Welcome to Organic Chemistry — the study of carbon-based compounds. Carbon is unique because it can form four bonds and create incredibly long chains and rings, giving rise to millions of different compounds.
In this module you will learn about hydrocarbons (alkanes and alkenes), how to name organic molecules using IUPAC nomenclature, understand crude oil and its fractions, explore polymers and alcohols, and examine key reactions like combustion, addition and fermentation.
Organic chemistry is everywhere — fuels, plastics, medicines, food and clothing all involve organic compounds. By the end of this module you will be able to draw structural formulas, predict products and explain key industrial processes. Let's dive into the world of carbon!
Organic chemistry makes up roughly 25–30% of the CSCA Chemistry paper. Expect questions on naming and drawing organic molecules, crude oil fractions, cracking equations, polymer repeat units, fermentation conditions, and pollution from fossil fuels. Extended response questions often link combustion to environmental issues.
Organic chemistry is the study of compounds containing carbon. Almost all organic compounds also contain hydrogen, and many contain oxygen, nitrogen or halogens.
Why Carbon is Special
Key Terms
Homologous Series
A family of organic compounds with the same general formula, similar chemical properties, and a gradual change in physical properties. Each member differs by CH₂.
The main homologous series at CSCA level:
IUPAC Naming
The number of carbons determines the prefix: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6). The suffix tells you the homologous series: -ane (alkane), -ene (alkene), -ol (alcohol), -oic acid (carboxylic acid).
💡Organic chemistry studies carbon compounds. Carbon forms 4 bonds, creating chains and rings. A homologous series shares a general formula, and naming uses carbon-count prefixes with series suffixes.
📋 Key Formulas
Alkanes: CₙH₂ₙ₊₂ | Alkenes: CₙH₂ₙ | Alcohols: CₙH₂ₙ₊₁OH | Prefixes: meth(1), eth(2), prop(3), but(4), pent(5), hex(6)
📝 Worked Example 1
Example 1: Name the compound with molecular formula C₃H₈.
3 carbons → prop-. All single bonds, fits CₙH₂ₙ₊₂ (3×2+2=8 ✓) → alkane. Name: propane.
📝 Worked Example 2
Example 2: Write the molecular formula of butene.
But- = 4 carbons. -ene = alkene. General formula CₙH₂ₙ = C₄H₈. Answer: C₄H₈.
📝 Worked Example 3
Example 3: Draw the displayed formula of ethanol (C₂H₅OH).
Two carbon chain: H-C-C-O-H with all remaining bonds filled with H atoms. C1 has 3H, C2 has 2H and is bonded to OH.
🧠Memorise the prefixes meth-, eth-, prop-, but-, pent-, hex- — they come up in nearly every organic question.
🧠Check your molecular formula against the general formula: if CₙH₂ₙ₊₂ → alkane, if CₙH₂ₙ → alkene, if CₙH₂ₙ₊₁OH → alcohol.
⚠️Forgetting that carbon forms 4 bonds: In displayed formulas, every carbon must have exactly 4 bonds (to C, H, O etc.).
⚠️Confusing molecular and structural formulas: C₂H₆O is a molecular formula. CH₃CH₂OH is a structural formula showing the arrangement.
🎯 Try This Yourself
Name the organic compound with formula C₅H₁₂ and state its homologous series.
Open and read all sections to complete this module