Organic Chemistry
11. Organic Chemistry
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Learning objectives
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- 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, is more environmentally sustainable in terms of the offset in carbon dioxide emission during burning by that taken in during plant growth (see also 12(e)).
- 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 unbranched alkanes, C1 to C3, and name the unbranched alkanes methane to propane
- 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 unbranched alkenes, C2 and C3, and name the unbranched alkenes ethene and propene
- 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 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 unbranched alcohols, C1 to C3, and name the unbranched alcohols methanol to propanol
- describe the reactions of alcohols in terms of combustion and oxidation to carboxylic acids
- describe the formation of ethanol by fermentation of glucose
- describe the carboxylic acids as a homologous series containing the –CO2H group
- describe the formation of ethanoic acid by the oxidation of ethanol by atmospheric oxygen or acidified potassium manganate(VII).
- describe polymers as large molecules built up from small units (monomers), different polymers having different units
- describe the formation of poly(ethene) as an example of addition polymerisation of ethene as the monomer (see also 11.2(i))
- state some uses of poly(ethene) as a typical plastic, e.g. plastic bags; clingfilm
- deduce the structure of the addition polymer product from a given monomer and vice versa
- 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)
- describe two methods of recycling plastics as — chemical method (exemplified by cracking of plastic waste into fuel)
- discuss the social, economic and environmental issues of recycling plastics.
This lesson links fuels, C1–C3 hydrocarbons, alcohols, ethanoic acid and addition polymers into one reaction map at G3 Science depth. Study it in four short passes—fuels, hydrocarbon families, alcohol chemistry and polymers—and pause to retrieve each part before continuing.
1. Definition
A homologous series is a family of organic compounds with the same functional group and general formula and similar chemical reactions. Consecutive members differ by CH₂.
- Alkanes are saturated hydrocarbons with formula CₙH₂ₙ₊₂.
- Alkenes are unsaturated hydrocarbons with formula CₙH₂ₙ.
- Alcohols contain the -OH functional group.
Use the prescribed C1–C3 structures and reactions. C4 structural isomers, esterification, ester naming and condensation polymers are G3 Chemistry extensions.
2. Key Ideas
| Family or process | Recognition | Required reaction or idea |
|---|---|---|
| Alkane | only C–C single bonds | combustion; substitution with halogen in UV |
| Alkene | contains C=C | bromine-water test; addition; hydrogenation |
| Alcohol | contains –OH | ethanol fermentation, combustion and oxidation |
| Carboxylic acid | contains –COOH | ethanoic acid forms by oxidising ethanol |
| Addition polymer | repeat unit from an alkene | C=C opens; no small-molecule by-product |
| Crude oil | hydrocarbon mixture | fractional distillation by boiling range |
Alkane substitution requires UV light. Alkene hydrogenation requires hydrogen and nickel. Ethanol oxidation requires an oxidising agent.
3. Detailed Explanations
Fuels, crude oil and cracking
Natural gas, which is mainly methane, and crude oil are non-renewable energy sources. Crude oil is a mixture of hydrocarbons separated by fractional distillation into fractions with different boiling ranges. Each fraction has competing uses: it can be burned as a fuel or used as a source of chemicals. Choosing one use reduces how much is available for the other.
Cracking breaks large hydrocarbon molecules into smaller hydrocarbons, including alkenes, and hydrogen. Refineries use it to meet the greater demand for fractions containing smaller molecules instead of treating every fraction as equally useful.
Bioethanol made from sugarcane is renewable because more plants can be grown. Burning it releases carbon dioxide, but plant growth takes in carbon dioxide and can offset some of that emission. A careful comparison also considers farming, processing, transport and land use; “renewable” does not mean “no environmental impact”.
Without looking back, explain why crude oil is a mixture rather than a compound, why its fractions have boiling ranges, and why some larger molecules are cracked.
Check your explanation
Crude oil contains different hydrocarbons that are not chemically bonded to one another, so it is a mixture. Each fraction still contains several hydrocarbons and therefore boils over a range. Cracking converts less-demanded large molecules into smaller hydrocarbons, including useful alkenes.
Homologous series and structures
Members of one homologous series have the same general formula and functional group, similar chemical properties, and a gradual change in physical properties. As molecular size and mass increase, melting point, boiling point and viscosity generally increase.
| Series | Names and condensed structures required here |
|---|---|
| alkanes | methane CH₄; ethane CH₃-CH₃; propane CH₃-CH₂-CH₃ |
| alkenes | ethene CH₂ = CH₂; propene CH₂ = CH-CH₃ |
| alcohols | methanol CH₃OH; ethanol CH₃CH₂OH; propanol CH₃CH₂CH₂OH |
When asked to draw a structure, show every carbon-to-carbon bond and place the double bond correctly. There is no one-carbon alkene because a C=C bond needs two carbon atoms.
Hydrocarbon reaction map
- complete combustion produces CO₂ and H₂O;
- limited oxygen may produce poisonous CO and/or soot;
- methane undergoes substitution with chlorine in UV light;
- alkenes decolourise aqueous bromine and undergo addition;
- hydrogenation adds H₂ across C=C using nickel.
Polyunsaturated means that a molecule contains more than one carbon–carbon double bond. Unsaturated vegetable oils can be changed into a more solid product for margarine by adding hydrogen across some C=C bonds, using nickel as a catalyst.
Ethane and ethene are both colourless gases. State how bromine water distinguishes them, then name the reagent and catalyst that convert ethene to ethane.
Check your answer
Ethene decolourises bromine water from orange to colourless because it contains C=C; ethane does not. Hydrogen adds across the ethene double bond using nickel as the catalyst, forming ethane.
Alcohols and ethanoic acid
The C1–C3 alcohols are methanol, ethanol and propanol. Ethanol can be made by fermentation of glucose using yeast under warm anaerobic conditions:
Alcohols burn in oxygen and are oxidised to carboxylic acids. Ethanol can be oxidised slowly by atmospheric oxygen or more quickly by warming with acidified potassium manganate(VII), which changes from purple to colourless. The product is ethanoic acid, a member of the carboxylic-acid homologous series containing the -CO₂H group.
Write a three-stage route from glucose to ethanoic acid. Include the condition for fermentation and the reagent and observation for the oxidation step.
Check your route
Ferment glucose with yeast under warm anaerobic conditions to form ethanol and carbon dioxide. Then warm the ethanol with acidified potassium manganate(VII); the solution changes from purple to colourless as ethanol is oxidised to ethanoic acid.
Addition polymers and plastics
A polymer is a large molecule built from many small units called monomers. In addition polymerisation, alkene double bonds open and many monomers join:
Poly(ethene) is used for plastic bags and clingfilm because it is flexible, waterproof and chemically unreactive in normal use. Its durability becomes a problem after disposal because it is non-biodegradable: it persists, accumulates on land or in water and can harm wildlife.
Two recycling routes are required:
- physical recycling: sort and clean poly(ethene), then recycle it by melting small pieces into pellets for new products;
- chemical recycling: crack plastic waste into smaller molecules that can be used as fuel or chemical feedstock.
Recycling can reduce waste and demand for new crude oil, but collection, sorting, cleaning and processing cost money and use energy. A good evaluation weighs social convenience and jobs, economic cost and product value, and environmental effects such as waste, emissions and resource use. Do not claim that one route is always best without evidence from the situation.
Before opening the answer, sketch the repeat unit of poly(ethene), then decide which recycling route better suits clean poly(ethene) film and which may suit mixed plastic waste.
Check your answer
The repeat unit is [-CH₂-CH₂-]ₙ with single bonds in the backbone. Clean poly(ethene) can be sorted, melted and formed into pellets by physical recycling. Mixed waste that is unsuitable for reliable remelting may instead be cracked into smaller useful molecules, although that process uses energy.
4. Common Mistakes
- Treating crude oil as a pure compound or giving a fraction one boiling point.
- Saying every crude-oil fraction is used only as a fuel.
- Confusing UV substitution of an alkane with addition to an alkene.
- Writing “clear” instead of “colourless” for bromine water.
- Saying fermentation produces ethanoic acid directly.
- Forgetting that ethanol oxidation forms water as well as ethanoic acid.
- Calling every vegetable oil polyunsaturated without evidence of more than one C=C bond.
- Leaving C=C in an addition-polymer repeat unit.
- Saying plastic recycling has no energy, cost or sorting trade-off.
- Adding esterification, C4 isomers or condensation polymers to a G3 Science answer.
5. Exam Tips
Name the starting family, state the reagent and condition, identify the bond or functional-group change, then name the product.
For an evaluation, write one supported benefit and one supported limitation before reaching a conclusion tied to the situation. For a structure, count carbon atoms first and check the general formula last.
Exam question 1: Explain an oxidation observationCore
Ethanol is warmed with acidified potassium manganate(VII). State the colour change and organic product.
Show Answer
The solution changes from purple to colourless. Ethanol is oxidised to ethanoic acid.
6. Worked Examples
Modelled example 1
Identify a C3 hydrocarbon
Problem
Study the worked solution
Interpret the test
Method
Infer that the hydrocarbon contains a C = C bond.Reason
At this course depth, an alkene undergoes addition with bromine and decolourises bromine water.Working
Positive bromine-water result → unsaturated hydrocarbon.Use the carbon range
Method
Name the C3 alkene propene.Reason
For n = 3, the alkene formula CₙH₂ₙ gives C₃H₆.Working
Propene, C₃H₆; bromine water changes from orange to colourless.
Guided practice 2
Read a structure before naming it
Problem
Use the bond and carbon count
Hints
Hint 1: functional group
Hint 2: formula check
View solution step by step
Identify and name
Method
Use the double bond and three-carbon chain.Reason
C=C gives the alkene ending; three carbons give prop-.Working
Alkene: propene.Check the formula
Method
Count atoms and compare with the general formula.Reason
For n = 3, CₙH₂ₙ gives six hydrogens.Working
C₃H₆.
Common misconception 3
Form an addition polymer
Learner claim
Track the bond and products
View solution step by step
Correct the bond change
Method
Open the C = C bond in each propene molecule.Reason
The released bonding capacity forms new C-C single bonds between monomer units.Working
Alkene double bonds become part of a single-bonded polymer backbone.Correct the product claim
Method
State that no small molecule is produced.Reason
All atoms from the alkene monomers remain in an addition polymer.Working
Propene → poly(propene); no water or other byproduct.
Examiner practice 4
Evaluate two plastic-waste routes
Examination question
Write a balanced comparison, not a slogan
View solution step by step
Describe physical recycling
1 markMethod
Sort clean poly(ethene), melt small pieces and form pellets.Reason
Keeping one polymer stream clean makes remelting practical.Working
sort → cut → melt → pelletsDescribe chemical recycling
1 markMethod
Crack suitable waste to form smaller molecules that may be used as fuel or chemical feedstock.Reason
This changes the polymer chemically rather than simply reshaping it.Working
plastic waste → (cracking) smaller useful moleculesCompare trade-offs
1 markMethod
State that physical recycling can use less processing but needs clean, sorted material; cracking can accept some difficult waste but uses energy.Reason
A supported comparison includes both benefit and limitation.Working
physical: simpler but needs clean sorting; chemical: handles difficult waste but uses energyReach a conditional choice
1 markMethod
Choose physical recycling for the clean single-polymer stream and consider cracking for otherwise unsuitable mixed waste.Reason
The evidence describes two different feedstocks, so one method need not suit both.Working
clean single polymer → physical; unsuitable mixed waste → consider chemical
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Credit a justified, conditional comparison rather than an unsupported claim that one route is always best.
Challenge 5
Use the fermentation equation
Equation-to-amount transfer
Read the mole ratio from coefficients
Hints
Hint 1: coefficients
Hint 2: ratio
View solution step by step
Extract the ratio
Method
Read the equation coefficients as 1:2:2.Reason
A balanced equation gives the stoichiometric mole relationship between substances.Working
glucose : ethanol : carbon dioxide = 1:2:2.Apply the stated amount
Method
Scale the ratio from one mole of glucose.Reason
The given glucose amount already matches the first coefficient, so no further scale factor is needed.Working
1 mol glucose forms 2 mol ethanol and 2 mol CO₂.
7. Mind Stretchers
Mind stretcher 1: Trace two transformationsExtension
Ethanol is left exposed to air and later forms ethanoic acid. Connect this change to the two homologous series and name the substance from air that causes the oxidation.
Show Answer
Ethanol belongs to the alcohol homologous series and is oxidised by oxygen in air. The product, ethanoic acid, belongs to the carboxylic-acid homologous series.
Mind stretcher 2: Evaluate bioethanol carefullyExtension
Why is “renewable” not enough to prove that bioethanol has lower climate impact?
Show Answer
Renewability describes replacement of the feedstock. Climate impact also depends on land use, fertiliser, processing energy, transport and how much carbon dioxide regrowth removes, so life-cycle data are needed.
Mind stretcher 3: Choose a plastic-waste routeExtension
A batch of clean poly(ethene) film and a batch of mixed, contaminated plastic waste are collected. Explain why the same recycling route may not suit both.
Show Answer
The clean poly(ethene) can be melted and formed into pellets by physical recycling. Mixed or contaminated waste may be difficult to sort and remelt into a reliable product, so chemical recycling by cracking may be considered. The final choice also depends on energy use, cost, emissions and the value of the products.
8. Quiz
Check 1: Check your understandingCore
Check C1–C3 compounds, their prescribed reactions, alcohol and ethanoic-acid chemistry, addition polymers and plastics evaluation.
Check your understanding