Higher Level Revision Notes

Leaving Cert Higher Level Chemistry

Leaving Cert Higher Level Chemistry Chapter 7: Organic Chemistry

You learn the main functional groups, the common reactions of organic compounds, and how to identify or make useful molecules.

What this page doesTurns a difficult chapter into clear notes, diagrams, and one simple interactive tool.

Student promisePlain English first, exam language second.

Curriculum

Irish Leaving Certificate (ILC)

Level

Higher Level

Subject

Chemistry

Chapter

Chapter 7 — Organic Chemistry

Simple English Summary

You learn the main functional groups, the common reactions of organic compounds, and how to identify or make useful molecules.

Teacher voice: Read the ideas first, then use the detailed notes and diagrams to lock in the exam wording.

What To Focus On

Recognise the major functional groups.
Follow addition, substitution, and elimination.
Understand polymer formation and chromatography.
Use simple organic reaction pathways.

DisclaimerThis publication is an independent educational resource developed by ExamsLogic and compiled for student revision. It is based on publicly available official curricula and is not endorsed by any examination board.

Open Simulators Jump To Notes

Quick Simpler Start

In one sentence

You learn the main functional groups, the common reactions of organic compounds, and how to identify or make useful molecules.

Exam habit

When you revise this chapter, ask yourself: "Can I explain this to a friend in one easy paragraph?" If yes, you are in a good place.

Interactive Simulator

This small tool gives you a quick visual check before you move deeper into the chapter notes.

Simulator

Functional Group Trainer

Pick a compound family and the tool gives the simple exam meaning.

Organic family

Choose a family to see the reaction idea.

1. Learning Objectives

Recognise and name the main organic functional groups at Higher Level.
Distinguish between tetrahedral carbon and planar carbon.
Draw and name structures of alcohols, chloroalkanes, aldehydes, ketones, carboxylic acids and esters.
Classify reactions as substitution, addition, elimination, esterification, hydrolysis and redox.
Explain HL mechanisms: free radical substitution and ionic addition.
Understand aromatic compounds, especially benzene.
Explain physical properties, boiling point trends and solubility.
Interpret GC, HPLC, mass spectrometry, IR and UV absorption at HL level.

2. Functional Groups: The Organic Chemistry Map

Family	Functional group	Example	Key property
Alcohol	-OH	Ethanol, C₂H₅OH	Can form hydrogen bonds; small alcohols are water-soluble.
Chloroalkane	C-Cl	Chloroethane	Often reacts by substitution.
Aldehyde	-CHO	Ethanal	Oxidised to carboxylic acid.
Ketone	C=O within chain	Propanone	Less easily oxidised than aldehydes.
Carboxylic acid	-COOH	Ethanoic acid	Weak acid; reacts with alcohols to form esters.
Ester	-COO-	Ethyl ethanoate	Fruity smell; formed by esterification.

Examiner Tip: The functional group controls most of the chemical behaviour. Once you identify it, the reaction pattern usually becomes much easier.

3. Tetrahedral Carbon and Planar Carbon HL

Tetrahedral Carbon

Carbon with four single bonds is tetrahedral. This is typical of alkanes, alcohols and chloroalkanes.

Examples: methane, ethanol, chloroethane

Planar Carbon

Carbon involved in a double bond is planar. This is found in alkenes, aldehydes, ketones, carboxylic acids and esters.

Examples: ethene, ethanal, propanone, ethanoic acid

Type of carbon	Bonding	Shape around carbon	Typical families
Tetrahedral carbon	Four single bonds	Tetrahedral	Alkanes, alcohols, chloroalkanes
Planar carbon	Contains C=C or C=O	Trigonal planar around that carbon	Alkenes, aldehydes, ketones, acids, esters

Examiner Trap: Not all carbon atoms in a molecule have the same geometry. One carbon may be tetrahedral while another is planar.

4. Nomenclature and Structural Formulae

Carbon number	Stem	Alkane	Alcohol	Carboxylic acid
1	meth-	methane	methanol	methanoic acid
2	eth-	ethane	ethanol	ethanoic acid
3	prop-	propane	propanol	propanoic acid
4	but-	butane	butanol	butanoic acid

Worked Example: Name CH₃CH₂COOH.
Step 1: Count carbons = 3 → prop-.
Step 2: Functional group = -COOH → carboxylic acid.
Answer: propanoic acid.

Common Mistake: The carbon in the -COOH group counts as part of the main carbon chain.

5. Physical Properties and Solubility

Family	Main intermolecular force	Boiling point trend	Water solubility
Alcohols	Hydrogen bonding	Higher than similar alkanes	Small members quite soluble
Chloroalkanes	Dipole-dipole + dispersion	Higher than corresponding alkanes	Poor to low solubility
Aldehydes / ketones	Dipole-dipole	Moderate	Small members reasonably soluble
Carboxylic acids	Strong hydrogen bonding	High	Lower members soluble
Esters	Dipole-dipole	Lower than acids	Usually limited solubility

Rule of thumb: As carbon chain length increases, water solubility usually decreases because the non-polar hydrocarbon part becomes more dominant.

6. Aromatic Compounds HL

Aromatic compounds contain a benzene ring. Benzene has formula C₆H₆ and is often shown as a hexagon with a circle inside, representing delocalised electrons.

HL Note: Benzene is more stable than expected because its electrons are delocalised. That is why benzene does not behave like a normal alkene.

Compound	Formula	Comment
Benzene	C₆H₆	Parent aromatic compound
Methylbenzene	C₆H₅CH₃	Also called toluene
Ethylbenzene	C₆H₅C₂H₅	Aromatic side-chain compound

7. Reaction Types

Reaction type	Meaning	Common example
Substitution	One atom or group replaces another.	Methane reacts with chlorine in UV light.
Addition	Atoms add across a double bond.	Ethene reacts with bromine.
Elimination	A small molecule is removed, often forming a double bond.	Ethanol → ethene + water.
Redox	Oxidation and reduction occur.	Alcohol oxidised to aldehyde or acid.
Esterification	Acid + alcohol forms ester + water.	Ethanoic acid + ethanol.
Hydrolysis	Bond broken by water.	Ester hydrolysis.

8. Chloroalkanes, Alcohols, Aldehydes, Ketones, Acids and Esters

Chloroalkanes

Chloroalkanes are important because they undergo substitution reactions and can be made from alkanes or alkenes in different ways.

Alcohols

Alcohols contain the -OH group and may be oxidised or dehydrated.

Ethanol → Ethanal → Ethanoic acid

Aldehydes and Ketones

Aldehydes contain the -CHO group and are more easily oxidised than ketones. Ketones contain the carbonyl group inside the chain.

Carboxylic acids

Carboxylic acids are weak acids and react with alcohols to form esters.

Esters

Esters are usually pleasant-smelling compounds formed by esterification.

9. Important Organic Reactions

Addition

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

Alkenes decolourise bromine water, which is a key test for unsaturation.

Free radical substitution HL

CH₄ + Cl₂ → CH₃Cl + HCl

This occurs in UV light and proceeds by initiation, propagation and termination steps.

Ionic addition HL

Alkenes undergo ionic addition when electrophiles add across the C=C bond.

Elimination

C₂H₅OH → C₂H₄ + H₂O

Removing water from an alcohol forms an alkene.

Esterification

Ethanoic acid + ethanol ⇌ ethyl ethanoate + water

Base hydrolysis of esters

When an ester reacts with aqueous base, the products are an alcohol and the salt of a carboxylic acid.

Oxidation and reduction

Primary alcohols may be oxidised to aldehydes and then to acids. Reduction reverses the direction in many simple organic schemes.

Examiner Tip: In organic schemes, always track the functional group change first. That usually tells you the reaction type immediately.

10. Basic Organic Synthesis and PVC

Higher Level questions often connect several steps together in short synthesis routes.

Start	Step	Product
Ethene	Addition of H₂O	Ethanol
Ethanol	Oxidation	Ethanal / ethanoic acid
Ethanoic acid + ethanol	Esterification	Ethyl ethanoate

PVC from ethene

Ethene → chloroethene (vinyl chloride) → poly(chloroethene) / PVC

This is a required Higher Level industrial-organic link.

11. Organic Natural Products and Extraction Techniques

Organic natural products may be extracted from plants and other natural sources.

Technique	Use	Idea
Solvent extraction	Separating desired organic substances	Uses difference in solubility
Steam distillation	Extracting essential oils	Useful for temperature-sensitive compounds

Example: Steam distillation can be used to isolate essential oils from plant material without heating them to very high temperatures.

12. Instrumentation HL

Chromatography

Method	Main use
GC	Separating volatile organic compounds
HPLC	Separating less volatile compounds

Mass Spectrometry

Gives molecular mass and fragmentation pattern information.

IR Spectrometry

Used to identify functional groups from characteristic bond absorptions.

UV Absorption Spectrometry

Useful for compounds that absorb ultraviolet light, especially in conjugated systems.

HL Note: At this level, the exam focus is usually what each technique is for, not deep instrument engineering detail.

13. Quick Comparison Table

Family	Main reaction tendency	Useful exam clue
Alkenes	Addition	Decolourise bromine water
Alkanes	Substitution	Need UV light with chlorine
Alcohols	Oxidation / elimination	-OH group
Aldehydes	Oxidation	-CHO group
Carboxylic acids	Esterification	-COOH group
Esters	Hydrolysis	Fruity smell

14. Examiner Secrets, Mistakes and Traps

Secret 1: Functional group first, name second, reaction type third.

Secret 2: If a question mentions C=C, think addition. If it mentions -COOH plus alcohol, think esterification.

Common Mistake: Mixing up aldehydes and ketones. Aldehydes have -CHO at the end of the chain.

Common Mistake: Forgetting that longer hydrocarbon chains usually lower water solubility.

Examiner Trap: Benzene is not treated like an ordinary alkene. Delocalisation makes it behave differently.

15. Last-Minute Revision Sheet

Tetrahedral carbon = four single bonds.
Planar carbon = carbon in C=C or C=O environment.
Alcohols, chloroalkanes, aldehydes, ketones, acids and esters are the key HL families.
Small polar molecules are more water-soluble than larger ones.
Alkenes undergo addition; alkanes undergo substitution.
Esterification forms ester + water.
Base hydrolysis breaks esters into alcohol + carboxylate salt.
PVC comes from chloroethene.
Steam distillation and solvent extraction are key natural-product methods.
GC, HPLC, MS, IR and UV must be recognised by purpose.

16. Self-Assessment Checklist

I can distinguish tetrahedral carbon from planar carbon.
I can identify and name the main functional groups.
I can explain addition, substitution, elimination, esterification and hydrolysis.
I can compare aldehydes, ketones, acids and esters.
I can explain PVC formation from ethene.
I can describe steam distillation and solvent extraction.
I can recognise the purpose of GC, HPLC, mass spectrometry, IR and UV.