Edexcel IGCSE Chemistry · Spec 4.30C, 4.35C
Alcohols and Carboxylic Acids up to Four Carbons (Paper 2)
Covers draw the first four alcohols, Draw the first four carboxylic acids and Use functional groups and general formulae.
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Alcohols and Carboxylic Acids up to Four Carbons (Paper 2)
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Explained
Drawing the first four of each family
Two homologous series appear here, and each is defined by its functional group. The alcohols have the hydroxyl group, O H. The carboxylic acids have the carboxyl group, C O O H.
A homologous series shares a general formula, so each member differs from the next by one CH2 unit, and all members react in the same ways because they share the same functional group.
The first four alcohols
Methanol, CH3OH. Ethanol, CH3CH2OH. Propan-1-ol, CH3CH2CH2OH. Butan-1-ol, CH3CH2CH2CH2OH.
The general formula is C n H 2n plus 1 O H. The number in the name of the last two says which carbon the O H group is attached to, counting from the end that gives the smaller number.
The first four carboxylic acids
Methanoic acid, HCOOH. Ethanoic acid, CH3COOH. Propanoic acid, CH3CH2COOH. Butanoic acid, CH3CH2CH2COOH.
The general formula is C n H 2n plus 1 C O O H. Note that the carbon of the C O O H group counts towards the name, so ethanoic acid has two carbons in total, one of which is the acid carbon. Methanoic acid is the exception where there is nothing but a hydrogen attached.
Drawing them
In a displayed formula every bond is shown as a line, including all the bonds to hydrogen.
For an alcohol, draw the carbon chain, fill the spare bonds with hydrogens, and replace one hydrogen with an O H, drawn as a line to O and another line from O to H.
For a carboxylic acid, the end carbon carries a double bond to one oxygen and a single bond to another oxygen, which then carries a hydrogen. That carbon has no hydrogen of its own, because all four of its bonds are used by the chain and the two oxygens.
Drawing the carboxyl group as C O O H in a displayed formula, without showing the double bond, is the most common way to lose the mark.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asking for a displayed formula states plainly beside the mark: must show ALL the bonds. There is no partial credit for a structure that is nearly complete.
The same paper marks the molecular formula, the empirical formula and the relative formula mass as three separate answers in three boxes, and says to ignore the calculation for the last of them. Each is a different question about the same molecule, and it is worth checking which one is being asked for before you answer.
On isomers, another mark scheme wants two things: that they have the same molecular formula, and that they have different structural or displayed formulae. It allows the same number of carbons and hydrogens for the first, and a different arrangement of atoms for the second.
An examiner report from the same series adds a warning about that pair. Writing that isomers have the same general formula is not correct, though the second half of the answer could still earn a mark, and it notes that isomers are often tested so the definitions should be learned.
Molecular formula and general formula are different things. Propane and propene share neither; butane and methylpropane share a molecular formula but are different substances.
Why the names look the way they do
The stem gives the number of carbons: meth for one, eth for two, prop for three, but for four. The ending gives the family: ane for an alkane, ene for an alkene, ol for an alcohol, oic acid for a carboxylic acid.
So butanoic acid has four carbons and a carboxyl group, and you can draw it from the name alone without having seen it before. That is the whole point of the naming system, and it is worth using deliberately rather than trying to memorise each compound separately.
Spec 4.30C, 4.35C
What you need to know
- Draw the first four alcohols
- Draw the first four carboxylic acids
- Use functional groups and general formulae
Active recall
Quick check
Answer each question before opening the answer.
What is the formula of butan-1-ol?
CH3CH2CH2CH2OH
Name CH3CH2COOH
Propanoic acid
How do successive homologues differ?
By CH2
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