Ch 2 · Naming Organic Compounds · O Level Chemistry 5070
Topic 2 of 8 in Organic Chemistry — O Level Chemistry 5070 — 48 lessons.
Introduction
An organic name is a code. The first part tells you how many carbon atoms are in the chain, the ending tells you which homologous series the compound belongs to, and a number tells you where the functional group sits. Once you can decode the name, you can draw the molecule; once you can read the formula, you can name it. Both directions are examined.
Key ideas — The five compounds you must be able to draw
In a displayed formula, methane is a carbon atom with four single bonds to hydrogen. Ethane is two carbon atoms joined by a single bond, each carrying three hydrogen atoms. Ethene is two carbon atoms joined by a double bond, each carrying two hydrogen atoms. Ethanol is ethane with one hydrogen replaced by an O–H group, with both the C–O and O–H bonds drawn. Ethanoic acid has a CH_3 carbon bonded to a second carbon that carries a C=O double bond and an O–H group.
🧑🏫 Teacher's explanation
Think of organic chemistry like building with LEGO bricks. The carbon atoms are the main blocks, and the hydrogen atoms are the smaller pieces used to fill every available connection point. Because carbon must always form exactly four bonds, the number of hydrogens changes depending on whether you are using single bonds, double bonds, or adding 'special' functional groups like oxygen.
📖 Definitions
🔍 Worked example — Easy
Draw the displayed formula for ethane.
🔍 Worked example — Standard
Identify the type of compound for CH_3CH_2OH.
🔍 Worked example — Exam-style
Draw the displayed formula for ethanoic acid.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Products of the syllabus reactions
You must also name and draw the compounds formed in the reactions met in this module:
🧑🏫 Teacher's explanation
Think of organic chemistry like building with LEGO bricks. The carbon atoms are the main blocks, and the functional groups (like -OH or -Cl) are special pieces that change how the whole structure behaves. Once you know the 'skeleton' of the molecule, you just need to identify the 'special pieces' to name it.
📖 Definitions
🔍 Worked example — Easy
Identify the product of the reaction: CH_4 + Cl_2 arrow CH_3Cl + ?
🔍 Worked example — Standard
Draw the structural formula for 1,2-dibromoethane formed from C_2H_4 + Br_2 arrow ?
🔍 Worked example — Exam-style
An organic compound has the formula CH_3COOCH_2CH_3. Name the two parent compounds used to make this ester.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Reading a name or a formula
The ending of a name gives the type of compound:
🧑🏫 Teacher's explanation
Think of organic chemistry names like a Lego set. The 'stem' tells you how many bricks are in the chain, and the 'suffix' (the ending) tells you what special attachment is on the end. If you see 'eth-', you know there are 2 carbons; if you see '-ol', you know there is an alcohol group.
📖 Definitions
🔍 Worked example — Easy
Identify the type of compound and name it for the molecular formula mathrmC_4H_10.
🔍 Worked example — Standard
Identify the functional group and name the compound C_3H_7OH.
🔍 Worked example — Exam-style
A molecule has the displayed formula showing a carbon atom double-bonded to an oxygen atom and single-bonded to an -OH group. It has 2 carbons. Name the compound.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Naming compounds with up to four carbon atoms
positions and unbranched chains. Carbon atoms are numbered from the end that gives the functional group the lowest possible number, and that number goes into the name.
🧑🏫 Teacher's explanation
Naming organic molecules is like giving someone directions to a specific house on a street. The 'street name' is the parent chain (the longest continuous chain of carbon atoms), and the 'house number' is the position of the functional group (the part of the molecule that determines its properties, like an alcohol or an acid).\n\nThink of the carbon chain as a row of houses. If you have a row of four houses and the 'special feature' (the functional group) is on the second house, you must tell people exactly where it is so they don't walk to the wrong one. In chemistry, we use numbers (locants) to do this. For alkanes, there is no special feature, so no number is needed. For alkenes, alcohols, and carboxylic acids, we number the chain from the end that gives the functional group the lowest possible number.
📖 Definitions
🔍 Worked example — Easy
Identify the type of compound and draw the displayed formula for CH_3CH_2OH.
🔍 Worked example — Standard
Name the following alkene: CH_2=CHCH_2CH_3.
🔍 Worked example — Exam-style
A molecule has the formula CH_3CH(OH)CH_2CH_3. Name it.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Compound P has the molecular formula C_3H_8. Compound Q has three carbon atoms and an -OH group on the middle carbon atom. (a) Name P and state the homologous series to which it belongs. [2] (b) Draw the displayed formula of ethanoic acid. [1] (c) Name Q and write its structural formula. [2]
🔍 Worked example — Easy
Identify the homologous series and name the compound with the formula C_2H_6.
🔍 Worked example — Standard
A molecule has 3 carbon atoms and a double bond between the first and second carbon. Name it.
🔍 Worked example — Exam-style
A compound has the molecular formula C_3H_6O. It contains a carbonyl group (C=O) and an -OH group on the same carbon. Name it.
⚠️ Common errors
Common mistakes
Practice
Answers (a) carboxylic acid [B1]; (b) alkene [B1]; (c) alcohol [B1]. Two carbon atoms joined by a double bond [B1]; each carbon bonded to two hydrogen atoms, six bonds in total [B1]. Chloromethane [B1]; CH_3Cl [B1]. mathrmCH_3CH=CHCH_3 [B1]; CH_3CH_2CH_2CH_2OH [B1]. Butanoic acid [B1]; C_4H_8O_2 or C_3H_7COOH [B1].