Alkanes

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.19 — The general formula for alkanes
• 4.20 — Why alkanes are classified as saturated hydrocarbons
• 4.21 — Structural and displayed formulae for alkanes up to five carbons, and naming unbranched isomers
• 4.22 — The reaction of alkanes with halogens in ultraviolet light, limited to mono-substitution

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The ALKANES are the simplest homologous series and the one that makes up most of crude oil. Every bond in an alkane is a single bond: carbon to carbon, or carbon to hydrogen, and nothing else. There is no functional group, which is the first clue to how they behave.

Where the general formula CₙH₂ₙ₊₂ comes from

1. Line up n carbon atoms in a chain. Each has four bonds to spend.
2. The carbons in the MIDDLE of the chain use two bonds joining to their neighbours, leaving two spare for hydrogen.
3. The two carbons at the ENDS use only one bond each on a neighbour, leaving three spare for hydrogen.
4. So the hydrogen count is 2 per carbon, plus one extra on each of the two ends: 2n + 2.
5. Check it on butane: n = 4 gives 2×4 + 2 = 10 hydrogens, and butane is C₄H₁₀.
6. The formula is not something to memorise blindly — it is the result of counting spare bonds on a chain.

That count is also the definition of SATURATED. An alkane already carries the largest number of hydrogen atoms its chain of carbons can possibly hold. There is no spare bond anywhere for anything else to attach to, so nothing can be ADDED to an alkane. The molecule is full.

Think of it like a bus with every seat taken

A full bus is not a broken bus. It runs perfectly well, it goes where buses go, and it does its job. What it cannot do is take another passenger — not because anything is wrong with it, but because there is nowhere to put one. A saturated hydrocarbon is a full bus. Every bond that could be holding a hydrogen already is. If you want a chlorine atom on board, somebody has to get off first, and that is exactly what happens in the reaction on the second half of this page.

The first five alkanes are worth knowing by heart, because questions assume them: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀ and pentane C₅H₁₂. Written as structural formulae they are CH₄, CH₃CH₃, CH₃CH₂CH₃, CH₃CH₂CH₂CH₃ and CH₃CH₂CH₂CH₂CH₃ — a chain of CH₂ units with a CH₃ capping each end.

Because they have no functional group and only strong single bonds, alkanes are relatively unreactive towards the reagents met earlier in this course: they are not attacked by acids, by alkalis or by aqueous solutions of most substances. That does not make them inert. They burn readily, which is the whole reason crude oil is worth anything, and they will react with a halogen — given one particular condition.

Statement 4.22 — substitution with a halogen

1. Mix methane with chlorine in the dark and nothing happens at all. This is the important control.
2. Now shine ULTRAVIOLET LIGHT on the mixture — sunlight will do — and a reaction proceeds.
3. The ultraviolet light supplies the energy needed to break the bond in the chlorine molecule and start the reaction.
4. Because methane is saturated, nothing can be added to it, so a chlorine atom can only take the place of a hydrogen atom.
5. One hydrogen is REPLACED by one chlorine, giving chloromethane, CH₃Cl.
6. The displaced hydrogen leaves combined with the other chlorine atom, as hydrogen chloride, HCl.
7. CH₄ + Cl₂ → CH₃Cl + HCl. Two products, which is the signature of a SUBSTITUTION reaction.

The specification restricts this to MONO-substitution — one hydrogen replaced, one product. In reality the reaction does not stop there and will go on to give dichloromethane and beyond, but you are not asked for that, and writing it will not gain marks.

Notice what the light is doing, because it is a genuinely unusual condition. Most reactions on this course are started by heating. Here the energy arrives as light of a particular kind, and without it the two gases sit together indefinitely. The reaction is not slow in the dark; it does not happen.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Saturated hydrocarbon

A hydrocarbon containing only single carbon-to-carbon bonds, so that it holds the maximum possible number of hydrogen atoms and nothing further can be added.

CₙH₂ₙ₊₂

n is the number of carbon atoms in the molecule

Units: No units — a formula relating atom counts, not a measured quantity

The first five alkanes

CH₄ + Cl₂ → CH₃Cl + HCl

Conditions: Ultraviolet light, and no reaction at all in the dark. A substitution reaction, restricted here to mono-substitution.

C₂H₆ + Br₂ → C₂H₅Br + HBr

Conditions: Ultraviolet light. The same substitution with a different alkane and a different halogen — one hydrogen replaced, hydrogen bromide released.

The words this page is marked on

Alkane

A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂.

Saturated

Containing only single carbon-to-carbon bonds, so nothing further can be added.

Substitution

A reaction in which an atom or group is replaced by a different atom or group, giving two products.

Mono-substitution

Substitution of exactly one hydrogen atom, which is all the specification requires here.

Model answer [4 marks]

Explain why alkanes are described as saturated, and what this means for the reactions they can undergo. [4]

Alkanes contain only single bonds between their carbon atoms, so every carbon atom is joined to as many hydrogen atoms as its four bonds allow. This means the molecule already holds the maximum number of hydrogen atoms possible for that chain, which is what saturated means, and is why the general formula is CₙH₂ₙ₊₂. Because there is no spare bond anywhere, nothing can be added to an alkane. The only way another atom can be introduced is for it to replace a hydrogen atom already present, which is why alkanes undergo substitution reactions rather than addition reactions.

Model answer [3 marks]

Describe what happens when methane and chlorine are mixed and left in sunlight, and name the products. [3]

In the dark no reaction occurs, but ultraviolet light in sunlight supplies the energy needed to break the bond in the chlorine molecule and start the reaction. A chlorine atom then replaces one hydrogen atom in the methane, because methane is saturated and nothing can be added to it. The products are chloromethane, CH₃Cl, and hydrogen chloride, HCl, and the equation is CH₄ + Cl₂ → CH₃Cl + HCl. Two products are formed, which is characteristic of a substitution reaction.

Conditions for the halogen reaction — all three are examined

• Ultraviolet light is essential; in the dark there is NO reaction
• The halogen is chlorine or bromine
• One hydrogen atom is replaced by one halogen atom — mono-substitution only
• TWO products are formed: the halogenoalkane, and a hydrogen halide such as HCl

Not this: Alkanes are unreactive, so nothing much happens to them.

This: They are unreactive towards acids, alkalis and aqueous reagents because they have no functional group. They are not inert: they burn readily, which is why they are used as fuels, and they react with halogens in ultraviolet light.

Mark-losing trap. No ultraviolet light means NO reaction — not a slow one. State the light as a condition or lose the mark.

Mark-losing trap. Alkanes undergo SUBSTITUTION, never addition. Being saturated, they have no spare bond to add anything to.

Mark-losing trap. Both products must be named. Forgetting the hydrogen chloride is the commonest way to lose a mark here.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is the general formula for the alkanes?
2. Grade 7 · Deduce [2 marks] — An alkane has seven carbon atoms. How many hydrogen atoms does it contain?
3. Grade 8 · Explain [3 marks] — Explain why a chlorine atom can only REPLACE a hydrogen atom in methane, rather than simply joining on.
4. Grade 9 · Explain [4 marks] — Select every statement that belongs in a full account of the reaction between methane and chlorine.
5. 9+ · Deduce [5 marks] — A saturated hydrocarbon has a relative molecular mass of 58. Deduce the number of carbon atoms in one molecule. (Aᵣ: H = 1, C = 12)

The people behind this science

Two ways into the same idea — the one who had to understand the first alkane well enough to make a mine safe, and the one who proved that light itself, and not warmth, can be what makes a reaction go. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one who had to understand the first alkane well enough to make a mine safe

Methane, the first member of this family, was known to miners as firedamp and killed them in large numbers. Davy was asked to do something about it, and to design the lamp he had to establish real chemistry about the gas: the proportions of methane and air that will explode, that flame will not pass through a fine metal gauze because the metal conducts the heat away, and that a flame's behaviour in the lamp warns of the gas before a man can smell it. He is the right figure to ask what an alkane does when it meets a flame, and what it does not do the rest of the time.

• “What did you have to learn about firedamp before designing the lamp?”
• “Why will a flame not pass through a wire gauze?”
• “How much of the gas has to be in the air before it explodes?”
• “Why is a gas that burns so fiercely so unreactive otherwise?”
• “Should you have taken a patent on the lamp?”

Jan Ingenhousz — the one who proved that light itself, and not warmth, can be what makes a reaction go

The strangest condition on this page is that methane and chlorine sit together unchanged in the dark and react in ultraviolet light. Ingenhousz established exactly that kind of claim first, and it was not obvious: in 1779 he showed that plants release their gas only in the light, and — crucially — he separated light from warmth, since a warm dark room does nothing. That experimental separation is the whole difficulty here too. He is the right person to ask how you demonstrate that light rather than heat is what supplies the energy.

• “How did you show it was the light and not the warmth?”
• “What happens to your plants when you put them in the dark?”
• “Can light really supply the energy a reaction needs?”
• “What control would you insist on in an experiment like this?”
• “Does the kind of light matter, or only the amount?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Jean-Baptiste Dumas in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Cracking
• Next lesson: Alkenes
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• All of Chemistry · Incandio Science