Alkanes

In a nutshell

Alkanes are the main compounds in crude oil, and they are the fuels that run cars, planes and power stations.

This subtopic follows them from the ground to the air: separating crude oil by fractional distillation, cracking big alkanes into more valuable small ones, burning them (and cleaning up the pollutants), and their one key reaction, free-radical substitution with chlorine.

Assumed knowledge: Introduction to organic chemistry, Bonding.

Core content

Alkanes are saturated hydrocarbons

Two words carry the marks here, and each needs the word only:

• Hydrocarbon: a compound containing carbon and hydrogen only.
• Saturated: contains single bonds only (no C=C double bonds).

So an alkane is a saturated hydrocarbon, with the general formula C_nH_2n+2.

Fractional distillation of crude oil

Petroleum (crude oil) is a mixture consisting mainly of alkane hydrocarbons. It is separated into fractions by fractional distillation.

A fraction is a mixture of hydrocarbons with similar boiling points (and so similar chain lengths).

How the industrial column works:

1. Crude oil is heated in a furnace until it is mostly vaporised.
2. The vapour is passed into the bottom of a fractionating column.
3. The column has a temperature gradient: it is hot at the bottom and cooler at the top.
4. Vapours rise. Each fraction condenses at a different height, where the temperature falls below its boiling point, and is tapped off there.
5. Larger molecules have higher boiling points, so they condense lower down. Small molecules with low boiling points rise to the top (the refinery gases leave as gas).

Going down the column, the fractions have longer chains, higher boiling points, and are more viscous and harder to ignite.

Why bigger molecules boil higher

Fractional distillation is a physical process. Boiling overcomes the van der Waals forces between molecules; no covalent bonds break.

• Longer chain → larger surface area (and more electrons) → stronger van der Waals forces between molecules → more energy needed → higher boiling point.
• Branched isomers have lower boiling points than straight-chain ones: they are more compact (more spherical), so the molecules cannot get as close and have a smaller surface area of contact.

Same molecular formula, same number of electrons, but more branching means weaker van der Waals forces between molecules and a lower boiling point.

Still don't get it? · why fractions separate (and why no bonds break)

Picture strips of Velcro. A long strip touches its neighbour along a lot of its length, so it takes a big pull to separate two long strips. Two short strips come apart easily. A strip that is scrunched into a ball touches its neighbour in only a small patch, so it also comes apart easily.

Now rebuild it for crude oil, one step at a time:

1. Crude oil is a mixture of alkane molecules of many different lengths.
2. Each molecule is held to its neighbours by weak attractions between the molecules (van der Waals forces). The strong covalent bonds inside each molecule are never touched.
3. A long chain touches its neighbours over a bigger area, so the attractions between molecules are stronger. It needs a higher temperature to escape into the gas: a higher boiling point.
4. In the column, every vapour rises until it reaches a level that is cooler than its own boiling point. There it turns back into a liquid and is collected.
5. Big molecules (high boiling point) reach that point low down. Small molecules (low boiling point) get near the top.

Exam wording: the column has a temperature gradient (cooler at the top); fractions condense at different heights because they have different boiling points; boiling point increases with chain length because there are stronger van der Waals forces between the molecules. Never write that bonds break: in distillation the only thing overcome is the forces between molecules.

Practical skills: fractional distillation of a crude oil substitute (skills opportunity AT a, d, k; PS 1.2)

In the lab you separate a crude oil substitute (a safe mixture of hydrocarbons) on a small scale. The method is not the industrial column: you heat a liquid in a flask and collect each fraction as it distils over in turn.

Method:

1. Put the crude oil substitute and a few anti-bumping granules in the flask; heat gently with an electric heater.
2. The lowest-boiling component boils first. Its vapour rises up the column and reaches the condenser first, while higher-boiling vapours condense in the column and run back into the flask.
3. The condenser cools the vapour back to a liquid, which is collected.
4. Record the temperature on the thermometer. Change the collecting tube each time the temperature rises past a chosen value, so each tube holds one fraction (one boiling range).
5. Compare the fractions: colour, viscosity (how runny), and how easily a small sample ignites and how smoky the flame is.

Expected results: higher-boiling fractions are darker, more viscous, harder to ignite and burn with a smokier (sootier) flame.

Safety (AT k): the fractions are flammable, so keep collected fractions away from any flame, never heat to dryness, and work in a well-ventilated area or fume cupboard.

Cracking: thermal and catalytic

Cracking is the breaking of C-C bonds in large alkane molecules to make smaller hydrocarbon molecules. It is a chemical process (covalent bonds break), so it needs a high temperature.

A long alkane does not have enough hydrogen to become two alkanes, so the products always include at least one alkene:

• C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)

The two types (mechanisms not required):

Learn each set of conditions as a pair. "High temperature" alone is not enough for thermal cracking, and catalytic cracking is not at high or low pressure: it is slight pressure.

Why crack alkanes: the economic reasons

Fractional distillation gives the wrong mix of fractions: too much of the long-chain, heavy fractions and not enough of the short-chain fractions such as petrol.

On data like this, the shortfall is in the short-chain fractions (demand exceeds supply), and the surplus is in the heavy fractions.

The economic reasons, point by point:

1. Long-chain fractions are produced in excess and have low value.
2. Cracking converts them into smaller molecules that are in greater demand and have a higher value.
3. It makes motor fuels (petrol) to meet the shortfall, and alkenes, which are used as a feedstock: they are polymerised to make plastics (such as poly(ethene)) and used to make other chemicals.

Say demand or value. "The products are more useful" is often ignored by examiners.

Combustion of alkanes

Alkanes are used as fuels because their combustion releases a lot of heat energy (it is highly exothermic).

Complete combustion happens in an excess of oxygen. The only products are carbon dioxide and water:

• C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O

Incomplete combustion happens when the supply of oxygen is limited. It produces carbon monoxide and/or carbon (soot), plus water:

• C₈H₁₈ + 8½O₂ → 8CO + 9H₂O
• C₈H₁₈ + 4½O₂ → 8C + 9H₂O

The same ideas apply to other organic compounds: complete combustion of any compound of C, H (and O) gives CO₂ and H₂O.

Incomplete combustion releases less energy per mole of fuel, so more fuel is used. The CO₂ from complete combustion is a greenhouse gas, contributing to global warming.

Pollutants from the internal combustion engine

The internal combustion engine (a car engine) produces several gaseous pollutants:

The nitrogen comes from the air, not from the fuel. N₂ has a very strong triple bond, so it only reacts with oxygen at the very high temperature inside the engine.

Catalytic converters

These gaseous pollutants are removed by a catalytic converter fitted to the exhaust.

• The catalyst is platinum, palladium or rhodium.
• It is coated as a thin layer on a ceramic honeycomb. This gives a large surface area, so the reaction is fast and less of the expensive metal is needed.

The converter turns the pollutants into CO₂, N₂ and H₂O:

• 2CO + 2NO → 2CO₂ + N₂
• C₈H₁₈ + 25NO → 8CO₂ + 12½N₂ + 9H₂O

Nitrogen is always written as N₂, never N.

Sulfur dioxide and flue-gas desulfurisation

Some fossil fuels contain sulfur impurities. When the fuel burns, the sulfur is oxidised to sulfur dioxide:

• S + O₂ → SO₂

SO₂ causes air pollution: it dissolves in rain water to form acid rain.

In power stations, SO₂ is removed from the flue gases (waste gases) by reacting it with calcium oxide or calcium carbonate. The reason, which is what the "explain" mark needs:

• SO₂ is an acidic gas.
• CaO and CaCO₃ are bases, so they neutralise the SO₂, forming a solid salt.

The equations:

• CaO + SO₂ → CaSO₃
• CaCO₃ + SO₂ → CaSO₃ + CO₂

(With extra oxygen, calcium sulfate, CaSO₄, can form instead.)

Chlorination of methane: free-radical substitution

Methane reacts with chlorine in ultraviolet (UV) light. A hydrogen atom is replaced by a chlorine atom:

• Overall: CH₄ + Cl₂ → CH₃Cl + HCl

The mechanism is free-radical substitution (both words are needed). A free radical is a species with an unpaired electron, shown by a dot. Write it as three kinds of step, as balanced equations, with no curly arrows and no charges.

Initiation (radicals are made):

• Cl₂ → 2Cl•
• UV light supplies the energy to break the Cl-Cl bond, the weakest bond present. The bond splits evenly, so each Cl atom keeps one electron.

Propagation (one radical in, one radical out):

• Cl• + CH₄ → HCl + •CH₃
• •CH₃ + Cl₂ → CH₃Cl + Cl•

The second step regenerates Cl•, which goes on to react with another CH₄. This is a chain reaction.

Termination (two radicals join, the chain stops):

• •CH₃ + Cl• → CH₃Cl
• •CH₃ + •CH₃ → C₂H₆ (so some ethane forms as a by-product)
• Cl• + Cl• → Cl₂

Two checks for every set of steps:

• Add the two propagation steps and cancel: you get the overall equation, with no radicals left.
• HCl is the inorganic product, never H₂ or H•.

Still don't get it? · the chain reaction

Think of a game of tag. Whoever is "it" tags someone, and that person becomes the new "it". The game never runs out of an "it": one starter can tag hundreds of people. It only ends when two people who are both "it" bump into each other and both drop out.

Now rebuild it step by step:

1. UV light splits one Cl₂ molecule into two chlorine atoms, each with an unpaired electron. These are the first two "its" (initiation).
2. A chlorine radical grabs a hydrogen from methane. It becomes HCl (a normal, stable molecule), but it leaves behind a methyl radical, •CH₃. The "it" has been passed on (propagation 1).
3. The methyl radical grabs a chlorine atom from a Cl₂ molecule. That makes the product, CH₃Cl, and leaves a new Cl• behind (propagation 2).
4. That new Cl• goes back to step 2. Steps 2 and 3 repeat thousands of times from one burst of light.
5. Radicals are rare, so only occasionally do two of them meet. When they do, their unpaired electrons pair up into a bond, no radical is left, and that chain stops (termination).

Exam wording: initiation makes radicals (Cl₂ → 2Cl• in UV light); propagation has one radical as a reactant and one as a product, and regenerates Cl•; termination joins two radicals to make a molecule. Name it free-radical substitution.

Further substitution gives a mixture

Free-radical substitution is hard to control. CH₃Cl can itself be attacked by Cl•, so further substitution gives CH₂Cl₂, CHCl₃ and CCl₄, and termination gives some C₂H₆.

• Excess chlorine pushes the reaction towards CCl₄: CH₄ + 4Cl₂ → CCl₄ + 4HCl
• Excess methane makes CH₃Cl the main product, because a Cl• is then most likely to hit a CH₄ molecule.

Brighter UV light increases the rate, because Cl• radicals are produced faster (a higher concentration of chlorine radicals).

Worked examples

1. Balancing combustion equations

Write equations for the complete combustion of heptane, C₇H₁₆, and for its incomplete combustion to carbon monoxide and to carbon.

Balance in the order C, then H, then O last.

Complete combustion. 7 C gives 7CO₂; 16 H gives 8H₂O. Now count oxygen atoms on the right:

$\text{O atoms} = (7 \times 2) + (8 \times 1) = 22$

Oxygen comes as O₂ molecules, so halve it:

$\frac{22}{2} = 11\ \text{O}_2$

C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O

Incomplete combustion to CO. Now each carbon takes only one O:

$\text{O atoms} = (7 \times 1) + (8 \times 1) = 15 \quad\Rightarrow\quad \frac{15}{2} = 7\tfrac{1}{2}\ \text{O}_2$

C₇H₁₆ + 7½O₂ → 7CO + 8H₂O

Incomplete combustion to carbon (soot). Only the water needs oxygen:

$\text{O atoms} = 8 \quad\Rightarrow\quad \frac{8}{2} = 4\ \text{O}_2$

C₇H₁₆ + 4O₂ → 7C + 8H₂O

Where marks are lost: writing 22O₂ (counting O atoms, not O₂ molecules) or 5½O₂ (forgetting that each CO₂ holds two O atoms). Halves such as 7½O₂ are fine; so are whole-number multiples of the whole equation.

2. Deducing the products of cracking

(a) Tetradecane, C₁₄H₃₀, is thermally cracked to give octane and ethene only. Write the equation.

Octane is C₈H₁₈. Subtract it to find what is left:

$\text{C}: 14 - 8 = 6 \qquad \text{H}: 30 - 18 = 12$

C₆H₁₂ left over, as ethene (C₂H₄):

$\frac{6}{2} = 3\ \text{C}_2\text{H}_4 \quad (\text{check H: } 3 \times 4 = 12)$

C₁₄H₃₀ → C₈H₁₈ + 3C₂H₄

(b) Hexadecane, C₁₆H₃₄, cracks into two molecules with the same number of carbon atoms. Write the equation.

Each product has 16 ÷ 2 = 8 carbons. Two alkanes would need 2 × 18 = 36 H, but there are only 34, so one product must be an alkene:

C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆ (octane + octene)

Where marks are lost: not reading "same number of carbons", or giving H₂ as a product when the question says hydrocarbons only. Always check the C and H totals on both sides.

3. Free-radical steps for a further substitution

Chloromethane reacts with chlorine in UV light to form dichloromethane. Write the initiation step, the two propagation steps and one termination step that makes CH₂Cl₂.

• Initiation: Cl₂ → 2Cl•
• First propagation (Cl• takes an H): CH₃Cl + Cl• → •CH₂Cl + HCl
• Second propagation (the radical takes a Cl and regenerates Cl•): •CH₂Cl + Cl₂ → CH₂Cl₂ + Cl•
• Termination: •CH₂Cl + Cl• → CH₂Cl₂

Check: propagation 1 + propagation 2, cancelling Cl•, gives CH₃Cl + Cl₂ → CH₂Cl₂ + HCl. Balanced, no radicals.

Where marks are lost: starting from CH₄ instead of CH₃Cl; inventing species such as CH₃Cl• or HCl•; putting the dot on the Cl of •CH₂Cl instead of on the carbon.

4. Putting the radical dot on the right carbon

Propane reacts with chlorine in UV light to form 2-chloropropane. Write the two propagation steps.

The Cl must end up on the middle carbon, so the H must be removed from the middle carbon, and the dot sits there:

• CH₃CH₂CH₃ + Cl• → CH₃CH•CH₃ + HCl
• CH₃CH•CH₃ + Cl₂ → CH₃CHClCH₃ + Cl•

Where marks are lost: writing C₃H₇• (ambiguous: it does not show which carbon carries the dot), or CH₃CH₂CH₂•, which would give 1-chloropropane.

5. Model answer: how fractional distillation separates crude oil

A model answer, one creditable point per line:

1. Crude oil is heated so that it vaporises.
2. The vapour is passed into a fractionating column.
3. The column has a temperature gradient: cooler at the top than at the bottom.
4. The fractions have different boiling points, so they condense at different heights (different temperatures) and are collected there.
5. Larger molecules have higher boiling points (stronger van der Waals forces between molecules), so they condense lower down; small molecules reach the top.

Where marks are lost: describing cracking instead; describing bubble caps and trays but never mentioning boiling point or the temperature gradient.

6. Model answers: NO in engines, and removing sulfur dioxide

How NO forms in a petrol engine:

1. Nitrogen and oxygen from the air react together.
2. This needs the high temperature (or spark) in the engine.
3. N₂ + O₂ → 2NO

Why calcium oxide removes sulfur dioxide from flue gases:

1. SO₂ is acidic and CaO is basic.
2. CaO neutralises the SO₂: CaO + SO₂ → CaSO₃

Where marks are lost: saying the nitrogen comes from the fuel (this scores zero for the whole part); naming calcium oxide but giving no reason it works.

Common exam mistakes

• Saturated and hydrocarbon: leaving out only. "Contains carbon and hydrogen" and "has single bonds" do not score; it is carbon and hydrogen only and single bonds only. Saying alkanes have double bonds confuses saturated with unsaturated.
• Boiling is not bond breaking: a large share of students say covalent bonds break when alkanes boil. Boiling overcomes van der Waals forces between molecules; write "between molecules", not "van der Waals bonds".
• Condensing: when a vapour condenses, van der Waals forces form, they do not break.
• Fractional distillation answers: confusing it with cracking; describing the column's trays but never mentioning boiling point or the temperature gradient; describing the industrial column when the question asks about the laboratory method (heat the mixture in a flask, collect each fraction as it distils over).
• Missing fraction: naphtha is the fraction most often forgotten; bitumen, fuel oil and refinery gases are common wrong answers for it.
• Cracking definition: "the chain is broken" without saying C-C bonds break, or not saying large molecules become smaller molecules. Some students think only intermolecular forces break in cracking.
• Cracking conditions: catalytic cracking at "high pressure" or "low pressure" (it is slight pressure); nickel or Al₂O₃ as the catalyst (it is a zeolite); thermal cracking with only one of high temperature and high pressure; naming the wrong type of cracking for the products (alkenes come from thermal cracking).
• Economic reasons: "the products are more useful" is often ignored; the mark needs greater demand or higher value. Cracking does not make plastics: it makes alkenes, which are polymerised into plastics.
• Incomplete combustion: caused by a limited supply of oxygen, not "no oxygen" or "absence of oxygen". The solid product is carbon, not CO. For carbon's effect, "pollutant" or "toxic" is ignored: say breathing problems/asthma or global dimming.
• Combustion equations: wrong oxygen coefficient (17 instead of 8½, or 8½ instead of 12½). Count oxygen atoms in both CO₂ and H₂O, then halve.
• NO_x: saying the nitrogen comes from the fuel (scores zero); saying NO forms because oxygen is limited (it forms because of the high temperature); writing N instead of N₂; writing NO₂ when the question asks for NO.
• Catalytic converters: naming NO₂ as a "harmless" product (the products are CO₂, N₂, H₂O); giving zeolite as the catalyst (it is Pt, Pd or Rh); saying the honeycomb increases surface area without adding that less expensive metal is needed.
• Sulfur: giving sulfur or SO as the pollutant (it is SO₂); writing an equation that makes elemental S; saying SO₂ is "toxic" without acid rain; suggesting water removes it; naming CaO without saying it is a base that neutralises acidic SO₂.
• Mechanism name: "free radical" alone; it must be free-radical substitution.
• Condition: vague "light" or "heat"; write UV light (sunlight is also accepted).
• Radical equations: missing dots; charges on radicals; species such as HCl•, CH₃Cl• or H•; H₂ as a product instead of HCl; a termination step written where the second propagation step is needed. The second propagation step is the one most often lost.
• Dot position: writing C₃H₇• or C₄H₉•, or putting the dot on the wrong carbon. Write the radical as a structural formula with the dot on the carbon that lost the H.
• Curly arrows: do not use them in free-radical mechanisms. Examiners advise against it because they cause more errors than they earn, and an ionic step that contradicts a correct radical mechanism loses credit.
• Rate in brighter light: vague answers ("more energy", "more collisions") do not score; the reason is that chlorine radicals are produced faster (higher concentration of Cl•).

Key definitions

• Hydrocarbon: a compound containing carbon and hydrogen only.
• Saturated: contains single bonds only (no C=C double bonds).
• Alkane: a saturated hydrocarbon, general formula C_nH_2n+2.
• Petroleum (crude oil): a mixture consisting mainly of alkane hydrocarbons.
• Fraction: a mixture of hydrocarbons with similar boiling points (similar chain lengths).
• Fractional distillation: separation of a mixture into fractions by their different boiling points, using a column with a temperature gradient.
• Cracking: the breaking of C-C bonds in large alkane molecules to form smaller hydrocarbon molecules.
• Thermal cracking: cracking at high temperature and high pressure; gives a high percentage of alkenes.
• Catalytic cracking: cracking at high temperature, slight pressure and with a zeolite catalyst; gives motor fuels and aromatic hydrocarbons.
• Complete combustion: burning in excess oxygen to form carbon dioxide and water only.
• Incomplete combustion: burning in a limited supply of oxygen, forming carbon monoxide and/or carbon (and water).
• Free radical: a species with an unpaired electron.
• Free-radical substitution: the mechanism by which an alkane reacts with chlorine in UV light, a hydrogen atom being replaced by a chlorine atom.
• Initiation: the step that forms radicals from a molecule (Cl₂ → 2Cl•).
• Propagation: a step in which a radical reacts with a molecule to give a product and a new radical, so the chain continues.
• Termination: a step in which two radicals combine to form a molecule, ending the chain.

Specification

• I can state that alkanes are saturated hydrocarbons, and define both words with "only".
• I can state that petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.
• I can explain how fractional distillation separates crude oil (temperature gradient, different boiling points, chain length and van der Waals forces between molecules).
• I can describe how to carry out fractional distillation of a crude oil substitute in the lab, and how the fractions differ.
• I can state that cracking involves breaking C-C bonds in alkanes, and write equations for cracking.
• I can give the conditions and main products of thermal cracking (high pressure, high temperature, high percentage of alkenes).
• I can give the conditions and main products of catalytic cracking (slight pressure, high temperature, zeolite catalyst, motor fuels and aromatic hydrocarbons).
• I can explain the economic reasons for cracking alkanes (supply and demand, value, alkenes as feedstock).
• I can write balanced equations for the complete and incomplete combustion of alkanes and other organic compounds.
• I can state that the internal combustion engine produces NO_x, CO, carbon and unburned hydrocarbons, and how each forms.
• I can explain how a catalytic converter removes these pollutants, with equations.
• I can state that burning hydrocarbons containing sulfur produces SO₂, which causes air pollution.
• I can explain why SO₂ can be removed from flue gases using calcium oxide or calcium carbonate.
• I can explain the reaction of methane with chlorine as a free-radical substitution mechanism, with initiation, propagation and termination steps.

Related notes

• Introduction to organic chemistry
• Bonding
• Energetics
• Group 2, the alkaline earth metals
• Halogenoalkanes
• Alkenes