Organic Chemistry (Singapore O-Level Chemistry 6092): alkanes as saturated fuels from crude oil, alkenes and their addition reactions and the bromine test, addition polymerisation, and the alcohol and carboxylic acid families linked by oxidation and esterification

What this topic is really about

Organic Chemistry is the chemistry of carbon compounds, organised into families called homologous series. The unifying idea is that the functional group, the reactive part of the molecule, decides the chemistry, while the carbon chain just gets longer down a family. Saturated alkanes are unreactive fuels; the double bond makes alkenes reactive towards addition; and the alcohol and acid families are linked by oxidation and joined by esterification. Learn the families and their key reactions and the topic becomes a small map rather than a long list. This guide ties the three dot points together and links to each one.

The complete set of dot-point pages for this topic, each with worked examples and questions, lives at /sg-o-level/chemistry/syllabus/organic-chemistry.

Alkanes: saturated fuels from crude oil

Fuels and alkanes introduces the alkanes as a homologous series of saturated hydrocarbons with the general formula $\text{C}_n\text{H}_{2n+2}$, obtained from crude oil by fractional distillation. They are relatively unreactive but burn well, which makes them fuels. Complete combustion in plenty of oxygen gives carbon dioxide and water; incomplete combustion in limited oxygen gives the toxic gas carbon monoxide and soot (carbon).

Alkenes: addition reactions and polymers

Alkenes and addition reactions explains that alkenes are unsaturated hydrocarbons with a carbon-carbon double bond and the general formula $\text{C}_n\text{H}_{2n}$. The double bond makes them reactive towards addition, in which atoms add across the bond as it opens up. The bromine test uses this: an alkene rapidly decolourises orange-brown bromine water, while an alkane does not. Many alkene molecules can join end to end by addition polymerisation to make polymers such as poly(ethene).

Alcohols and carboxylic acids

Alcohols and carboxylic acids covers ethanol and ethanoic acid. Ethanol is made by fermentation of sugars or by the addition of steam to ethene; it burns and can be oxidised to ethanoic acid by an oxidising agent or by bacteria in air. Ethanoic acid is a typical weak acid showing the usual acid reactions, and it reacts with an alcohol to form a sweet-smelling ester in esterification.

How this topic is examined

• Use general formulae and homologous-series language. State the general formula and the trend in properties; identify a family from its functional group.
• Apply the bromine test correctly. Rapid decolourising means unsaturated (alkene); no change means saturated (alkane).
• Link the families. Show the oxidation of ethanol to ethanoic acid (orange to green dichromate) and the esterification of an acid with an alcohol.

Check your knowledge

A mix of recall, reasoning and reaction questions covering Organic Chemistry. Attempt them under timed conditions, then check against the solutions.

1. State the general formula of the alkanes and the general formula of the alkenes. (2 marks)
2. Write a word equation for the complete combustion of methane, and name the extra product formed during incomplete combustion. (3 marks)
3. Describe the bromine test and state the result for an alkene and for an alkane. (3 marks)
4. State two ways ethanol can be made. (2 marks)
5. Name the type of reaction between ethanoic acid and ethanol, the catalyst used, and the type of product formed. (3 marks)