Organic chemistry
Fuels and crude oil, alkanes, alkenes, alcohols, carboxylic acids, esters and polymers for G3 Pure and O-Level Chemistry.
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Before you begin
Petrol, plastics, vinegar and the alcohol in hand sanitiser are all organic compounds: compounds of carbon. There are millions of them, but they fall into a few families, and each family reacts in its own predictable way. This topic starts with the families and the fuels we get from crude oil, then works through alkanes, alkenes, alcohols, carboxylic acids and esters, and ends with the polymers made from them and what happens to plastic waste.
Be comfortable with: covalent bonding; writing and balancing chemical equations; and acids, bases and salts (needed for carboxylic acids).
Learning goals
- Name natural gas, mainly methane, and crude oil as non-renewable energy sources
- Describe crude oil as a mixture of hydrocarbons separated by fractional distillation into fractions used as fuels and as a source of chemicals
- Describe biofuels, such as bioethanol from sugarcane, as a renewable alternative to natural gas and crude oil
- Explain how biofuels can be more sustainable than fossil fuels in terms of carbon dioxide emissions
- Describe a homologous series as compounds with a general formula, similar chemical properties and physical properties that change gradually as the molecules get bigger
- Describe the alkanes as a homologous series of saturated hydrocarbons with general formula CₙH₂ₙ₊₂
- Draw branched and unbranched alkanes with one to four carbons and name methane to butane
- Define isomerism and recognise isomers
- Describe alkanes, such as methane, as generally unreactive apart from combustion and substitution by chlorine
- Describe the alkenes as a homologous series of unsaturated hydrocarbons with general formula CₙH₂ₙ
- Draw branched and unbranched alkenes with two to four carbons and name ethene to butene
- Describe how cracking makes alkenes and hydrogen, and why cracking is needed to meet demand for fractions of smaller molecules
- Distinguish saturated from unsaturated hydrocarbons by their structures and with aqueous bromine
- Describe the combustion, polymerisation and addition reactions with bromine, steam and hydrogen of alkenes such as ethene
- State what polyunsaturated means on food products
- Describe how margarine is made by adding hydrogen to unsaturated vegetable oils to form a solid
- Describe alcohols as a homologous series with the –OH functional group
- Draw branched and unbranched alcohols with one to four carbons and name methanol to butanol
- Describe the combustion of alcohols and their oxidation to carboxylic acids
- Describe how ethanol is made by the catalysed addition of steam to ethene and by fermenting glucose
- Describe the carboxylic acids as a homologous series containing the –CO₂H group
- Draw carboxylic acids with one to four carbons and name methanoic acid to butanoic acid
- Describe carboxylic acids as weak acids that react with carbonates, bases and some metals
- Describe how ethanol is oxidised to ethanoic acid by oxygen in air or acidified potassium manganate(VII)
- Describe how a carboxylic acid reacts with an alcohol to form an ester, such as ethyl ethanoate
- Deduce an ester's name and formula from its unbranched acid and alcohol, each with one to four carbons, and the reverse
- Describe polymers as large molecules built from small monomer units, with different polymers having different units or linkages
- Describe how ethene monomers form poly(ethene) by addition polymerisation
- State uses of poly(ethene) as a typical plastic, such as plastic bags and cling film
- Deduce a polymer's structure from its monomer, and the reverse
- Describe nylon, a polyamide, and Terylene, a polyester, as condensation polymers and recognise their partial structures
- State typical uses of man-made fibres such as nylon and Terylene, such as clothing, curtains, fishing line, parachutes and sleeping bags
- Describe the pollution caused by disposing of non-biodegradable plastics
- Describe physical and chemical methods of recycling plastics
- Describe physical recycling of plastics, such as melting poly(ethene) waste into pellets
- Describe chemical recycling of plastics by cracking into fuel and depolymerisation into monomers, such as acid hydrolysis of polyesters
- Discuss the social, economic and environmental issues of recycling plastics
Syllabus statements covered
- name natural gas, mainly methane, and crude oil as non-renewable sources of energy
- describe crude oil as a mixture of hydrocarbons and its separation by fractional distillation to yield fractions which have competing uses as fuels and as a source of chemicals (see also 1.2(a))
- describe biofuel (exemplified by bioethanol from sugarcane) as a renewable alternative to natural gas and crude oil
- describe how biofuel, when compared to fossil fuels, can be more environmentally sustainable in terms of carbon dioxide emission (see also 12(g)).
- describe a homologous series as a group of compounds with a general formula, similar chemical properties and showing a gradation in physical properties as a result of increase in the size and mass of the molecules, e.g. melting and boiling points; viscosity
- describe the alkanes as a homologous series of saturated hydrocarbons with the general formula CnH2n+2
- draw the structures of branched and unbranched alkanes, C1 to C4, and name the unbranched alkanes methane to butane
- define isomerism and identify isomers
- describe alkanes (exemplified by methane) as being generally unreactive except in terms of combustion and substitution by chlorine
- describe the alkenes as a homologous series of unsaturated hydrocarbons with the general formula CnH2n
- draw the structures of branched and unbranched alkenes, C2 to C4, and name the unbranched alkenes ethene to butene
- describe the manufacture of alkenes and hydrogen by cracking hydrocarbons and recognise that cracking is essential to match the demand for fractions containing smaller molecules from the refinery process
- describe the difference between saturated and unsaturated hydrocarbons from their molecular structures and by using aqueous bromine
- describe the reactions of alkenes (exemplified by ethene) in terms of combustion, polymerisation (see also 11.4(b)), and the addition with bromine, steam and hydrogen
- state the meaning of polyunsaturated when applied to food products
- describe the manufacture of margarine by the addition of hydrogen to unsaturated vegetable oils to form a solid product.
- describe the alcohols as a homologous series containing the –OH group
- draw the structures of branched and unbranched alcohols, C1 to C4, and name the unbranched alcohols methanol to butanol
- describe the reactions of alcohols in terms of combustion and oxidation to carboxylic acids
- describe the formation of ethanol by the catalysed addition of steam to ethene and by fermentation of glucose
- describe the carboxylic acids as a homologous series containing the –CO2H group
- draw the structures of carboxylic acids, C1 to C4, and name the unbranched acids methanoic acid to butanoic acid
- describe the carboxylic acids as weak acids, reacting with carbonates, bases and some metals
- describe the formation of ethanoic acid by the oxidation of ethanol by atmospheric oxygen or acidified potassium manganate(VII)
- describe the reaction of a carboxylic acid with an alcohol to form an ester, e.g. ethyl ethanoate
- deduce the name and formula of an ester from the unbranched carboxylic acid, C1 to C4, and alcohol, C1 to C4, and vice versa.
- describe polymers as large molecules built up from small units (monomers), different polymers having different units and/or different linkages
- describe the formation of poly(ethene) as an example of addition polymerisation of ethene as the monomer (see also 11.2(j))
- state some uses of poly(ethene) as a typical plastic, e.g. plastic bags; clingfilm
- deduce the structure of the polymer product from a given monomer and vice versa
- describe nylon, a polyamide, and Terylene, a polyester, as condensation polymers, the partial structure of nylon being represented as and the partial structure of Terylene as (details of manufacture and mechanisms of these polymerisations are not required)
- state some typical uses of man-made fibres such as nylon and Terylene, e.g. clothing; curtain materials; fishing line; parachutes; sleeping bags
- describe the pollution problems caused by the disposal of non-biodegradable plastics
- describe two methods of recycling plastics as
- physical method (exemplified by melting small pieces of poly(ethene) waste into pellets)
- chemical method (exemplified by depolymerisation and cracking of plastic waste into chemical feedstock and fuel respectively) — describe depolymerisation as a process in which polymers are broken down into their monomers, exemplified by hydrolysis of polyesters using acid as a catalyst (details of mechanisms are not required)
- discuss the social, economic and environmental issues of recycling plastics.
Lessons
Work through them in order.
Foundations and fuels
- Introduction to organic chemistryRecognise homologous series and functional groups, use general formulae, and name simple organic compounds.
- Crude oil and fractional distillationExplain why natural gas and crude oil are non-renewable, and how fractional distillation separates crude oil.
- Crude-oil fractions and their usesMatch crude-oil fractions to their uses, and explain why they are wanted both as fuels and as raw materials.
- Fossil fuels: combustion and emissionsWrite equations for complete and incomplete combustion, and link each pollutant to its source and effect.
- Biofuels and carbon emissionsExplain why sugarcane bioethanol is renewable, and judge claims that it is carbon neutral.
Homologous families and reactions
- Alkanes: structures and isomersDraw and name the alkanes from methane to butane, explain their boiling points, and identify isomers.
- Reactions of alkanesWrite equations for the combustion of alkanes and the substitution of methane by chlorine in UV light.
- Alkenes: structures and the bromine testDraw and name the alkenes from ethene to butene, identify their isomers, and use the bromine-water test.
- CrackingExplain why large hydrocarbons are cracked, and write cracking equations that give alkenes and hydrogen.
- Addition reactions of alkenesDescribe the addition of bromine, hydrogen and steam to ethene, and how margarine is made.
- Alcohols: structures and isomersDraw and name the alcohols from methanol to butanol, including their branched isomers.
- Making ethanolDescribe and compare making ethanol by fermentation and by adding steam to ethene.
- Reactions of ethanolWrite the equation for burning ethanol, and describe its oxidation to ethanoic acid.
- Carboxylic acidsName the carboxylic acids from methanoic to butanoic acid, explain why they are weak, and write their reactions as acids.
- Esters: formation and namingMake and name an ester from an acid and an alcohol, and work back from an ester to its acid and alcohol.
Polymers
- Addition polymersDraw an addition polymer from its alkene monomer, and work back from a polymer to its monomer.
- Condensation polymersRecognise nylon and Terylene as condensation polymers from their amide and ester linkages, and give their uses.
- Plastic waste and recyclingDescribe the pollution caused by plastics, and how they are recycled physically and by depolymerisation or cracking.
Practise and check
Or choose
Topic reference
Quick reference
| Series or process | Functional group or key fact | Example |
|---|---|---|
| Alkanes | CₙH₂ₙ₊₂; saturated, single bonds only | ethane, CH₃CH₃ |
| Alkenes | CₙH₂ₙ; C = C double bond | ethene, CH₂ = CH₂ |
| Alcohols | CₙH₂ₙ₊₁OH; -OH group | ethanol, CH₃CH₂OH |
| Carboxylic acids | -COOH group; weak acids | ethanoic acid, CH₃COOH |
| Esters | -COO– linkage; name: alcohol part (-yl) then acid part (-oate) | ethyl ethanoate, CH₃COOCH₂CH₃ |
| Test for C = C | aqueous bromine: orange to colourless with an alkene; stays orange with an alkane | |
| Alkane + chlorine | substitution, in UV light | CH₄ + Cl₂ → CH₃Cl + HCl |
| Alkene + hydrogen | addition; nickel catalyst, heat | ethene → ethane |
| Alkene + steam | addition; H₃PO₄ catalyst, high temperature and pressure | ethene → ethanol |
| Fermentation | glucose → ethanol + carbon dioxide; yeast, warm, no air | |
| Oxidation of ethanol | by air (bacteria) or acidified potassium manganate(VII), purple to colourless | ethanol → ethanoic acid |
| Esterification | acid + alcohol ⇌ ester + water; concentrated H₂SO₄ catalyst | |
| Cracking | large alkanes → smaller alkanes + alkenes (+ hydrogen); high temperature, catalyst | C₁₀H₂₂ → C₈H₁₈ + C₂H₄ |
| Addition polymer | nalkene → polymer only; open the C = C | ethene → poly(ethene) |
| Condensation polymer | monomers join, giving off water at each link | nylon (amide), Terylene (ester) |
| Depolymerisation | polymer → its monomers; polyester + water (acid catalyst) → diol + dicarboxylic acid |
Core knowledge to remember
- A homologous series is a group of compounds with the same general formula and similar chemical properties, showing a gradation in physical properties, such as melting point, boiling point and viscosity, as the molecules increase in size and mass.
- Isomers are compounds with the same molecular formula but different structural formulae.
- A saturated hydrocarbon has only single carbon–carbon bonds; an unsaturated hydrocarbon has at least one carbon–carbon double bond.
- Polyunsaturated describes food fats and oils whose molecules contain several carbon–carbon double bonds. Margarine is made by adding hydrogen to unsaturated vegetable oils to form a solid product.
- Crude oil is a mixture of hydrocarbons, separated by fractional distillation into fractions with different boiling ranges, used as fuels and as a source of chemicals.
- Cracking breaks large hydrocarbon molecules into smaller alkanes, alkenes and hydrogen, to match the demand for smaller molecules.
- Bioethanol from sugarcane is a renewable alternative to fossil fuels.
- Carboxylic acids are weak acids: they ionise only partly in water, but still react with metals, bases and carbonates.
- Polymers are large molecules built up from small units (monomers). Different polymers have different units, different linkages, or both.
- Depolymerisation is a process in which polymers are broken down into their monomers.
Common mistakes
- Writing the bromine-water colour change backwards. It goes from orange to colourless with an alkene.
- Mixing up substitution (alkanes, in UV light) and addition (alkenes, across the double bond).
- Mixing up fractional distillation (separates molecules) and cracking (breaks them).
- Leaving out the H of the –OH group, or the carbon of the –COOH group, when counting atoms.
- Swapping the conditions for making ethanol: yeast and warmth for fermentation; steam, H₃PO₄, high temperature and pressure for hydration.
- Naming an ester with the acid part first. The alcohol part (-yl) comes first in the name.
- Keeping the double bond in the repeat unit of an addition polymer.
- Calling nylon or Terylene an addition polymer. Both are condensation polymers.
- Saying a weak acid is a dilute acid. “Weak” means only partly ionised.