Alkanes: fractional distillation, cracking and combustion

Saturated hydrocarbons and their boiling points

The gas in a Bunsen burner, the liquid sloshing in a camping-stove canister and the wax of a candle are all alkanes. Methane, CH₄, has one carbon atom; the butane in the canister has four; candle wax has twenty to forty. They belong to one homologous series, with the general formula CₙH₂ₙ₊₂, and each member differs from the next by CH₂.

An alkane is a hydrocarbon, made of carbon and hydrogen only, and it is saturated: every bond between carbon atoms is a single bond, so each carbon forms four single bonds and no further atoms can be added. The four bonds round each carbon point to the corners of a tetrahedron at 109.5°, so a carbon chain is a zigzag, not a straight line.

Alkanes are unreactive towards acids, alkalis and most oxidising agents. C–C and C–H bonds are strong, and they are almost non-polar, because carbon and hydrogen have similar electronegativities, so there is no δ+ or δ− site for an ion to attack. Two reactions matter at A level: alkanes burn, and they react with halogens in ultraviolet light.

Hydrocarbon

A compound that contains hydrogen and carbon atoms only.

Saturated

Containing only single bonds between carbon atoms, so that no more atoms can be added to the molecule.

Homologous series

A family of compounds with the same functional group and general formula, in which each member differs from the next by CH₂.

Boiling points rise along the series, from −162 °C for methane to 69 °C for hexane. Molecules of an alkane are held to one another only by van der Waals forces, the attractions between induced dipoles. A longer chain has more electrons and a larger surface in contact with its neighbours, so the induced dipoles are larger and there are more points of contact between molecules. The forces between molecules are stronger and more energy is needed to separate them. Boiling an alkane breaks none of its covalent bonds. Write 'van der Waals forces between molecules' in an answer, not 'bonds': the word suggests that the C–C or C–H bonds break, and they do not.

Branching lowers the boiling point. Pentane, 2-methylbutane and 2,2-dimethylpropane are isomers with the same number of electrons, yet they boil at 36 °C, 28 °C and 10 °C. A branched molecule is more compact, closer to a sphere, and cannot lie alongside its neighbours as a zigzag chain can. The surface in contact is smaller, so the van der Waals forces between the molecules are weaker.

Fractional distillation of crude oil

Crude oil is a mixture of hundreds of hydrocarbons, most of them alkanes, with anything from one carbon atom to more than fifty. As it comes out of the ground it is no use to anyone. Separated into fractions, groups of hydrocarbons with similar chain lengths and so similar boiling points, it gives bottled gas, petrol, jet fuel, diesel, lubricating oil and the bitumen on a road.

The oil is heated in a furnace to about 350 °C, which vaporises most of it, and fed into the bottom of a tall column. The column is hottest at the bottom and coolest at the top. Vapour rises through it, bubbling up through a series of trays. At each tray the vapour meets liquid at that tray's temperature: hydrocarbons with boiling points above it condense, and the rest carry on upwards. Each tray therefore collects a liquid with a narrow range of boiling points, and it is drawn off at the side.

Describe the drawing

A tall fractionating column, narrower at the top, with six horizontal trays inside, each carrying two bubble caps. Crude oil is heated in a furnace at the bottom left and enters the base of the column as vapour, which rises up the centre. A temperature arrow on the left runs from about 25 °C at the top down to about 350 °C at the bottom. Pipes lead off to the right: refinery gas, C₁ to C₄, from the top, then petrol, naphtha, kerosene and diesel from the upper four trays, fuel and lubricating oil, C₂₀ to C₅₀, from the lowest two, and the bitumen residue from the very bottom.

The smallest molecules, with the weakest van der Waals forces, never condense in the column and leave the top as refinery gas. The largest do not vaporise at 350 °C at all; they leave the bottom as a residue. That residue is distilled a second time at reduced pressure, which lowers its boiling points enough to separate it without heating it to the temperature at which its molecules would break up.

Fraction

A mixture of hydrocarbons with similar chain lengths, and so with boiling points in a narrow range.

Fractional distillation

Separation of a mixture into fractions by boiling point, in a column with a temperature gradient that is hottest at the bottom.

Cracking

Fractional distillation gives whatever proportions the crude oil contains, and those are not the proportions that sell. A typical crude yields more of the heavy fractions than the market takes, and less petrol and naphtha than it wants. Cracking breaks long-chain alkanes from the surplus fractions into shorter, more valuable molecules. It also makes alkenes, which crude oil barely contains and which the chemical industry needs as the starting point for polymers.

Cracking breaks C–C bonds. A long alkane does not have enough hydrogen to make two shorter alkanes, so at least one product is always an alkene:

C₁₀H₂₂(g) → C₈H₁₈(g) + C₂H₄(g)decane gives octane, for petrol, and ethene, for poly(ethene)

Thermal cracking uses a high temperature, 400 to 900 °C, and a high pressure, up to 7000 kPa, for about a second, so that the products do not decompose further. The C–C bonds break homolytically and the reaction goes through radicals. The products contain a high proportion of alkenes, ethene in particular, and thermal cracking is the main source of the alkenes the polymer industry uses.

Catalytic cracking passes the vapour over a zeolite catalyst, a crystalline aluminosilicate riddled with pores, at about 450 °C and a slight pressure. The catalyst lets the reaction run at a lower temperature, which saves energy. The reaction goes through ions rather than radicals, and the products are mainly branched alkanes, cycloalkanes and aromatic hydrocarbons. Those are the molecules a petrol engine needs, because they burn smoothly without igniting too early, so catalytic cracking is the main source of motor fuel.

Describe the drawing

One long alkane, decane, drawn as a zigzag at the top left, feeds two processes. The thermal cracking box gives 400 to 900 °C, up to 7000 kPa and no catalyst, with a flame and a sketch of a C–C bond splitting into two radicals by two half-headed arrows; it leads to ethene and propene, a high proportion of alkenes for polymers. The catalytic cracking box gives about 450 °C and slight pressure over a porous zeolite catalyst; it leads to a branched alkane, cyclohexane and methylbenzene, for petrol.

Both kinds are chosen by what the refinery needs more of. Where the demand is for ethene and propene, thermal cracking is run; where it is for petrol, catalytic cracking. The economic reason has two halves, and an answer needs both: the surplus heavy fractions sell for less than the products, and the products are ones crude oil cannot supply in the amounts wanted.

Cracking

The breaking of C–C bonds in long-chain alkanes to form shorter alkanes and alkenes.

Combustion and what comes out of the exhaust

Burned in plenty of oxygen, an alkane gives carbon dioxide and water and releases a great deal of energy, and that energy is what the alkanes are burned for. This is complete combustion:

C₈H₁₈(l) + 12½O₂(g) → 8CO₂(g) + 9H₂O(l)complete combustion of octane

In a limited supply of oxygen, as in a gas fire with a blocked flue or an engine running short of air, there is not enough oxygen to take all the carbon to carbon dioxide. Carbon monoxide forms, and so does carbon itself, as soot. That is incomplete combustion:

CH₄(g) + 1½O₂(g) → CO(g) + 2H₂O(l)carbon monoxide

CH₄(g) + O₂(g) → C(s) + 2H₂O(l)soot

Carbon monoxide is toxic. It binds to the iron in haemoglobin more strongly than oxygen does, so the blood carries less oxygen, and it has neither colour nor smell, so a home with a gas boiler needs a detector to warn of it. Soot particles irritate the airways, worsen asthma and are linked to lung cancer.

Two more pollutants come from something other than the fuel's carbon. The spark and the high temperature in an engine cylinder make nitrogen and oxygen from the air react, so the nitrogen in nitrogen oxides, NOₓ, was never in the petrol. Sulfur compounds are impurities in crude oil and coal, and burn to sulfur dioxide. Both oxides dissolve in rain to make it acidic.

N₂(g) + O₂(g) → 2NO(g)in the cylinder; outside, 2NO(g) + O₂(g) → 2NO₂(g)

Nitrogen is four-fifths of the air and passes through an ordinary flame untouched. The N≡N triple bond, 945 kJ mol⁻¹, is one of the strongest bonds there is, and the molecule is non-polar, so its reaction with oxygen has a very high activation energy. Only at the temperature of an engine cylinder or a lightning bolt do enough collisions have that energy. Lightning is the natural source of NOₓ; engines and power stations are the man-made ones.

Unburnt hydrocarbons are fuel that went through the engine without burning, either because the mixture was too rich or because the flame went out near the cold cylinder walls. They are a waste of fuel, some are toxic, and with NOₓ in sunlight they make smog.

Complete combustion

Burning in a plentiful supply of oxygen, in which the carbon in a fuel is oxidised to carbon dioxide and the hydrogen to water.

Incomplete combustion

Burning in a limited supply of oxygen, giving carbon monoxide or carbon as well as, or instead of, carbon dioxide.

Catalytic converters, desulfurisation and the greenhouse effect

A car's catalytic converter sits in the exhaust pipe. Inside is a ceramic honeycomb, which gives a large surface area for a small mass of catalyst, coated with a thin layer of platinum, palladium and rhodium. The exhaust gases are adsorbed onto the metal, react and leave. Carbon monoxide, nitrogen oxides and unburnt hydrocarbons are all removed together:

2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)

C₈H₁₈(g) + 25NO(g) → 8CO₂(g) + 12½N₂(g) + 9H₂O(g)

In both reactions nitrogen monoxide is the oxidising agent. Its oxygen oxidises the carbon monoxide or the hydrocarbon, and its nitrogen leaves as N₂, which is harmless. The converter does nothing to the carbon dioxide; it adds to it. It also works only when hot, so the first few minutes of a journey from cold pass through it largely unchanged.

Describe the drawing

Cutaway of a catalytic converter in an exhaust pipe, with gas flowing left to right through a ceramic honeycomb of many small channels. Carbon monoxide, nitrogen monoxide and unburnt hydrocarbon enter on the left; carbon dioxide, nitrogen and water leave on the right. A circular inset magnifies one channel, its walls dotted with platinum, palladium and rhodium. Below: 2CO + 2NO gives 2CO₂ + N₂, and C₈H₁₈ + 25NO gives 8CO₂ + 12½N₂ + 9H₂O.

Sulfur dioxide from road traffic has largely gone, because sulfur is removed from petrol and diesel at the refinery. Sulfur cannot be taken out of coal so easily, so coal-fired power stations remove the sulfur dioxide from the flue gases instead: flue-gas desulfurisation. The gases pass through a wet slurry of calcium oxide or calcium carbonate. Sulfur dioxide is an acidic oxide and these are bases, so they react to give calcium sulfite, which is oxidised to calcium sulfate and sold as gypsum for plasterboard.

CaO(s) + SO₂(g) → CaSO₃(s)

CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)

Carbon dioxide, water vapour and methane are greenhouse gases. The Earth's surface, warmed by sunlight, emits infrared radiation. Molecules of these gases absorb some of it, because infrared makes their bonds vibrate, and re-emit it in all directions, some of it back towards the surface, so the lower atmosphere stays warmer than it otherwise would.

Burning fossil fuels has raised the carbon dioxide in the air from about 280 parts per million before industry to more than 420 today, and the stronger greenhouse effect is the cause of the warming measured since. No catalyst removes carbon dioxide from an exhaust; it can be cut only by burning less fuel, or captured and stored.

Greenhouse gas

A gas whose molecules absorb infrared radiation emitted by the Earth's surface and re-emit it, warming the lower atmosphere.

Other boards: reforming, smog and nitrogen dioxide as a catalyst

Edexcel examines reforming, which changes the shape of a molecule without shortening it. Straight-chain alkanes from the naphtha fraction pass over a platinum catalyst at about 500 °C and come out as branched alkanes, cycloalkanes and arenes, with hydrogen as the by-product. Hexane can become its isomer 2-methylpentane, or close into a ring and then lose more hydrogen:

C₆H₁₄(g) → C₆H₁₂(g) + H₂(g)hexane to cyclohexane

C₆H₁₂(g) → C₆H₆(g) + 3H₂(g)cyclohexane to benzene

The reason is the one behind catalytic cracking. Squeezed in a petrol engine, a straight-chain alkane can ignite from the compression alone, before the spark, which makes the engine knock. Branched and cyclic molecules resist that, so the fuel burns smoothly and the engine can run at a higher compression, which gets more work from each litre. The hydrogen is used at the refinery to strip sulfur out of fuels.

Reforming

The conversion of straight-chain alkanes into branched alkanes and cyclic hydrocarbons, over a platinum catalyst, to give fuels that burn more efficiently.

Cambridge examines two more things the nitrogen oxides do. In sunlight, NOₓ and unburnt hydrocarbons react together to make photochemical smog, the brown haze over a city on a still summer afternoon. Among its components are ozone at ground level and peroxyacetyl nitrate, PAN, CH₃CO₃NO₂, which stings the eyes and damages lungs and leaves.

Nitrogen dioxide also speeds up the oxidation of sulfur dioxide in the air, and is remade as it does so:

SO₂(g) + NO₂(g) → SO₃(g) + NO(g)

NO(g) + ½O₂(g) → NO₂(g)NO₂ regenerated

Nitrogen dioxide is used in the first step and returned by the second, so it is a catalyst, and the sulfur trioxide dissolves in rain as sulfuric acid, SO₃(g) + H₂O(l) → H₂SO₄(aq). Nitrogen dioxide makes rain acidic directly as well, by dissolving to give nitric acid. A Cambridge answer on acid rain from a power station is expected to give both equations and to call NO₂ a catalyst.

Photochemical smog

A haze formed when nitrogen oxides and unburnt hydrocarbons react in sunlight, containing ground-level ozone and PAN.