Naming, Drawing And Classifying

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.4 — Naming compounds using IUPAC rules, up to six carbon atoms
• 4.5 — Writing possible structural and displayed formulae from a molecular formula
• 4.6 — Classifying organic reactions as substitution, addition or combustion

Specification section 4(b) Crude oil — also taught in Crude Oil And Its Fractions, Burning Fuels.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Before chemistry was systematic, substances had names that told you nothing: oil of vitriol, spirit of hartshorn, flowers of zinc. Modern organic names are the opposite. They are built to be read, and a name assembled by the international rules tells you how many carbon atoms the molecule has, what family it belongs to, and where the interesting part sits.

How an organic name is assembled

1. The STEM counts the carbons in the longest chain: meth- 1, eth- 2, prop- 3, but- 4, pent- 5, hex- 6. These six are all this course requires.
2. The ENDING names the family: -ane for an alkane, -ene for an alkene, -ol for an alcohol, -oic acid for a carboxylic acid.
3. A NUMBER is inserted where the family allows more than one position, counting from whichever end gives the smaller number.
4. So propan-1-ol is a three-carbon alcohol with the –OH on an end carbon, and propan-2-ol is the same chain with the –OH in the middle.
5. Read backwards, a name builds the molecule: 'but-' means four carbons in a row, '-ene' means one carbon-to-carbon double bond, and 'but-2-ene' puts that double bond between the second and third carbons.

Think of it like a postal address rather than a nickname

A nickname identifies someone only if you already know them. An address is built from parts that anyone can decode without knowing the person at all: the number, the street, the town. An organic name works the same way. Nobody has to have met pent-2-ene to know exactly what it is — five carbons in a row, a double bond starting at the second one — because every part of the name is an instruction. That is the whole point of the international rules: they replace names you have to be told with names you can read.

The second skill on this page is drawing every isomer of a given molecular formula, and the trap is that students draw the same molecule twice in different orientations and count it as new. A molecule bent round on the paper is not a different compound. The way to avoid this is to work systematically rather than by inspiration.

Finding every isomer of C₅H₁₂ systematically

Draw and name all the structural isomers with the molecular formula C₅H₁₂.

1. Start with the LONGEST possible chain: five carbons in a row. Fill every spare bond with hydrogen. This is pentane.
2. Now shorten the chain to FOUR carbons, and attach the fifth carbon as a branch. It cannot go on an end carbon — that would just make the chain five long again.
3. So the branch goes on carbon 2, giving 2-methylbutane. Putting it on carbon 3 gives the same molecule viewed from the other end, so it is not new.
4. Now shorten to THREE carbons and attach the remaining two as branches. Both must go on the middle carbon, giving 2,2-dimethylpropane.
5. A two-carbon chain cannot hold the remaining three carbons without recreating a longer chain, so the search is finished.
6. Three isomers: pentane, 2-methylbutane and 2,2-dimethylpropane. All are C₅H₁₂ and all boil at different temperatures.

Answer: Three. Work down from the longest chain, one carbon at a time, and stop when shortening further only rebuilds a chain you already have.

The third thing this page does is classify reactions, and the specification asks for only three categories. The useful trick is to count the products rather than to memorise examples.

Telling the three reaction types apart by counting

1. COMBUSTION is the easy one to spot: the other reactant is oxygen, and the reaction releases heat. Complete combustion of a hydrocarbon gives carbon dioxide and water.
2. ADDITION: two molecules join to make ONE product and nothing else. This can only happen where there is a double bond to open, so it belongs to alkenes.
3. SUBSTITUTION: one atom or group is swapped for another, so there are always TWO products — the new organic compound, and the small molecule carrying away what was displaced.
4. So count the products. One product means addition. Two products, one of them small, means substitution.
5. Ethene plus bromine gives dibromoethane and nothing else — one product, so addition. Methane plus chlorine gives chloromethane AND hydrogen chloride — two products, so substitution.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The three reaction types, exam-exact

Substitution

A reaction in which an atom or group in a molecule is replaced by a different atom or group.

Addition

A reaction in which two molecules join to form a single product, with nothing else formed.

Combustion

A reaction of a substance with oxygen that releases heat energy.

Structural isomers

Compounds with the same molecular formula whose atoms are joined in a different order.

The six stems — learn them in order

• meth- one carbon
• eth- two carbons
• prop- three carbons
• but- four carbons
• pent- five carbons
• hex- six carbons

What the ending tells you

Finding every isomer of a molecular formula without missing one

1. Draw the longest unbranched chain the formula allows, and fill every spare bond with hydrogen.
2. Shorten the chain by one carbon and attach that carbon as a branch, never on an end carbon.
3. Move the branch along the shortened chain to each position that is genuinely different, ignoring any that is the same molecule seen from the other end.
4. Shorten the chain again and repeat with two branches.
5. Stop when shortening further would only rebuild a chain you have already drawn.

Addition against substitution

A. ADDITION: two molecules become ONE. Nothing is given off. Requires a double bond, so it is an alkene reaction. Ethene + bromine → dibromoethane.

B. SUBSTITUTION: one atom is exchanged for another, giving TWO products. It happens to saturated molecules. Methane + chlorine → chloromethane + hydrogen chloride.

Model answer [4 marks]

Draw and name the three structural isomers of C₅H₁₂, and explain how you know there are no others. [4]

The isomers are pentane, an unbranched chain of five carbons; 2-methylbutane, a four-carbon chain with a methyl branch on the second carbon; and 2,2-dimethylpropane, a three-carbon chain with two methyl branches on the middle carbon. Working systematically from the longest chain downwards guarantees completeness: a branch on an end carbon simply lengthens the chain again, and a branch position that mirrors one already drawn is the same molecule viewed from the other end. Once the chain is too short to hold the remaining carbons without recreating a longer chain, every possibility has been found.

Model answer [4 marks]

Classify each of these as substitution, addition or combustion, giving a reason: methane reacting with chlorine in ultraviolet light; ethene reacting with bromine. [4]

Methane with chlorine is substitution: a chlorine atom replaces a hydrogen atom in the methane, and there are two products, chloromethane and hydrogen chloride. Methane is saturated, so nothing can be added to it. Ethene with bromine is addition: the carbon-to-carbon double bond opens and both bromine atoms join on, so the two molecules become a single product, dibromoethane, with nothing else formed. Counting the products distinguishes them: one product means addition, two means substitution.

Not this: Turning a drawn molecule round, or bending the chain on the paper, gives a different isomer.

This: It gives the same molecule drawn differently. An isomer must have the atoms joined in a genuinely different ORDER. Before counting a structure as new, check whether it is one you have already drawn viewed from the other end.

Mark-losing trap. Number from the end that gives the SMALLER number. It is but-1-ene, never but-3-ene.

Mark-losing trap. The stem counts the LONGEST chain, not all the carbons. A branched five-carbon molecule can still be a butane.

Mark-losing trap. Addition makes one product and substitution makes two. Count the products before you name the reaction type.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — How many carbon atoms are in the longest chain of a molecule whose name begins with pent-?
2. Grade 7 · Name [2 marks] — Give the name of the alkene with four carbon atoms in a chain and the double bond between the first and second carbons.
3. Grade 8 · Classify [3 marks] — Ethene reacts with bromine to form only dibromoethane. What type of reaction is this, and how do the products tell you?
4. Grade 9 · Explain [4 marks] — Select every statement that belongs in a full answer explaining why C₄H₁₀ has exactly two structural isomers and not three.
5. 9+ · Deduce [5 marks] — A compound has the molecular formula C₃H₈O and contains an –OH group attached to a chain of three carbons. How many structural isomers of this description are there?

The people behind this science

Two ways into the same idea — the one who decided a chemical name should state what the substance contains, and the one who made a molecule something you draw rather than merely something you weigh. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Antoine Lavoisier — the one who decided a chemical name should state what the substance contains

The idea behind every name on this page — that a name should be built from parts that describe the substance, rather than being an arbitrary label you have to be taught — is Lavoisier's. In 1787, with three colleagues, he replaced the whole vocabulary of chemistry: oil of vitriol became sulfuric acid, flowers of zinc became zinc oxide, and for the first time a name told you what was in the bottle. Organic nomenclature is that same reform applied to carbon compounds a century later, and it works for exactly the reason he argued it would.

• “Why did the old names for substances have to go?”
• “How should a name be built so that anyone can read it?”
• “What did chemists lose when you abolished their vocabulary?”
• “Does naming a thing properly change how you think about it?”
• “How much resistance was there to renaming everything at once?”

August Kekulé — the one who made a molecule something you draw rather than merely something you weigh

Drawing isomers is only possible because Kekulé established that a molecule has a definite structure and that the structure can be represented on paper. He introduced the structural formula as chemistry's working language, and the systematic method on this page — longest chain first, then shorten and branch — is the direct descendant of his way of reasoning about what four bonds per carbon will and will not permit. He also presided over the 1860 congress at which chemists finally agreed on the formulae these structures are built from.

• “How can you be certain no further arrangement is possible?”
• “Why should a molecule be drawn rather than only written?”
• “What does a structure forbid that a formula cannot?”
• “How did chemists argue about structures before they agreed?”
• “Which came first for you, the drawing or the reasoning?”

Carry on through the course

• Previous lesson: What Organic Chemistry Is
• Next lesson: Crude Oil And Its Fractions
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• Cracking — Breaking long molecules nobody wants into short ones everybody does — and the reason a refinery has to.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification