Edexcel International A Level Chemistry

Topic 4: Introductory Organic Chemistry and Alkanes

The language of organic chemistry (hazards, families, formulae, names, reaction types, bond breaking) and the first family, the alkanes. Their isomers, their use as fuels, their combustion and the free radical substitution mechanism.

The one idea

Organic chemistry is the chemistry of carbon compounds, and it is organised by families. Every family has a functional group, the part that reacts, and a carbon chain, which mostly sets the physical properties. The alkanes are the family with no functional group: only C–C and C–H single bonds. This makes them a good first family to study. They burn, and they react with halogens in UV light.

Before the chemistry of any family, you need a shared language. A name tells you the structure. A formula can show it in four ways. A reaction type says what kind of change happens. A fission says how a bond breaks, and the particles that result decide the mechanism. This topic builds that language first, then uses it on the alkanes.

Where to find things

How to use these notes

• Read the pages in order the first time. Names and formulae (pages 3 and 4) are used on every later page, and the mechanism (page 16) uses the bond-breaking language of page 6.
• Each page opens with its one idea in a sentence. If you can explain it to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

This topic has no core practical. Safe working (page 1) is a concept, not a practical. Page 15 describes a simple test tube demonstration of bromine and hexane in light and dark, because the observation is a common exam question. It is not a listed practical.

The skills that earn most marks here are definitions quoted accurately (hazard, risk, homologous series, functional group, structural isomers, free radical, electrophile, carbon neutral), naming and drawing structures correctly, balanced equations (combustion, cracking, radicals with a dot) and explanations that give the reason as well as the fact.

Boiling temperatures are quoted to the nearest degree and carbon ranges for the crude oil fractions are typical values. Books and data booklets differ slightly, so learn the trends, not the last digit.

Pages

1. Hazard and risk

   A hazard is what a substance could do to you. Risk is how likely it is to happen. Safe working means finding the hazards, judging the risk and cutting it down before you start.

   12 min 4.14.24.34.3(i)4.3(ii)4.3(iii)

2. Homologous series and functional groups

   Organic compounds come in families. The functional group sets what a family does in a reaction, and the length of the carbon chain sets the gradual trend in physical properties.

   10 min 4.4

3. Ways of writing a formula

   Molecular, structural, displayed and skeletal formulae show different amounts of detail. You must be able to write one from another and know what each one shows.

   13 min 4.54.5(ii)

4. Naming organic compounds

   IUPAC names are built from a stem for the longest carbon chain, plus the names and positions of whatever is attached to it. The same rules let you draw a molecule from its name.

   15 min 4.54.5(i)4.5(ii)

5. Types of organic reaction

   Most organic reactions fall into five types. You classify a reaction by looking at what happens to the reactants, and a single reaction can belong to more than one type.

   9 min 4.6

6. Bond breaking, free radicals and electrophiles

   A covalent bond is a shared pair of electrons. It can break evenly, one electron to each atom, to make free radicals, or unevenly, both electrons to one atom, to make ions.

   13 min 4.74.7(i)4.7(ii)4.8

7. Alkanes and cycloalkanes

   Alkanes and cycloalkanes are hydrocarbons with only single bonds. Their general formulae let you check any formula and tell a chain from a ring.

   9 min 4.9

8. Structural isomerism

   Structural isomers have the same molecular formula but different structural formulae. There are three ways the atoms can be arranged differently, and naming a drawing is the way to check that two drawings are really different molecules.

   10 min 4.10

9. Isomers of alkanes and cycloalkanes

   A method for finding every structural isomer of an alkane or cycloalkane up to six carbons, by starting with the longest chain and shortening it one carbon at a time. The answers to learn are 2 for C₄H₁₀, 3 for C₅H₁₂ and 5 for C₆H₁₄.

   15 min 4.11

10. Crude oil and fractional distillation

    Crude oil is a mixture of hydrocarbons, mostly alkanes. Fractional distillation separates it into fractions by boiling temperature, and the fractions are the fuels we burn.

    10 min 4.12

11. Cracking and reforming

    Cracking breaks long alkane chains into shorter alkanes and alkenes. Reforming rearranges straight chains into branched and cyclic molecules. Both change the molecules, so both need balanced equations.

    13 min 4.12

12. Combustion of alkanes

    Alkanes burn in plenty of oxygen to give carbon dioxide and water. With limited oxygen they give carbon monoxide or carbon as well. You must balance both kinds of equation, and use them in calculations.

    13 min 4.174.17(i)

13. Pollutants from burning fuels

    Burning alkane fuels releases carbon monoxide, carbon particulates, unburned hydrocarbons, oxides of nitrogen and sulfur dioxide. Carbon monoxide is toxic because it binds to haemoglobin, and the oxides of nitrogen and sulfur make acid rain.

    13 min 4.134.14

14. Alternative fuels and carbon neutrality

    Alkane fuels are finite and release pollutants and carbon dioxide. An alternative fuel is judged by whether it is renewable and by its overall carbon dioxide emissions, counting how it is made as well as how it burns.

    13 min 4.154.16

15. Alkanes and halogens

    In ultraviolet light, an alkane and a halogen react by substitution. A hydrogen atom is replaced by a halogen atom, and a hydrogen halide is made. Without light there is no reaction.

    9 min 4.174.17(ii)

16. Free radical substitution: the mechanism

    The reaction of an alkane with a halogen is a chain reaction in three stages. Initiation makes radicals, propagation regenerates them as the product forms, and termination removes them.

    15 min 4.184.18(i)4.18(ii)4.18(iii)

17. Further substitution and its limits

    A chlorine radical can attack the product as well as the alkane, so the reaction goes on to make a mixture. A mixture means a low yield of any one product, which is why free radical substitution is of limited use in synthesis.

    11 min 4.184.18(iv)

Also in this topic: Topic 4 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.