Chemguide

Extra material needed for the 2010 exam - Chapter 19

This page lists the material from the old Edexcel syllabus which isn't covered by my new Edexcel IGCSE Chemistry book.

The chlorination of methane

If you look at the bottom of page 157, you will find a discussion of the reaction between methane and bromine. The reaction with chlorine is exactly the same, and needs exactly the same conditions - a source of UV light (including sunlight). This is commented on in the bottom two lines of the page.

In the chlorine case, you start off with a mixture which will be faintly green because of the chlorine. The colour will fade, and a mixture of chloromethane and hydrogen chloride gases is formed. The reaction will also go further in exactly the same way as that described for bromine in the margin box.

The oxidation of ethanol to ethanoic acid

You are probably familiar with the fact that if alcoholic drinks are left exposed to the air, they turn sour, and produce vinegar. Vinegar is commonly said to contain acetic acid, but the modern name is ethanoic acid, CH₃COOH.

Ethanoic acid is a member of a family of compounds known as carboxylic acids. The carboxyl group is the -COOH group. The name, ethanoic acid, comes from the fact that the longest carbon chain in the compound has two carbons (including the one in the -COOH group) - hence "eth". The "an" shows that there are no carbon-carbon double bonds.

The change from ethanol to ethanoic acid is an example of oxidation, and is commonly written for simplicity as:

The O in square brackets is taken to mean "oxygen from an oxidising agent". In the lab, the usual oxidising agent for this reaction is a mixture of potassium dichromate(VI) solution and dilute sulfuric acid.

Ethanol is warmed with an excess of the oxidising mixture in a flask with a condenser placed vertically in the neck. This is described as "heating under reflux", and prevents any liquids escaping until the reaction is complete. Any hot ethanol which escapes before it has been oxidised, for example, condenses in the vertical condenser and trickles back into the flask again. For a university-standard description of how to set up a simple reflux, you could watch this short YouTube video.

The oxidising mixture turns from orange to green as the ethanoic acid is formed. After a short while, when the reaction is complete, the ethanoic acid can be distilled off from the remains of the oxidising mixture.

The equation for this reaction is seriously complicated. The simple version above is fine at this level.

The reaction between ethanol and ethanoic acid

Heating a mixture of ethanoic acid and ethanol with a few drops of concentrated sulphuric acid produces a sweet smelling liquid called ethyl ethanoate. This is one member of a family (homologous series) of compounds called esters.

The concentrated sulphuric acid isn't written into the equation because it is a catalyst and isn't used up in the reaction.

Notice that this is a reversible reaction. The ethyl ethanoate has the lowest boiling point of any of the substances present, and is distilled off as soon as it is formed. This prevents the back reaction from occurring.

GCSE (and A level) students often have more trouble with the names and structures of esters than with any other organic compounds. You may like to read my introduction to esters in the main part of Chemguide. Just read the "What are esters?" bit. You can ignore the rest unless you are interested.

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