Organic Chemistry

Organic hub: functional groups, fuels, alkanes and alkenes, alcohols, carboxylic acids, esters and polymers. Practise the topic with its check.

SEC G3 Pure Chemistry 2027
17 lessons

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Before you begin

Organic chemistry is the study of carbon compounds. Start by recognising organic families and reading their structures. Then explore crude-oil separation and biofuel carbon accounting before studying hydrocarbon reactions, alcohols, acids, esters and polymers.

Be comfortable with: covalent bonding, writing and balancing chemical equations and acids, bases and salts, for carboxylic acids.

Learning goals

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.

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

Homologous families and reactions

Polymers

Practise and check

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Organic Chemistry structured questions · SEC G3 Pure Chemistry

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Topic reference

Quick Reference

Item	Quick rule / reminder
Homologous series	Same functional group; similar reactions; gradual trend
Alkanes	Saturated; general formula CₙH₂ₙ₊₂
Alkenes	Unsaturated (C=C); general formula CₙH₂ₙ
Test for unsaturation	Bromine water: orange → colourless with alkenes; alkanes show no change (no UV)
Substitution (alkanes)	Requires UV/light; halogen replaces H (e.g. chlorination)
Fractions (crude oil)	Fractions have boiling ranges (crude oil is a mixture)
Cracking	heat + catalyst → shorter alkanes + alkenes (alkenes are feedstock for polymers)
Alcohols	Functional group -OH; general formula CₙH₂ₙ₊₁OH
Alcohol structures	Draw branched and unbranched alcohols with 1–4 carbons; name the unbranched members methanol to butanol
Carboxylic acids	Functional group -COOH; general formula CₙH₂ₙ₊₁COOH
Esters	Name = alkyl (from alcohol) + carboxylate (from acid); use the name to deduce the structure and vice versa
Ethanol oxidation	ethanol → ethanoic acid (oxidation; see Alcohols)
Esterification	alcohol + carboxylic acid ⇌ ester + water (sweet smell)
Monomer ↔ polymer	For addition polymers: add the C = C back to get the monomer
Condensation polymers	Join monomers while eliminating a small molecule; recognise nylon and Terylene linkages
Depolymerisation	Break polymer chains into useful smaller molecules, for example by acid-catalysed hydrolysis of a polyester

Core knowledge to remember

Homologous series: family with the same functional group and general formula.
Functional group: atom/group that determines reactions (e.g. -OH).
Saturated: contains only single C–C bonds.
Unsaturated: contains at least one C=C bond.
Cracking: breaking large alkanes into smaller alkanes and alkenes.
Addition reaction: atoms add across a C = C bond (alkene → alkane-type product).
Addition polymerisation: many alkene molecules join to form a polymer.
Esterification: carboxylic acid + alcohol ⇌ ester + water.
Fermentation: making ethanol from sugar using yeast (anaerobic).
Condensation polymerisation: monomers join and a small molecule is eliminated at each link.
Depolymerisation: polymer chains are broken into smaller molecules that can be reused.

Common mistakes

Alkane vs alkene: alkene has C=C and decolourises bromine water.
General formula mix-ups: CₙH₂ₙ₊₂ (alkanes) vs CₙH₂ₙ (alkenes).
Polymerisation: in addition polymerisation, the double bond opens.
Ester naming order: alkyl (from alcohol) + carboxylate (from acid), e.g. ethyl ethanoate.
Combustion: complete gives CO₂ and H₂O; incomplete gives CO/soot.
Fractions: fractions have boiling ranges (do not write a single boiling point for crude oil fractions).
Monomer from polymer: use the repeating unit, then add the C = C back (addition polymer).
Bromine water wording: write the colour change (orange → colourless), not “it reacts”.
Substitution vs addition: alkanes do substitution (needs UV); alkenes do addition across C = C.
Polymers drawing: put brackets around the repeating unit and write “n”.
Ethanol manufacture: don’t mix fermentation with hydration conditions (see Alcohols).
Alcohol names: the unbranched C3 and C4 members are propanol and butanol; a displayed structure may place the -OH group on a different carbon or use a branched skeleton.
Condensation polymers: do not use the addition-polymer shortcut. Look for the ester or amide links and the small molecule eliminated.
Chemical recycling: depolymerisation breaks chains; acid-catalysed hydrolysis of a polyester forms a diol and a dicarboxylic acid.