Edexcel IGCSE Chemistry · Spec 4.2, 4.4
Organic Formulae and Naming up to Six Carbons
Covers distinguish five formula types, Use carbon-chain prefixes to six carbons and Apply functional-group suffixes.
Chemistry revision video
Organic Formulae and Naming up to Six Carbons
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Explained
Five formulae, and a naming system that tells you what to draw
The same molecule can be written five ways, and a question will specify which one it wants. Answering with the wrong type is a mark lost on something you knew, so the first job is to be able to tell them apart.
The five types
Empirical formula: the simplest whole number ratio of atoms. Butane, C4H10, has an empirical formula of C2H5.
Molecular formula: the actual number of atoms of each element. C4H10.
General formula: the pattern for a whole homologous series, with n standing for the number of carbons. CnH2n plus 2 for alkanes.
Structural formula: the arrangement written on one line, using brackets for branches. CH3CH2CH2CH3.
Displayed formula: every atom and every bond drawn out.
Two of those catch people out. The empirical formula of ethene, C2H4, is CH2, which is not a real molecule but is still the correct answer, because a ratio is all that was asked for. And a displayed formula must show the bonds to hydrogen as well as those between the carbons; leaving them out is the most common way of losing that mark.
Building a name
The stem counts the carbons in the longest chain: meth, eth, prop, but, pent, hex for one to six.
The ending names the family: ane, ene, ol, oic acid.
A number locates the functional group or the double bond, counting from the end that gives the lower number. So the OH in an alcohol on the second of three carbons makes propan-2-ol, not propan-3-ol, even if you happened to draw it from the other end.
Read a name from right to left when you are working out what to draw. The ending tells you which family, so what to put on; the stem tells you how many carbons, so how long a chain; the number tells you where.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks what makes two compounds isomers, for two marks: that they have the same molecular formula, and that they have different structural or displayed formulae.
Its notes then allow two looser wordings. Same number of carbons and hydrogens is accepted for the first, and different arrangement of atoms for the second.
Those alternatives are worth having, because they say the same thing without the vocabulary. What is not accepted is an answer using only one of the two ideas. Same formula on its own describes identical compounds; different structure on its own describes almost any two compounds. Isomerism is the combination, and both halves have to be present.
The same paper tests the numbering directly, giving four options for where a pair of substituents sit on a chain and asking which is correct. The answer is the 1 and 2 arrangement, and the mark scheme dismisses 1 and 1, 2 and 1, and 2 and 2 in one line each.
Notice that 2 and 1 is refused even though it describes the same molecule as 1 and 2. Names run from the end that gives the lowest numbers, so the pair is written in ascending order. It is a convention rather than chemistry, and it is marked as strictly as the chemistry is.
Isomers in practice
Two molecules with the same molecular formula and different arrangements are structural isomers.
C4H10 has two: butane, a straight chain of four carbons, and methylpropane, a chain of three with one carbon branching off the middle.
C4H8 as an alkene has but-1-ene and but-2-ene, differing in where the double bond sits.
C3H8O has propan-1-ol and propan-2-ol, differing in where the OH sits.
When a question asks you to draw an isomer, check your answer against the original before moving on. The same molecule drawn flipped or rotated is not an isomer, and it is an easy mistake with branched chains, where a branch on the second carbon from the left is the same as one on the second from the right.
Working systematically
To find all the isomers of a formula, start with the longest straight chain, then shorten it by one and put the spare carbon on as a branch, then shorten it again.
For a functional group, keep the chain fixed and move the group along it one position at a time, stopping when the positions start repeating from the other end.
Then count the hydrogens on every structure you have drawn. Each carbon needs four bonds in total, and a structure with a carbon short of one is a drawing error rather than a new isomer.
Spec 4.2, 4.4
What you need to know
- Distinguish five formula types
- Use carbon-chain prefixes to six carbons
- Apply functional-group suffixes
Active recall
Quick check
Answer each question before opening the answer.
What does a displayed formula show?
Every atom and every covalent bond
What prefix represents six carbons?
Hex-
Name CH3CH2CH2OH
Propan-1-ol
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