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Curriculum: Irish Leaving Certificate (ILC)
Level: Ordinary Level (OL)
Subject: Chemistry
Chapter 7: Organic Chemistry
Version: Complete Chapter File
Prepared By: ExamsLogic Academic Team
1. Learning Objectives
Explain what organic chemistry is and why carbon forms so many compounds.
Recognise and name common organic families at Ordinary Level.
Identify functional groups in alkanes, alkenes, alkynes, alcohols, chloroalkanes, aldehydes, ketones, carboxylic acids and esters.
Describe structures, simple physical properties and common uses of the required compounds.
Classify organic reactions as substitution, addition, polymerisation, elimination, redox and combustion.
Describe basic syntheses such as making PVC from ethene and forming esters from acids and alcohols.
Outline extraction techniques for natural products.
Explain chromatography, TLC, GC, HPLC and mass spectrometry at Ordinary Level.
Big idea: In Chapter 7, the fastest route to the answer is usually: spot the functional group → identify the family → predict the properties and reaction type.
Organic chemistry is the study of carbon compounds. Carbon is special because it can form four covalent bonds and can join to other carbon atoms to make chains, branches and rings.
Organic chemistry = chemistry of carbon compounds
Methane, CH₄, is the simplest example showing carbon’s valency of 4.
Examiner Tip: If asked why carbon forms many compounds, mention both four covalent bonds and carbon-carbon bonding.
4. Functional Groups
A functional group is the atom or group of atoms in an organic molecule that gives that molecule its characteristic chemical reactions.
Functional group = part of the molecule responsible for characteristic reactions
Family
Functional Group / Feature
Example
Name
Alkane
C–C single bonds only
CH₄
Methane
Alkene
C=C
C₂H₄
Ethene
Alkyne
C≡C
C₂H₂
Ethyne
Alcohol
–OH
C₂H₅OH
Ethanol
Chloroalkane
–Cl
CH₃Cl
Chloromethane
Aldehyde
–CHO
CH₃CHO
Ethanal
Ketone
C=O within chain
CH₃COCH₃
Propanone
Carboxylic acid
–COOH
CH₃COOH
Ethanoic acid
Ester
–COO–
CH₃COOCH₂CH₃
Ethyl ethanoate
Aromatic compound
Benzene ring
C₆H₆
Benzene
Examiner Trap: Do not confuse –CHO, –COOH and –COO–. Those three are one of the most common organic-chemistry mix-ups.
5. Naming Rules and Structural Formulae
Organic names are built from the carbon-chain root plus the family ending.
Carbon-chain roots
Carbon atoms
Root
1
meth-
2
eth-
3
prop-
4
but-
5
pent-
Common endings
Ending
Family
-ane
Alkane
-ene
Alkene
-yne
Alkyne
-ol
Alcohol
-al
Aldehyde
-one
Ketone
-oic acid
Carboxylic acid
A structural formula makes the functional group much easier to spot.
Worked example: CH₃CH₂OH has 2 carbon atoms, so the root is eth-. It contains –OH, so the ending is -ol. Name = ethanol.
Common Mistake: The correct spelling is ethanol, not “ethaneol”.
6. Tetrahedral Carbon: Alkanes, Alcohols and Chloroalkanes
Alkanes
Alkanes are saturated hydrocarbons. They contain carbon-carbon single bonds only and follow the general formula CnH2n+2.
Alkane general formula = CₙH₂ₙ₊₂
Name
Formula
Structural formula
Physical state at room temperature
Methane
CH₄
CH₄
Gas
Ethane
C₂H₆
CH₃–CH₃
Gas
Propane
C₃H₈
CH₃–CH₂–CH₃
Gas
Butane
C₄H₁₀
CH₃–CH₂–CH₂–CH₃
Gas / easily liquefied
Property reminder: Simple organic compounds are usually insoluble in water but dissolve better in non-polar organic solvents.
Alcohols
Alcohols contain the –OH functional group.
Alcohol functional group = –OH
Alcohol
Formula
Structural formula
Use
Methanol
CH₃OH
CH₃–OH
Fuel / solvent
Ethanol
C₂H₅OH
CH₃–CH₂–OH
Solvent / fuel / drinks chemistry
Propanol
C₃H₇OH
CH₃–CH₂–CH₂–OH
Solvent
Butanol
C₄H₉OH
CH₃–CH₂–CH₂–CH₂–OH
Solvent / fuel interest
Chloroalkanes
Chloroalkanes are formed when a hydrogen atom in an alkane is replaced by chlorine.
Name
Formula
Structural formula
Chloromethane
CH₃Cl
CH₃–Cl
Chloroethane
C₂H₅Cl
CH₃–CH₂–Cl
7. Planar Carbon: Alkenes, Aldehydes, Ketones, Carboxylic Acids and Esters
Alkenes
Alkenes are unsaturated hydrocarbons containing a C=C double bond. Their general formula is CnH2n.
Alkene general formula = CₙH₂ₙ
Name
Formula
Structural formula
Ethene
C₂H₄
CH₂=CH₂
Propene
C₃H₆
CH₃–CH=CH₂
Butene
C₄H₈
CH₃–CH₂–CH=CH₂
Aldehydes
Aldehydes contain the –CHO group at the end of the chain.
Name
Formula
Structural formula
Methanal
HCHO
H–CHO
Ethanal
CH₃CHO
CH₃–CHO
Propanal
CH₃CH₂CHO
CH₃–CH₂–CHO
Ketones
Ketones contain a carbonyl group C=O within the chain.
Name
Formula
Structural formula
Propanone
CH₃COCH₃
CH₃–CO–CH₃
Butanone
CH₃COCH₂CH₃
CH₃–CO–CH₂–CH₃
Carboxylic Acids
Carboxylic acids contain the –COOH group and are weak acids.
Name
Formula
Common note
Methanoic acid
HCOOH
Found in ant stings
Ethanoic acid
CH₃COOH
Main acid in vinegar
Propanoic acid
CH₃CH₂COOH
3-carbon acid
Esters
Esters contain the –COO– group and often have pleasant fruity smells.
Carboxylic acid + Alcohol ⇌ Ester + Water
Name
Formula
Typical use
Methyl ethanoate
CH₃COOCH₃
Solvent / fragrance
Ethyl ethanoate
CH₃COOCH₂CH₃
Perfumes / flavourings
Examiner Trap: Aldehydes and ketones both contain C=O, but only aldehydes have –CHO at the end of the chain.
8. Aromatic Compounds
Aromatic compounds contain a benzene ring. The three key examples at this level are benzene, methylbenzene and ethylbenzene.
Compound
Formula
Description / note
Benzene
C₆H₆
Parent aromatic hydrocarbon
Methylbenzene
C₆H₅CH₃
Also called toluene
Ethylbenzene
C₆H₅CH₂CH₃
Benzene ring + ethyl side chain
At OL level, focus on recognition, simple naming and physical properties of the required aromatic examples.
Property note: Aromatic hydrocarbons are usually non-polar, insoluble in water, and better dissolved in organic solvents.
9. Organic Reaction Types
Reaction Type
Simple Meaning
Typical Example
Substitution
One atom or group is replaced by another.
Methane + chlorine in UV light.
Addition
Atoms add across a double bond.
Ethene + bromine.
Polymerisation
Many small molecules join to form a large molecule.
Ethene → poly(ethene).
Elimination
Atoms are removed, often forming a double bond.
Ethanol → ethene + water.
Redox
Oxidation or reduction occurs.
Ethanol oxidised to ethanoic acid.
Combustion
Burning in oxygen.
Hydrocarbon + oxygen → carbon dioxide + water.
Substitution
CH₄ + Cl₂ → CH₃Cl + HCl
One hydrogen atom in methane is replaced by chlorine. This reaction needs UV light.
Addition and Bromine Water
C₂H₄ + Br₂ → C₂H₄Br₂
In an addition reaction, atoms add across the double bond of an alkene.
Substance
Bromine water result
Meaning
Alkane
Stays orange/brown
No C=C double bond
Alkene
Orange/brown → colourless
C=C present
Elimination
C₂H₅OH → C₂H₄ + H₂O
Removing water from ethanol forms ethene. This is dehydration, which is an elimination reaction.
Redox and Combustion
Ethanol + oxygen → Ethanoic acid + water
Hydrocarbon + oxygen → carbon dioxide + water
Acid Reactions of Carboxylic Acids
Carboxylic acids behave as weak acids. They react with bases to form salts and water, and with alcohols to form esters.
10. Polymerisation and PVC from Ethene
Polymerisation is the joining of many small molecules called monomers to make a large molecule called a polymer.
n(CH₂=CH₂) → [–CH₂–CH₂–]ₙ
Ethene can first be changed into chloroethene (vinyl chloride), and chloroethene can polymerise to form PVC, poly(chloroethene).
A syllabus-friendly route: ethene → chloroethene → PVC.
Examiner Tip: PVC is not made directly from ethene in one step in the way the syllabus usually phrases it. The important named monomer is chloroethene.
11. Organic Natural Products and Extraction Techniques
Many useful organic compounds come from plants. Two common extraction techniques at this level are solvent extraction and steam distillation.
Solvent extraction
A suitable solvent dissolves the desired organic compound.
The extract is separated from the solid plant material.
The solvent can then be evaporated off.
Steam distillation
Used for heat-sensitive fragrant oils.
Steam carries the volatile oil over.
The vapour condenses and the oil is separated.
Examples: perfume oils, flavouring compounds and plant fragrances are often obtained using extraction techniques.
11A. Extra Skills: Alcohols, Indicators and Purification
The syllabus-linked wording also expects a few common laboratory and naming ideas to be understood clearly.
Primary and secondary alcohols
Type
Simple meaning
Example
Primary alcohol
The carbon with the -OH group is attached to only one other carbon.
Ethanol
Secondary alcohol
The carbon with the -OH group is attached to two other carbons.
Propan-2-ol
Indicators often used in chemistry tests
Indicator
What it is useful for
Phenolphthalein
Common in acid-base titrations; colourless in acid and pink in alkali.
Methyl orange
Common in acid-base titrations; red in acid and yellow in alkali.
Recrystallisation and melting point
Recrystallisation is a way to purify a solid organic product. The solid is dissolved in a suitable hot solvent and then crystals form again as the solution cools.
Recrystallisation helps separate a purer solid from impurities.
Pure solids usually have a sharp melting point.
Examiner Tip: If a solid melts over a wide range, it is often impure. A pure substance melts sharply.
12. Chromatography
Chromatography separates a mixture because different substances move at different speeds between a stationary phase and a mobile phase.
Paper / thin-layer ideas
A small spot of mixture is placed near the bottom of the stationary phase.
The solvent rises and carries the substances upward.
Different substances travel different distances.
TLC — Thin-Layer Chromatography
In TLC, the stationary phase is a thin coating on a plate rather than paper. It usually gives sharper separations.
More strongly retained substances move less; more soluble substances move further with the solvent.
Uses: inks, dyes, plant pigments, food colourings and forensic samples can all be separated by chromatography.
13. Instrumentation: GC, HPLC and Mass Spectrometry
Technique
Simple purpose
Typical use
GC (Gas Chromatography)
Separates volatile substances
fuel mixtures, fragrances, solvents
HPLC
Separates substances in liquid mixtures
drugs, food components, biological samples
Mass spectrometry
Gives information about particle masses
identification of unknown compounds
GC
Useful for volatile organic mixtures. Different components leave at different times.
HPLC
Useful for dissolved samples where gas chromatography would not suit the sample.
Mass spectrometry
Helps identify compounds by their mass patterns.
Forensic and analytical uses: checking drugs, explosives residues, fuel samples, perfumes, food flavourings and unknown organic liquids.
14. Worked Examples
Example 1: CH₃COOH contains the functional group –COOH, so it belongs to the carboxylic acid family and is called ethanoic acid.
Example 2: CH₂=CH₂ contains a C=C bond, so it is an alkene called ethene. It decolourises bromine water.
Example 3: CH₃CH₂OH contains –OH, so it is an alcohol called ethanol. It can be oxidised to ethanoic acid.
Example 4: Ethanol can lose water to form ethene. This is elimination because atoms are removed and a double bond is formed.
15. Examiner Secrets, Mistakes and Traps
Examiner Secret: Organic questions often look hard only because of the names. If you identify the functional group first, the question usually becomes much easier.
Common Mistake: Mixing up ethanol (CH₃CH₂OH) and ethanoic acid (CH₃COOH).
Common Mistake: Forgetting that alkenes decolourise bromine water but alkanes do not.
Examiner Trap: PVC is linked to chloroethene, not just any chlorine-containing compound.
Examiner Trap: Esters and carboxylic acids both contain oxygen, but only carboxylic acids have –COOH.
16. Final Revision Sheet
Family
Must-know feature
Must-know reaction / test
Alkane
C–C single bonds
Substitution with chlorine in UV light
Alkene
C=C
Decolourises bromine water
Alkyne
C≡C
Unsaturated hydrocarbon
Alcohol
–OH
Oxidation / elimination
Chloroalkane
–Cl
Substitution-type compound
Aldehyde
–CHO
Can be oxidised further
Ketone
C=O inside chain
Recognise by structure
Carboxylic acid
–COOH
Weak acid; forms esters
Ester
–COO–
Fruity smell
Aromatic compound
Benzene ring
Recognise benzene, methylbenzene, ethylbenzene
17. Self-Assessment Checklist
I can identify the main organic functional groups.
I can name simple organic compounds using root + ending.
I can tell the difference between alkane, alkene and alkyne.
I can identify alcohols, aldehydes, ketones, acids and esters from structure.
I can explain substitution, addition, elimination, polymerisation and redox at OL level.
I know the bromine water test for unsaturation.
I know how esters form and why they are important.
I can outline solvent extraction and steam distillation.
I know the basic purpose of TLC, GC, HPLC and mass spectrometry.
18. Mixed Exam Practice Questions
State what is meant by a functional group.
Name the family and functional group in CH₃CH₂OH.
What observation shows that a compound contains a carbon-carbon double bond?
Write an equation showing methane reacting with chlorine in UV light.
Explain what is meant by elimination using ethanol as an example.
Name the compounds CH₃CHO, CH₃COCH₃ and CH₃COOH.
Write a word equation for ester formation.
What is the monomer used to form PVC?
State one difference between solvent extraction and steam distillation.
Give one use of chromatography in analysis or forensics.
Mark Scheme
1. Atom/group responsible for characteristic organic reactions.
2. Alcohol, –OH.
3. Bromine water decolourises from orange/brown to colourless.
4. CH₄ + Cl₂ → CH₃Cl + HCl.
5. Removal of water from ethanol to form ethene / double bond formed.
6. Ethanal, propanone, ethanoic acid.
7. Carboxylic acid + alcohol ⇌ ester + water.
8. Chloroethene.
9. Solvent extraction uses a solvent; steam distillation uses steam for volatile oils.
10. Any valid example: inks, drugs, explosives residue, perfumes, food dyes, unknown mixtures.