Ch 1 · Formulae, Functional Groups and Terminology · O Level Chemistry 5070
Topic 1 of 8 in Organic Chemistry — O Level Chemistry 5070 — 48 lessons.
Introduction
Organic chemistry is the chemistry of carbon compounds. Before you can name alkanes, alkenes, alcohols and carboxylic acids or predict how they react, you need a shared language: how to write formulae, what a functional group is, and what makes a family of compounds a homologous series. Examiners expect the exact definitions here, and a displayed formula loses its mark for a single missing hydrogen atom or bond.
Key ideas — Molecular, structural and displayed formulae
A molecular formula gives the number of atoms of each element in a molecule: ethanol is C_2H_6O. A structural formula shows how the atoms are grouped without drawing every bond: CH_3CH_2OH. A displayed formula shows all the atoms and all the bonds, with each single bond drawn as one line and a double bond as two lines.
🧑🏫 Teacher's explanation
Imagine you are describing a house. A molecular formula is like a shopping list: '4 walls, 2 doors, 10 windows'. It tells you what you have, but not where they go. A structural formula is like a blueprint: 'The doors are on the front wall'. It shows the arrangement but skips the tiny details. A displayed formula is like a high-definition photo: you see every single brick, every nail, and every window frame exactly where they sit.
📖 Definitions
🔍 Worked example — Easy
Draw the displayed formula for ethane, C_2H_6.
🔍 Worked example — Standard
A compound has the molecular formula C_3H_8O. It is an alcohol. Write its structural formula.
🔍 Worked example — Exam-style
Identify the structural isomers of butane, C_4H_10, that have a different carbon skeleton.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Draw the displayed formula for this compound. [2] (b) State the name of the functional group present in this molecule. [1] [3 marks]
Key ideas — General formulae
Each homologous series has a general formula that gives the molecular formula of any member once you know n, the number of carbon atoms in the chain.
🧑🏫 Teacher's explanation
Think of a homologous series like a brand of cars. Every car from 'Brand X' has the same chassis and engine design (the functional group), which means they all drive similarly (chemical properties). However, some cars are short hatchbacks and others are long limousines. The only difference is how many extra segments (the CH_2 unit) have been added to the middle.
📖 Definitions
🔍 Worked example — Easy
A hydrocarbon belongs to the alkane series. If it contains 5 carbon atoms, determine its molecular formula.
🔍 Worked example — Standard
An unknown compound has the molecular formula C_3H_6. Identify the homologous series it belongs to and provide its name.
🔍 Worked example — Exam-style
A carboxylic acid has a molecular formula of C_4H_8O_2. Using the general formula C_nH_2n+1COOH, determine the value of n and write the molecular formula in a simplified form.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Identify the homologous series to which this compound belongs. [1] (b) Write the name of this compound. [1] (c) Draw the displayed formula for this compound. [2] [4 marks]
Key ideas — Functional groups and homologous series
A functional group is an atom or group of atoms that determine the chemical properties of a homologous series. Alkenes have the mathrmC=C double bond, alcohols the -OH group and carboxylic acids the -COOH group. Alkanes contain only C–C and C–H single bonds and no functional group, which is why they are so unreactive.
🧑🏫 Teacher's explanation
Think of a homologous series like a brand of cars. Every car in the 'Sedan' series has a steering wheel and four wheels (the functional group), which means they all drive the same way (chemical properties). However, one sedan might be a compact model, while another is a long limousine. The 'limousine' is just the 'compact' model with extra segments added to the middle. In chemistry, those extra segments are CH_2 units.
📖 Definitions
🔍 Worked example — Easy
An organic compound has the general formula C_nH_2n+2. If the compound has 4 carbon atoms, calculate its relative molecular mass (M_r). (H=1, C=12)
🔍 Worked example — Standard
Identify the functional group and the general formula for a series where the first member is CH_3 extOH and the second member is C_2H_5OH.
🔍 Worked example — Exam-style
Draw the structural isomers of C_4H_8 that contain a $ ext{C=C}$ double bond (alkenes).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Structural isomers
structural isomers. Structural isomers are compounds with the same molecular formula but different structural formulae. mathrmC_4H_10 has two: butane, the unbranched chain CH_3CH_2CH_2CH_3, and methylpropane (2-methylpropane), a three-carbon chain with a CH_3 branch on the middle carbon, CH_3CH(CH_3)CH_3. C_4H_8 has three alkene isomers: but-1-ene, mathrmCH_2=CHCH_2CH_3; but-2-ene, mathrmCH_3CH=CHCH_3; and methylpropene, mathrmCH_2=C(CH_3)CH_3. Isomers have different physical properties (butane boils at -1 °C, methylpropane at -12 °C) but similar chemical properties when they share the same functional group.
🧑🏫 Teacher's explanation
Imagine you have a set of LEGO bricks: exactly four red 2 imes 2 blocks and ten small white 1 imes 1 blocks. You can snap them together to make a long, straight tower, or you can snap them together to make a short, wide, chunky shape. Even though you used the exact same number of bricks for both shapes, the two shapes look different and might behave differently if you tried to stack them.
📖 Definitions
🔍 Worked example — Easy
Identify the molecular formula of a compound that has two structural isomers: butane and methylpropane.
🔍 Worked example — Standard
A compound has the molecular formula C_3H_6. Draw (describe) its only possible structural isomer that is an alkene.
🔍 Worked example — Exam-style
Compare the boiling points of butane (-1 °C) and methylpropane (-12 °C). Explain why they have different boiling points despite having the same molecular formula.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the name of the functional group present in these compounds. [1] (b) Compound A is but-1-ene. Draw the structural formula for its structural isomer, but-2-ene. [2] (c) Explain why the boiling points of these two compounds are different. [1] [4 marks]
Worked example
Question. Hydrocarbon X is a member of the alkane homologous series and has a relative molecular mass of 72. (a) Deduce the molecular formula of X. [2] (b) Name the unbranched isomer of X and describe its displayed formula. [2] (c) State two pieces of evidence that X and ethane belong to the same homologous series. [2]
🔍 Worked example — Easy
Calculate the relative molecular mass (M_r) of the second member of the alkene homologous series.
🔍 Worked example — Standard
A hydrocarbon has the molecular formula C_4H_10. Draw its structural isomer that is branched.
🔍 Worked example — Exam-style
Hydrocarbon X is a member of the alkane homologous series and has a relative molecular mass of 72. (a) Deduce the molecular formula of X. (b) Name the unbranched isomer of X. (c) State two pieces of evidence that X and ethane belong to the same homologous series.
⚠️ Common errors
Common mistakes
Practice
Answers An atom or group of atoms that determine the chemical properties of a homologous series. [B1] mathrmC_8H_16 [B1]; C_3H_7OH (or C_3H_8O) [B1]. Any three of: same functional group; same general formula; differ by a CH_2 unit from one member to the next; trend in physical properties; similar chemical properties. [B1 each] Propanoic acid has one more CH_2 unit than ethanoic acid [B1]; their M_r values differ by 14 [B1]. Compounds with the same molecular formula but different structural formulae [B1]; butane CH_3CH_2CH_2CH_3 [B1]; methylpropane CH_3CH(CH_3)CH_3 [B1].