Understanding Organic chemistry: Introduction to Alkenes: Edexcel IGCSE Chemistry Explained

Organic chemistry is the study of carbon compounds, and this first block of four topics takes you from fundamental definitions through to the reactions that distinguish saturated hydrocarbons from unsaturated ones.

The organic chemistry section of the Edexcel IGCSE Chemistry specification builds logically. You start with the language of organic chemistry, including what homologous series, functional groups and isomers actually mean. From there you move into crude oil (the raw material), alkanes (saturated hydrocarbons) and alkenes (unsaturated hydrocarbons). This progression is deliberate: each topic depends on the one before it.

These edexcel igcse chemistry revision notes cover the organic chemistry: introduction to alkenes edexcel igcse material with worked examples and the detail the igcse 4ch1 exam rewards.

Introduction to organic chemistry

A hydrocarbon is a compound containing hydrogen and carbon only. Every organic molecule you meet in this specification is either a hydrocarbon or a derivative of one.

Organic molecules can be represented in several ways:

A homologous series is a family of organic compounds with the same functional group, the same general formula, and similar chemical properties. Members differ by CH₂. A functional group is the atom or group of atoms responsible for the characteristic reactions of that homologous series.

Isomers are compounds with the same molecular formula but different structural formulae. For example, butane (CH₃CH₂CH₂CH₃) and methylpropane (CH₃CH(CH₃)CH₃) are both C₄H₁₀ but have different arrangements of atoms.

IUPAC naming rules: The specification expects names for compounds up to six carbon atoms. The root tells you the chain length (meth- = 1, eth- = 2, prop- = 3, but- = 4, pent- = 5, hex- = 6). The suffix tells you the homologous series (-ane for alkanes, -ene for alkenes, -ol for alcohols, -oic acid for carboxylic acids).

Reaction types: At this level, organic reactions are classified as substitution (one atom or group replaces another), addition (atoms add across a double bond), or combustion (burning in oxygen).

Crude oil

Crude oil is a mixture of hydrocarbons, mostly alkanes. It is separated into useful fractions by fractional distillation. The oil is heated until it vaporises, then the vapour rises up a fractionating column. The column is hot at the bottom and cool at the top. Hydrocarbons with high boiling points condense near the bottom; those with low boiling points rise further before condensing.

The main fractions, from top to bottom of the column:

The trends going down the column: boiling point increases, viscosity increases, colour darkens (from pale yellow to black), and the molecules get larger.

Combustion of hydrocarbons:

• Complete combustion (excess oxygen): produces carbon dioxide and water. Example: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
• Incomplete combustion (limited oxygen): produces carbon monoxide and/or carbon (soot) and water. Carbon monoxide is poisonous because it binds to the oxygen-carrying molecules in blood, reducing the blood's capacity to transport oxygen.

Pollutants from burning fuels:

• Oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperatures inside car engines.
• Sulfur dioxide forms when sulfur impurities in the fuel burn.
• Both sulfur dioxide and oxides of nitrogen dissolve in rainwater to form acid rain, which damages buildings, vegetation and aquatic life.

Cracking: Long-chain alkanes from crude oil are in less demand than short-chain alkanes and alkenes. Catalytic cracking breaks long-chain alkanes into shorter, more useful molecules. Conditions: a catalyst of silica or alumina at 600 to 700 degrees C. The products are shorter-chain alkanes and alkenes.

Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Cracking is necessary because the demand for short-chain hydrocarbons (petrol, alkenes for making polymers) is greater than the supply from fractional distillation alone.

Alkanes

Alkanes are saturated hydrocarbons: every carbon-carbon bond is a single bond. Their general formula is C_nH_2n+2.

Alkanes are saturated because they contain only single bonds. All bonding positions are occupied by hydrogen atoms; there are no double bonds, so no more atoms can be added without removing existing ones.

Reactions of alkanes with halogens: In the presence of ultraviolet (UV) radiation, alkanes undergo substitution reactions with halogens. A hydrogen atom is replaced by a halogen atom:

CH₄ + Cl₂ → CH₃Cl + HCl (in UV light)

Only mono-substitution is required at this level. No mechanism is needed.

Alkenes

Alkenes contain the functional group C=C (a carbon-carbon double bond). Their general formula is C_nH_2n.

Alkenes are unsaturated because they contain a double bond. One of those bonds can break to allow additional atoms to add across it, which is why alkenes undergo addition reactions.

Reaction with bromine: Alkenes react with bromine to form dibromoalkanes. This is an addition reaction:

C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)

The bromine water test distinguishes alkenes from alkanes. Shake the hydrocarbon with bromine water (which is orange-brown). If the hydrocarbon is an alkene, the bromine water decolourises (turns from orange-brown to colourless) because bromine adds across the double bond. If it is an alkane, the bromine water stays orange-brown because no reaction occurs.

Pay particular attention to the vocabulary of combustion. Complete combustion requires a plentiful supply of oxygen and produces only carbon dioxide and water. Incomplete combustion occurs when oxygen is limited and produces carbon monoxide (toxic), carbon/soot (visible as black particles), and water. The exam frequently tests whether you can distinguish between the two and explain why incomplete combustion is dangerous. Carbon monoxide binds to the molecules in blood that normally carry oxygen, reducing the blood's oxygen-carrying capacity. You do not need to name haemoglobin at this level, but you do need to describe the mechanism clearly.

Cracking questions often ask you to balance the equation and identify the products. A useful check: count carbons and hydrogens on both sides. If the products include an alkene (general formula C_nH_2n), you can test it with bromine water to confirm it is unsaturated.

Common mistakes

• Confusing structural and displayed formulae. A structural formula groups atoms (e.g. CH₃CH₂OH). A displayed formula shows every individual bond drawn out. The exam may ask for one specifically, and giving the other loses the mark.
• Writing "ethane" when you mean "ethene" (or vice versa). One letter changes the compound from a saturated alkane (ethane, C₂H₆) to an unsaturated alkene (ethene, C₂H₄). Proofread your answers.
• Saying bromine water turns "clear" instead of "colourless." Clear means you can see through it (clear orange is still clear). Colourless means it has no colour. The distinction matters in the edexcel mark scheme.
• Forgetting that cracking produces both alkanes and alkenes. Students sometimes write that cracking produces only smaller alkanes. The alkene product is the reason cracking is industrially important: alkenes are monomers for polymers.

How organic chemistry fits into the exam

The edexcel igcse chemistry organic chemistry: introduction to alkenes content appears on both Paper 1 and Paper 2. Paper 1 might ask you to name a compound, draw a displayed formula, or write a balanced combustion equation. Paper 2 might give you data from a cracking experiment and ask you to identify the products, or present an unfamiliar hydrocarbon and ask you to classify it as saturated or unsaturated and justify your answer using the general formulae.

Strong candidates treat the igcse 4ch1 organic chemistry: introduction to alkenes material as a connected system. Crude oil provides alkanes by fractional distillation. Cracking converts long-chain alkanes into shorter alkanes and alkenes. Alkenes undergo addition reactions that alkanes cannot. Each topic leads naturally to the next, and the exam exploits this by asking multi-part questions that follow the same logical sequence.

The edexcel igcse chemistry explained above covers every specification point for these four topics. What it cannot do is replace active practice. Draw displayed formulae until you can do it without thinking. Write combustion equations until balancing them is automatic. Do the bromine water test in your head for every hydrocarbon you encounter. The edexcel igcse chemistry practice questions in the self-check section below are a starting point. For a larger bank of questions mapped to the specification, use the Green Bridge CBT platform.

Self-check questions

1. Write the molecular formula and structural formula for pentane.
2. Draw the displayed formula for propene, clearly showing the double bond.
3. Write a balanced equation for the complete combustion of propane (C₃H₈).
4. Describe the test you would use to distinguish between hexane and hexene. State what you would observe in each case.
5. A long-chain alkane C₁₂H₂₆ is cracked to produce ethene (C₂H₄) and one other product. Determine the molecular formula of the other product.

Self-check questions

1. Draw the structural formula of propene and name the functional group it contains.
2. Write a balanced equation for the complete combustion of ethene, including state symbols.
3. Describe the test you would use to distinguish between ethane and ethene, stating the reagent, the method and the expected result for each.
4. Explain why alkenes are described as unsaturated hydrocarbons, and state one industrial reaction that depends on this property.
5. A student adds bromine water to an unknown hydrocarbon. The bromine water remains orange. What does this tell you about the hydrocarbon?

For more edexcel igcse chemistry notes on the organic chemistry: introduction to alkenes edexcel igcse topics, visit the Green Bridge CBT platform.