Learning Objectives
9 objectivesBy the end of this note, you should be able to:
- Distinguish between the terms hazard and risk for organic substances.
- Explain why risk assessments are needed for hazardous organic materials.
- Suggest practical methods to reduce risk in organic procedures.
- Define homologous series and functional group with general features.
- Apply IUPAC nomenclature to compounds containing up to ten carbons.
- Draw organic compounds using structural, displayed and skeletal formulae.
- Classify reactions as addition, substitution, oxidation, reduction or polymerisation.
- Distinguish homolytic from heterolytic bond breaking by electron movement.
- Define free radical and electrophile in terms of electronic behaviour.
Hazard, Risk and Risk Assessment
A hazard describes the intrinsic potential of a substance to cause harm, whereas risk describes the chance of that harm actually occurring during use.
Risk depends on hazard, quantity, exposure conditions and procedure. A highly flammable solvent used in a fume cupboard at small scale carries lower risk than the same solvent used in a large open beaker near a flame.
Risk assessments are essential because organic compounds are frequently flammable, toxic, irritant, corrosive, carcinogenic or harmful to the environment. The assessment identifies hazards, evaluates likelihood of harm, and specifies controls.
| GHS pictogram | Hazard meaning | Typical organic example |
|---|---|---|
| Flame | Flammable | ethanol, hexane, propanone |
| Skull and crossbones | Acute toxicity | methanol |
| Corrosion | Corrosive | concentrated ethanoic acid |
| Exclamation mark | Irritant or harmful | many halogenoalkanes |
| Health hazard | Carcinogen, mutagen | benzene |
| Flame over circle | Oxidising | concentrated nitric acid |
| Environment | Aquatic toxicity | chlorinated solvents |
Three exam-recognised methods reduce risk:
- Working on a smaller scale, lowering the quantity of hazardous material.
- Taking precautions specific to the hazard, such as fume cupboards for volatile or toxic vapours, gloves and goggles for corrosives, and removing ignition sources for flammables.
- Using an alternative method or substance with lower hazard, such as heating under reflux instead of an open flame, or replacing a toxic reagent.
MisconceptionHazard and risk are not the same. A substance keeps its hazard regardless of how it is used, but the risk changes with quantity, conditions, and precautions taken.
Exam TipState both the hazard property and the conditions affecting risk in your answer.
Homologous Series and Functional Groups
A homologous series is a family of organic compounds sharing the same general formula and functional group, with successive members differing by a –CH₂– unit.
Members of a homologous series share the same chemical reactions because the functional group dictates reactivity. Physical properties such as boiling point change gradually with chain length.
A functional group is the atom or group of atoms responsible for the characteristic chemical behaviour of a molecule.
| Functional group | Suffix or prefix | Condensed example | General formula of series |
|---|---|---|---|
| Alkane (C–C) | -ane | CH₃CH₃ | CₙH₂ₙ₊₂ |
| Alkene (C=C) | -ene | CH₂=CH₂ | CₙH₂ₙ |
| Halogenoalkane (C–X) | halo- prefix | CH₃Cl | CₙH₂ₙ₊₁X |
| Alcohol (–OH) | -ol | CH₃CH₂OH | CₙH₂ₙ₊₁OH |
| Aldehyde (–CHO) | -al | CH₃CHO | CₙH₂ₙO |
| Ketone (>C=O) | -one | CH₃COCH₃ | CₙH₂ₙO |
| Carboxylic acid (–COOH) | -oic acid | CH₃COOH | CₙH₂ₙO₂ |
| Ester (–COO–) | -oate | CH₃COOCH₃ | — |
| Amine (–NH₂) | -amine | CH₃NH₂ | CₙH₂ₙ₊₁NH₂ |
| Nitrile (–CN) | -nitrile | CH₃CN | — |
IUPAC Nomenclature and Drawing Formulae
IUPAC nomenclature assigns systematic names using prefixes for chain length and suffixes for the principal functional group, ensuring every organic compound has a unique identifier.
The stem prefix indicates the number of carbons in the longest chain.
| Carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| Stem | meth | eth | prop | but | pent | hex | hept | oct | non | dec |
Naming proceeds by these steps:
- Identify the longest carbon chain containing the principal functional group; this becomes the parent name.
- Number the chain to give the principal functional group the lowest possible locant.
- Identify side groups (alkyl branches such as methyl, ethyl; halogens as fluoro, chloro, bromo, iodo).
- Where there is a choice, number to give the lowest set of locants to all substituents.
- Cite substituents alphabetically with their locants, using multiplying prefixes (di, tri, tetra) for repeats.
- Separate numbers with commas, and numbers from letters with hyphens.
Three formula representations are examinable.
| Formula type | What it shows | Example for butane |
|---|---|---|
| Structural | Atom arrangement in a condensed line | CH₃CH₂CH₂CH₃ |
| Displayed | Every atom and every bond drawn explicitly | drawn formula with all C–H and C–C bonds visible |
| Skeletal | Lines for C–C bonds; C and H on C not shown | zigzag line of three segments |
In a skeletal formula, each vertex and line-end represents a carbon atom, and hydrogen atoms attached to carbon are not drawn. Heteroatoms (O, N, halogens) and their attached hydrogens are always shown.
Examiner InsightNumbering choice is a frequent mark loss point. Compare both numbering directions and pick the set giving the lowest locants overall for the principal group, then substituents.
Exam TipWrite the locant set both ways and choose the lower one before naming.
Classifying Organic Reactions
Organic reactions are classified into five main types based on the change in connectivity or oxidation state of the carbon framework: addition, substitution, oxidation, reduction and polymerisation.
| Reaction type | Definition | Example |
|---|---|---|
| Addition | Two reactant molecules combine to form one product, often by breaking a multiple bond | CH₂=CH₂ + Br₂ → CH₂BrCH₂Br |
| Substitution | One atom or group is replaced by another atom or group | CH₄ + Cl₂ → CH₃Cl + HCl |
| Oxidation | Gain of oxygen, loss of hydrogen, or increase in oxidation number | CH₃CH₂OH → CH₃CHO (with [O]) |
| Reduction | Loss of oxygen, gain of hydrogen, or decrease in oxidation number | CH₃CHO → CH₃CH₂OH (with [H]) |
| Polymerisation | Many small monomer molecules join to form a long-chain polymer | n CH₂=CH₂ → (–CH₂–CH₂–)ₙ |
The symbols [O] and [H] denote a generic oxidising or reducing agent in organic equations where the exact reagent is not specified.
Homolytic and Heterolytic Bond Breaking
Covalent bond breaking is classified as either homolytic fission, which produces neutral radicals, or heterolytic fission, which produces ions, depending on how the shared electron pair is distributed.
| Feature | Homolytic fission | Heterolytic fission |
|---|---|---|
| Electron split | Each atom takes one electron | Both electrons go to one atom |
| Products | Two free radicals (neutral) | A cation and an anion |
| Curly arrow | Half-arrow / fish-hook (single-headed) | Full curly arrow (double-headed) |
| Typical example | Cl₂ → 2Cl• (UV light) | H–Br → H⁺ + Br⁻ (in solution) |
| Conditions favoured | Non-polar bonds, gas phase, UV | Polar bonds, polar solvent |
A half-headed curly arrow (fish-hook) shows the movement of a single electron in homolytic fission. A full-headed curly arrow shows the movement of an electron pair in heterolytic fission.
Examiner InsightCurly arrow type is examined strictly. Half-arrows are used only for radical processes, full arrows only for ionic mechanisms. Mixing them up loses the mechanism mark.
Exam TipCheck every curly arrow you draw matches the type of fission required.
Free Radicals and Electrophiles
A free radical is a species with an unpaired electron, whereas an electrophile is an electron-pair acceptor that is attracted to regions of high electron density.
A free radical is highly reactive because the unpaired electron seeks to pair with another electron. Free radicals form by homolytic fission and are typically uncharged.
An electrophile is electron-deficient. Electrophiles include positively charged ions and neutral molecules with a δ⁺ atom that can accept a lone pair.
| Term | Definition | Examples |
|---|---|---|
| Free radical | Species with an unpaired electron | Cl•, CH₃•, •OH |
| Electrophile | Electron-pair acceptor; attracted to electron-rich regions | H⁺, NO₂⁺, Br⁺, the δ⁺ carbon in HBr |
QUICK RECAP
Key Points
- Hazard is intrinsic; risk depends on use, quantity and precautions.
- Risk assessments identify hazards and specify controls before practical work.
- Reduce risk by smaller scale, hazard-specific precautions, or safer alternatives.
- Homologous series share general formula, functional group, and –CH₂– increment.
- Functional group dictates the chemical reactions of the molecule.
- IUPAC stems run meth, eth, prop, but, pent, hex, hept, oct, non, dec.
- Number the chain to give the principal functional group the lowest locant.
- Substituents are listed alphabetically using locants and multiplying prefixes.
- Structural shows arrangement; displayed shows all bonds; skeletal shows lines only.
- Five reaction types: addition, substitution, oxidation, reduction, polymerisation.
- Homolytic fission: each atom takes one electron, forming radicals.
- Heterolytic fission: one atom takes both electrons, forming ions.
- Half-headed curly arrows for radicals; full curly arrows for ionic mechanisms.
- A free radical has an unpaired electron and is highly reactive.
- An electrophile is an electron-pair acceptor attracted to electron-rich sites.
CAN I…? PROGRESS CHECK
Self-Assessment
- Can I distinguish hazard from risk using a worked example?
- Can I list three exam-recognised methods to reduce risk in organic procedures?
- Can I define homologous series and list four characteristic features?
- Can I recognise and name the major functional groups by suffix or prefix?
- Can I apply IUPAC rules to name and draw compounds up to C₁₀?
- Can I convert confidently between structural, displayed and skeletal formulae?
- Can I classify a given reaction as addition, substitution, oxidation, reduction or polymerisation?
- Can I describe homolytic and heterolytic fission with the correct curly-arrow type?
- Can I define free radical and electrophile and give an example of each?