Alkanes

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2 guided lessons · 18 checks · 4. Organic Chemistry → Alkanes

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The saturated family and its formula

Alkanes are the quiet workhorses of organic chemistry, unreactive but everywhere. This lesson fixes the two labels the exam rewards: their general formula and the word saturated.

What you know already

From Cracking you already know that alkanes are the single-bonded products the column delivers in bulk. This note formalises the family: the general formula that generates every member, why the family is called saturated, and the names and drawings of the first five alkanes.

Key points of understanding

Alkanes are a group of saturated hydrocarbons, meaning they have single carbon-carbon bonds only.
No double bonds exist anywhere in an alkane molecule, which is what saturated means here.
The general formula of the alkanes is CnH2n+2, with n a whole number.
Alkanes are generally unreactive but burn, can be cracked, and react with halogens in light.
Methane, the first alkane, is the major component of natural gas.

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1.The saturated family and its formula

4CH1-4.194CH1-4.20

Alkanes are the quiet workhorses of organic chemistry, unreactive but everywhere. This lesson fixes the two labels the exam rewards: their general formula and the word saturated.

Key points

Alkanes are a group of saturated hydrocarbons, meaning they have single carbon-carbon bonds only.
No double bonds exist anywhere in an alkane molecule, which is what saturated means here.
The general formula of the alkanes is CnH2n+2, with n a whole number.
Alkanes are generally unreactive but burn, can be cracked, and react with halogens in light.

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2.Naming and drawing the first five

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The first five alkanes are the exam currency of this family. This lesson lines up methane through pentane with their molecular formulae and shows how the displayed drawings follow the formula.

Key points

The first five alkanes are methane CH4, ethane C2H6, propane C3H8, butane C4H10 and pentane C5H12.
Each displayed formula draws every bond: four single bonds round each carbon atom.
Methane has one carbon, so its displayed formula shows that carbon bonded to four hydrogens.
Ethane, propane, butane and pentane extend the carbon chain one carbon at a time.
Exams ask you to name, draw and give the formula for the first five alkanes.

First pass

Learn the run of five: methane, ethane, propane, butane, pentane, with hydrogens 4, 6, 8, 10 and 12. Each drawing simply shows single bonds joining the carbons and hydrogens.

Analogy — It is a ladder where each rung adds one carbon block and two hydrogen rungs; climb from CH4 to C5H12 without skipping.

Exam pointer — Practise drawing each displayed formula with every bond shown; half-drawn structures lose the mark.

Going deeper

The homologous run runs methane CH4, ethane C2H6, propane C3H8, butane C4H10 and pentane C5H12, each obeying CnH2n+2. Drawing a displayed formula means every single bond appears, so each carbon shows four bonds whether to neighbouring carbons or to hydrogens.

Analogy — Think of carbon as a four-way junction: in every alkane drawing, all four arms must be filled, and the chain grows by inserting extra junctions one at a time.

Exam pointer — Check every carbon in your drawing carries four bonds before moving on; a carbon with three or five is the classic lost mark.

Checks

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1.Which molecular formula belongs to pentane?
2.How many hydrogen atoms appear in the displayed formula of ethane?
3.Which name follows propane in the alkane series?
4.In the displayed formula of propane, what bonds does the middle carbon carry?
5.What must a full displayed formula of butane show that a molecular formula does not?
6.Name the alkane whose molecules contain three carbon atoms, and give its molecular formula.
Written response · recall
Your answer
7.How many bonds surround each carbon atom in a displayed formula of any alkane?
Written response · recall
Your answer
8.Explain what changes from one named alkane to the next in the series from methane to pentane.
Written response · explain why
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9.A student draws a displayed formula for ethane but joins the two carbon atoms with a double line. Name the hydrocarbon family the drawing now suggests.
Written response · application
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