Worked examples: Carbon Compounds

Work through each example by covering the answer, attempting it yourself, then checking your method against the solution. Every example flags the slip that most often costs marks. These are original questions in SPM style, not past-year papers.

Example 1, Naming from a structure

Question. Name the compound CH₃CH₂CH₂OH.

Solution. The longest chain has three carbon atoms (stem prop-) and the –OH group means it is an alcohol (suffix -ol), so the name is propan-1-ol. The –OH is on carbon 1, so the locant is 1.

Common slip: forgetting the locant for the –OH position, or writing “propanol” without checking which carbon carries the group.

Example 2, Deducing a molecular formula

Question. An alkene has four carbon atoms. State its molecular formula and name one member.

Solution. Alkenes follow CₙH₂ₙ, so for n = 4 the formula is C₄H₈. One member is but-1-ene, CH₂=CHCH₂CH₃.

Common slip: using the alkane formula CₙH₂ₙ₊₂ by habit and writing C₄H₁₀, that is butane, not an alkene.

Example 3, Drawing structural isomers

Question. Draw and name the two structural isomers of C₄H₁₀.

Solution. The first is butane, a straight chain: CH₃–CH₂–CH₂–CH₃. The second is 2-methylpropane, a three-carbon chain with a methyl branch on the middle carbon. Both have the formula C₄H₁₀ but a different arrangement of atoms.

Common slip: drawing the “same” chain bent a different way and counting it as a new isomer. Isomers must differ in the order the atoms are joined, not merely in how the drawing is turned.

Example 4, An addition reaction of an alkene

Question. Ethene is bubbled through bromine water. State the observation and write the equation.

Solution. The brown bromine water is decolourised at room temperature. The double bond opens and bromine adds across it: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane).

Common slip: calling this a substitution reaction. Alkenes undergo addition; only alkanes undergo substitution, and that needs ultraviolet light.

Example 5, Distinguishing two hydrocarbons

Question. You are given separate samples of ethane and ethene. Describe a chemical test to tell them apart.

Solution. Add a few drops of bromine water to each and shake. The ethene decolourises the brown bromine water quickly at room temperature; the ethane produces no change. (Acidified potassium manganate(VII) works the same way: decolourised by ethene, unchanged by ethane.)

Common slip: giving the reagent but not the observation for both samples. The mark needs the positive result and the negative one.

Example 6, Oxidation of an alcohol

Question. Ethanol is heated with acidified potassium dichromate(VI). State the colour change, name the organic product and write an equation using [O] for the oxidising agent.

Solution. The colour changes from orange to green. Ethanol is oxidised to ethanoic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.

Common slip: quoting the manganate(VII) colour change (purple to colourless) while naming dichromate(VI) as the reagent. Match the colour change to the reagent named.

Example 7, Esterification

Question. Ethanoic acid is warmed with ethanol and a little concentrated sulfuric acid. Name the ester formed, write the equation and state the role of the sulfuric acid.

Solution. The ester is ethyl ethanoate. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The concentrated sulfuric acid is a catalyst (it also removes water, helping the forward reaction).

Common slip: naming the ester the wrong way round. The alkyl part comes from the alcohol (ethyl, from ethanol) and the -anoate part from the acid (ethanoate, from ethanoic acid).

Example 8, Preparing ethanol by fermentation

Question. Describe how ethanol is produced from glucose by fermentation, and write the equation.

Solution. Glucose solution is mixed with yeast and kept warm in the absence of air. Enzymes in the yeast convert the glucose to ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The ethanol is then separated by distillation.

Common slip: saying oxygen is needed. Fermentation is anaerobic, air is excluded so the sugar is not oxidised all the way to carbon dioxide and water.

Using these examples

Notice the pattern: name the functional group, state the exact condition and reagent, give the observation or colour change, and write a balanced equation. Those four moves earn the marks in every Carbon Compounds question. Once the method feels automatic, move on to the practice questions and mark yourself the same way. A one-to-one teacher can check that your equations balance and your ester names run the right way round, the details that separate a partial answer from a full-mark one across the SPM Chemistry written papers.

Example 9, Reaction of a carboxylic acid

Question. Ethanoic acid is added to sodium carbonate. State what you observe, name the products and write a balanced equation.

Solution. Effervescence is seen as a colourless gas is released; the gas turns limewater milky, showing it is carbon dioxide. The products are a salt (sodium ethanoate), water and carbon dioxide: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.

Common slip: forgetting that a carboxylic acid, though weak, still shows all the ordinary acid reactions, with metals, carbonates and alkalis.

Example 10, Dehydration of ethanol

Question. Ethanol vapour is passed over hot aluminium oxide. Name the organic product, write the equation and state the type of reaction.

Solution. The product is ethene: C₂H₅OH → C₂H₄ + H₂O. This is a dehydration reaction, water is removed from the alcohol. The aluminium oxide is the catalyst; heating with concentrated sulfuric acid gives the same product.

Common slip: confusing dehydration with oxidation. Dehydration removes water to give an alkene; oxidation adds oxygen to give a carboxylic acid.

Example 11, Incomplete combustion

Question. Explain why a Bunsen burner with the air hole closed gives a yellow, sooty flame, while an open air hole gives a clean blue flame.

Solution. With the air hole closed, the supply of oxygen is limited, so incomplete combustion occurs and produces soot (carbon) and carbon monoxide, giving a yellow, luminous flame. With the air hole open, there is plenty of oxygen for complete combustion to carbon dioxide and water, giving a hotter, clean blue flame.

Common slip: naming the products of incomplete combustion as “less carbon dioxide” only, and omitting carbon monoxide and soot, which are the marking points.

Example 12, Naming a branched alkene

Question. Name the compound CH₂=C(CH₃)–CH₃.

Solution. The longest chain that includes the double bond has three carbons (prop-), the double bond starts at carbon 1 (prop-1-ene), and there is a methyl branch on carbon 2. The name is 2-methylprop-1-ene.

Common slip: numbering the chain from the wrong end, so the double bond and branch get the higher locants. Always number to give the double bond the lowest possible position.

From examples to marks

These twelve examples cover the whole chapter and drill the same disciplined habits: read the structure, identify the functional group, apply the correct reagent and condition, describe what you would see, and finish with a balanced equation. Rehearse them until the reasoning is automatic, then test yourself with the practice questions for this chapter and mark against these solutions.