Organic Chemistry
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!
What the Exam Tests
The syllabus lists basic organic compounds (hydrocarbons and their derivatives) under Properties and Reactions of Substances. This module teaches hydrocarbons, functional groups, the main organic reactions (substitution, addition, oxidation, esterification), polymers and isomerism, at Chinese senior-high-school level.
Questions can ask you to identify a functional group, predict the product of a reaction, count the isomers of a formula, find the monomer of a polymer, or choose a test that tells two organic compounds apart.
In our mock papers Substances and their Reactions, which also covers inorganic substances, redox and ionic reactions, gets 12 of the 48 questions. The CSCA publishes the topics but not how many questions each gets, so this split is our own reading of the syllabus. Reaction mechanisms, Friedel–Crafts reactions and E/Z naming go beyond the syllabus: they are marked in practice and left out of mocks.
Organic chemistry is the study of compounds containing carbon. Almost all organic compounds also contain hydrogen, and many contain oxygen, nitrogen or halogens. A few simple carbon compounds are classed as inorganic (无机物): CO, CO₂, H₂CO₃ and the carbonates, cyanides and carbides.
Why Carbon is Special
- Carbon has 4 valence electrons, so it forms 4 covalent bonds.
- It can bond to other carbon atoms forming chains, branches and rings.
- These chains can be almost any length, creating millions of compounds.
Key Terms
- Molecular formula (分子式): Shows the number of each type of atom. E.g., C₂H₆.
- Structural formula (结构简式): Shows how atoms are arranged. E.g., CH₃CH₃.
- Displayed formula (结构式): Shows every bond. Draw all C-H and C-C bonds explicitly.
A paper sat in Chinese uses 分子式, 结构简式 and 结构式; members of one homologous series are called 同系物.
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:
- Alkanes: CₙH₂ₙ₊₂ (single bonds only)
- Alkenes: CₙH₂ₙ (one C=C double bond)
- Alcohols: CₙH₂ₙ₊₁OH (contain -OH group)
- Carboxylic acids: CₙH₂ₙ₊₁COOH (contain -COOH group)
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).
In Chinese the carbon counts 1–6 are 甲, 乙, 丙, 丁, 戊, 己 and the series are 烷 (alkane, 烷烃), 烯 (alkene, 烯烃), 醇 (alcohol) and 酸 (carboxylic acid, 羧酸): 丙烷 = propane, 乙烯 = ethene, 乙醇 = ethanol, 乙酸 = ethanoic acid.
Key takeaway: 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.
Exam tip: Memorise the prefixes meth-, eth-, prop-, but-, pent-, hex- — you need them to name or recognise almost any organic compound.
Exam tip: Check your molecular formula against the general formula: if CₙH₂ₙ₊₂ → alkane; if CₙH₂ₙ → alkene or cycloalkane (e.g. C₃H₆ is propene or cyclopropane); if CₙH₂ₙ₊₂O → alcohol or ether (e.g. C₂H₆O is ethanol or dimethyl ether). A molecular formula alone does not fix the series.
Common mistake: Forgetting that carbon forms 4 bonds: In displayed formulas, every carbon must have exactly 4 bonds (to C, H, O etc.).
Common mistake: Confusing molecular and structural formulas: C₂H₆O is a molecular formula. CH₃CH₂OH is a structural formula showing the arrangement.
Try This Yourself
Name the straight-chain (unbranched) compound with formula C₅H₁₂ and state its homologous series.
Module Summary
Open and read all sections to complete this module