What you'll learn
This revision guide covers the essential carbon chemistry topics tested in CXC CSEC Integrated Science examinations. You'll master the structure and properties of hydrocarbons, understand how organic compounds are named and classified, and explore their practical applications in Caribbean industries. The content aligns directly with the CSEC specification requirements for organic chemistry.
Key terms and definitions
Hydrocarbon — A compound containing only carbon and hydrogen atoms
Saturated hydrocarbon — A hydrocarbon containing only single carbon-carbon bonds (alkanes)
Unsaturated hydrocarbon — A hydrocarbon containing at least one carbon-carbon double or triple bond (alkenes and alkynes)
Homologous series — A family of organic compounds with the same general formula, similar chemical properties, and a gradual change in physical properties
Functional group — A specific atom or group of atoms that determines the characteristic chemical properties of an organic compound
Isomer — Compounds with the same molecular formula but different structural arrangements
Polymerization — The chemical process of joining many small molecules (monomers) to form large molecules (polymers)
Combustion — A chemical reaction where a substance burns in oxygen, releasing energy as heat and light
Core concepts
Structure and bonding in carbon compounds
Carbon atoms can form four covalent bonds, allowing them to create chains, branches, and rings. This unique property explains why millions of carbon compounds exist.
Carbon's bonding characteristics:
- Four valence electrons enabling four covalent bonds
- Can bond with other carbon atoms to form long chains
- Forms strong, stable bonds with hydrogen, oxygen, nitrogen, and halogens
- Creates both single bonds (C—C) and multiple bonds (C=C, C≡C)
Structural representation:
- Full structural formula shows every atom and bond
- Condensed structural formula groups atoms together (e.g., CH₃CH₂CH₃)
- Molecular formula shows only the number of each atom type (e.g., C₃H₈)
Alkanes: Saturated hydrocarbons
Alkanes form a homologous series with the general formula CₙH₂ₙ₊₂. Each member differs from the next by a CH₂ unit.
First ten alkanes:
- Methane (CH₄)
- Ethane (C₂H₆)
- Propane (C₃H₈)
- Butane (C₄H₁₀)
- Pentane (C₅H₁₂)
- Hexane (C₆H₁₄)
- Heptane (C₇H₁₆)
- Octane (C₈H₁₈)
- Nonane (C₉H₂₀)
- Decane (C₁₀H₂₂)
Physical properties:
- Boiling points increase with chain length due to stronger intermolecular forces
- First four members (methane to butane) are gases at room temperature
- Pentane to C₁₇ are liquids
- Beyond C₁₈ are solids
- All alkanes are less dense than water
- Insoluble in water but soluble in organic solvents
Chemical properties:
- Relatively unreactive due to strong C—C and C—H bonds
- Undergo combustion in oxygen
- Complete combustion: alkane + oxygen → carbon dioxide + water + energy
- Incomplete combustion (limited oxygen): produces carbon monoxide or carbon (soot)
Caribbean applications: Trinidad and Tobago's natural gas industry processes methane for export and domestic use. Liquefied petroleum gas (LPG), containing propane and butane, is widely used for cooking throughout the Caribbean.
Alkenes: Unsaturated hydrocarbons
Alkenes contain at least one carbon-carbon double bond (C=C). The general formula is CₙH₂ₙ.
First five alkenes:
- Ethene (C₂H₄)
- Propene (C₃H₆)
- Butene (C₄H₈)
- Pentene (C₅H₁₀)
- Hexene (C₆H₁₂)
Key characteristics:
- More reactive than alkanes due to the C=C double bond
- Undergo addition reactions where the double bond breaks and new atoms add on
- Burn with a smokier flame than alkanes due to incomplete combustion
Test for unsaturation: Add bromine water (brown/orange) to the hydrocarbon:
- Alkenes decolorize bromine water (turns colourless)
- Alkanes show no visible change
- This test distinguishes saturated from unsaturated hydrocarbons
Addition reactions:
Hydrogenation: Alkene + hydrogen → alkane (requires catalyst, heat)
- C₂H₄ + H₂ → C₂H₆
Halogenation: Alkene + halogen → dihaloalkane
- C₂H₄ + Br₂ → C₂H₄Br₂
Hydration: Alkene + steam → alcohol (requires acid catalyst)
- C₂H₄ + H₂O → C₂H₅OH
Alcohols and their properties
Alcohols contain the hydroxyl functional group (—OH) attached to a carbon chain. The general formula is CₙH₂ₙ₊₁OH.
First five alcohols:
- Methanol (CH₃OH)
- Ethanol (C₂H₅OH)
- Propanol (C₃H₇OH)
- Butanol (C₄H₉OH)
- Pentanol (C₅H₁₁OH)
Physical properties:
- Lower members are liquids at room temperature
- Soluble in water due to hydrogen bonding with the —OH group
- Higher boiling points than corresponding alkanes
- Boiling points increase with chain length
Chemical properties:
Combustion: Alcohol + oxygen → carbon dioxide + water + energy
- C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Oxidation: Primary alcohols can be oxidized to carboxylic acids
- Ethanol + oxygen → ethanoic acid + water
- C₂H₅OH + O₂ → CH₃COOH + H₂O
Caribbean applications: Ethanol production through fermentation of sugar cane molasses occurs in several Caribbean nations. Jamaica and other islands produce rum through fermentation and distillation. Industrial ethanol serves as a fuel additive, reducing dependence on imported petroleum.
Carboxylic acids
Carboxylic acids contain the carboxyl functional group (—COOH). The general formula is CₙH₂ₙ₊₁COOH.
Common examples:
- Methanoic acid (HCOOH) — found in ant stings
- Ethanoic acid (CH₃COOH) — vinegar contains 4-8% ethanoic acid
- Propanoic acid (C₂H₅COOH)
- Butanoic acid (C₃H₇COOH)
Properties:
- Weak acids that partially ionize in water
- React with bases to form salts and water
- React with carbonates to produce carbon dioxide
- Sour taste (but never taste laboratory chemicals)
- Lower members are liquids with sharp odours
Esterification: Carboxylic acid + alcohol → ester + water (requires acid catalyst, heat)
- CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
Esters have sweet, fruity smells and are used in flavourings and perfumes.
Polymerization and plastics
Polymerization occurs when many small molecules (monomers) join to form large molecules (polymers).
Addition polymerization: Many alkene molecules join by breaking their double bonds:
- n(C₂H₄) → (—CH₂—CH₂—)ₙ
- Ethene monomers form polyethene (polythene)
Common polymers:
Polyethene (PE):
- Monomer: ethene
- Uses: plastic bags, bottles, containers
- Widespread use in Caribbean supermarkets and shops
Polypropene (PP):
- Monomer: propene
- Uses: ropes, carpets, plastic furniture
- Used in Caribbean fishing industry for nets and ropes
Polyvinyl chloride (PVC):
- Monomer: chloroethene (vinyl chloride)
- Uses: water pipes, electrical insulation
- Common in Caribbean construction and water distribution systems
Polystyrene (PS):
- Monomer: styrene
- Uses: insulation, disposable containers
- Used for fish boxes and food packaging in Caribbean markets
Environmental concerns:
- Most plastics are non-biodegradable
- Accumulate in landfills and oceans
- Caribbean beaches face plastic pollution from local and ocean currents
- Improper disposal blocks drains, contributing to flooding during hurricanes
- Burning plastics releases toxic gases
Worked examples
Example 1: Identifying hydrocarbons
Question: A hydrocarbon has the molecular formula C₅H₁₀.
(a) Is this hydrocarbon saturated or unsaturated? Explain your answer. (2 marks)
(b) Name one chemical test to confirm your answer and state the expected result. (2 marks)
(c) Write the molecular formula of the saturated hydrocarbon with five carbon atoms. (1 mark)
Solution:
(a) Unsaturated (1 mark). The general formula for alkanes is CₙH₂ₙ₊₂. For n=5, this gives C₅H₁₂. Since C₅H₁₀ has fewer hydrogen atoms, it must contain a double bond and is therefore unsaturated (1 mark).
(b) Add bromine water to the hydrocarbon (1 mark). The brown/orange colour of the bromine water will turn colourless/decolorize (1 mark).
(c) C₅H₁₂ (1 mark)
Example 2: Combustion calculations
Question: Propane gas is commonly used for cooking in Caribbean households.
(a) Write the molecular formula for propane. (1 mark)
(b) Write a balanced equation for the complete combustion of propane. (3 marks)
(c) State one environmental advantage of using propane over wood for cooking. (1 mark)
Solution:
(a) C₃H₈ (1 mark)
(b) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (3 marks: correct formulae - 1 mark, balanced - 2 marks)
(c) Produces less smoke/particulates/carbon monoxide OR reduces deforestation OR produces more energy per gram (1 mark)
Example 3: Alcohol oxidation
Question: Ethanol can be oxidized to form ethanoic acid.
(a) Write the molecular formula for ethanol and ethanoic acid. (2 marks)
(b) Name one substance that can oxidize ethanol to ethanoic acid. (1 mark)
(c) Caribbean rum producers sometimes allow rum to age in wooden barrels. Suggest why this might slightly increase the ethanoic acid content. (2 marks)
Solution:
(a) Ethanol: C₂H₅OH or C₂H₆O (1 mark) Ethanoic acid: CH₃COOH or C₂H₄O₂ (1 mark)
(b) Oxygen/air OR acidified potassium dichromate/permanganate OR any suitable oxidizing agent (1 mark)
(c) Oxygen from air can pass through the wooden barrel (1 mark) and slowly oxidize the ethanol to ethanoic acid (1 mark).
Common mistakes and how to avoid them
Confusing molecular and structural formulae: Remember that C₃H₈ is molecular (showing only atom types and numbers) while CH₃CH₂CH₃ is structural (showing arrangement). Practice converting between both forms.
Incorrectly applying the bromine water test: Students often state "bromine water turns clear" instead of "decolorizes" or "turns colourless." Use precise scientific terminology. Also remember this test only works for unsaturated compounds.
Mixing up addition and substitution reactions: Alkenes undergo addition reactions (double bond breaks, atoms add on). Alkanes undergo substitution reactions (atoms replace hydrogen). Know which type applies to each hydrocarbon family.
Wrong general formulae: Memorize the general formulae: alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH). Check your molecular formula matches the compound type.
Unbalanced combustion equations: Always balance oxygen and water carefully. Count all atoms on both sides. For complete combustion, products are always CO₂ and H₂O only.
Forgetting functional groups in naming: Ethanol, not ethane-ol. Ethanoic acid, not ethane-oic acid. The functional group name is part of the compound name, not separated.
Exam technique for Carbon Chemistry: Hydrocarbons and Simple Organic Compounds
Command word precision: "State" requires a brief answer without explanation (1 mark). "Explain" requires a reason or mechanism (typically 2 marks). "Describe" requires several linked points about a process.
Structural formula questions: Draw all bonds clearly when asked for structural formulae. Use standard conventions: straight lines for bonds, clear angles. Condensed formulae like CH₃CH₂OH are acceptable unless the question specifically asks for "full structural formula."
Multi-step calculations: Show all working steps. Write the balanced equation first, even if not explicitly asked. State what you're calculating at each step. Examiners award method marks even if the final answer is incorrect.
Application questions: CXC frequently tests ability to apply knowledge to Caribbean contexts (petroleum industry, rum production, plastics pollution). Read scenarios carefully and link your organic chemistry knowledge to the practical situation described.
Quick revision summary
Carbon forms four covalent bonds, creating millions of organic compounds. Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons with single bonds; alkenes (CₙH₂ₙ) are unsaturated with C=C double bonds that decolorize bromine water. Alcohols contain the —OH functional group and oxidize to carboxylic acids (—COOH). All organic compounds undergo combustion. Addition polymerization joins alkene monomers into plastics like polyethene. These concepts apply directly to Caribbean industries including natural gas production, rum manufacturing, and plastic use, with environmental implications for waste management and ocean pollution.