unreactivity of alkanes, including towards polar reagents in terms of the strength of the C–H bonds and their relative lack of polarity14.1.6Combustion pollutants and catalysts• Recognise the environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the combustion of alkanes in the internal combustion engine and of their catalytic removal

Prepare alkanes by hydrogenating alkenes or cracking longer alkanes

Route Reactant and change Reagent / condition Example
hydrogenation H₂ adds across an alkene C=C to give an alkane H₂(g), Pt or Ni catalyst, heat CH₂=CH₂ + H₂ → CH₃CH₃
cracking a longer-chain alkane splits into smaller hydrocarbons, including an alkane product heat with Al₂O₃ C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Hydrogenation preserves the carbon skeleton and removes the C=C by forming two C–H bonds. A proposed cracking equation is acceptable only when its carbon and hydrogen totals balance; cracking can give a mixture rather than one unique product pair.

A catalyst changes the reaction rate but is not consumed in the overall equation. Do not use the hydrogenation conditions for cracking or assume that both cracking products must be alkanes.

Alkanes combust and undergo UV-initiated halogen substitution

Oxygen supply Carbon-containing product(s) Other product
sufficient CO₂ H₂O
limited CO and/or C H₂O

For complete combustion, balance carbon atoms as CO₂, hydrogen atoms as H₂O, then oxygen. For ethane: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Incomplete combustion is not one fixed equation because the proportions of CO and carbon depend on the oxygen supply.

In ultraviolet light, Cl₂ or Br₂ reacts with ethane by free-radical substitution: C₂H₆ + Cl₂ → C₂H₅Cl + HCl. A C–H bond is replaced by C–Cl; further substitutions can occur, so excess halogen can produce a mixture.

UV light initiates the radical chain; it is not written as a reactant or catalyst. Halogen substitution replaces H and is not addition across a C=C bond.

Track radicals through initiation, propagation and termination

Initiation: ultraviolet light causes homolytic fission of the halogen bond, so each chlorine atom takes one bonding electron and becomes a radical.

ClX2→UV2 Cl ⋅ \ce{Cl2 ->[UV] 2Cl.}

Propagation: a chlorine radical removes H from ethane, then the ethyl radical removes Cl from another Cl₂ molecule. One radical is consumed and another is produced in each step, so the chain continues.

Cl ⋅ +CX2HX6→HCl+CX2HX5 ⋅ \ce{Cl. + C2H6 -> HCl + C2H5.}

CX2HX5 ⋅ +ClX2→CX2HX5Cl+Cl ⋅ \ce{C2H5. + Cl2 -> C2H5Cl + Cl.}

Termination occurs when two radicals collide and form a molecule: Cl• + Cl• → Cl₂, C₂H₅• + Cl• → C₂H₅Cl, or 2C₂H₅• → C₄H₁₀. No radical is regenerated in a termination step.

If electron movement is drawn, a single-headed (fish-hook) curly arrow represents one electron. A full curly arrow represents an electron pair and must not be substituted into a radical step.

Cracking converts heavy oil fractions into more useful smaller molecules

Cracking breaks C–C bonds in long-chain hydrocarbons to make shorter alkanes and alkenes with lower relative molecular masses.

Demand for petrol and alkene feedstocks can exceed the natural fraction distribution. Choose a cracking equation that balances carbon and hydrogen atoms and includes the required heat/catalyst conditions.

C₁₀H₂₂ can crack to C₈H₁₈ + C₂H₄. The alkane is a fuel-range product and the alkene is a useful petrochemical feedstock.

Cracking is not complete combustion and does not produce one fixed product from every heavy fraction.

Strong, almost non-polar C–H bonds explain alkane unreactivity

Breaking a C–H bond requires substantial energy because it is strong. At ordinary conditions, many possible reactions therefore have a high activation energy and are too slow to observe.

Carbon and hydrogen have similar electronegativities, so a C–H bond has relatively little polarity. An alkane has no strongly δ+ or δ− reaction centre for a polar nucleophile or electrophile to attack.

Alkanes are generally unreactive, not absolutely inert. Combustion supplies heat and oxygen, radical substitution uses UV light to generate radicals, and cracking uses high temperature with Al₂O₃; these routes overcome or bypass the ordinary kinetic barrier.

Do not explain unreactivity merely by saying “alkanes are non-polar”. Both the strength of C–H bonds and their relative lack of polarity are required.

Link engine pollutants to their consequences and catalytic removal

Pollutant from an internal-combustion engine Formation / consequence Catalytic-converter change
CO incomplete combustion; toxic because it reduces the blood's ability to transport O₂ oxidised to CO₂
nitrogen oxides, NOₓ N₂ and O₂ react at high engine temperatures; contribute to respiratory harm, acid rain and photochemical smog reduced to N₂
unburnt hydrocarbons fuel passes through without complete combustion; contributes to photochemical smog oxidised to CO₂ and H₂O

2 CO+2 NO→2 COX2+NX2\ce{2CO + 2NO -> 2CO2 + N2}

A catalytic converter accelerates simultaneous redox: CO and hydrocarbons are oxidised while nitrogen oxides are reduced. The catalyst is not used up and does not make the original fuel use pollution-free.

Catalytic removal changes pollutants after they form; it is different from preventing incomplete combustion or lowering the engine temperature at which NOₓ forms.