Chemguide: Support for CIE A level Chemistry

Learning outcome 16: Hydroxy compounds

16.1: Alcohols

Statement 16.1.4

This statement is about using the triiodomethane (iodoform) reaction to identify the CH3CH(OH)- group in alcohols.

Before you go on, you should find and read the statement in your copy of the syllabus.

You will find what you need on the page about the triiodomethane (iodoform) reaction with alcohols

For CIE purposes, you only need to learn the method using iodine and sodium hydroxide solution.

The most important section on this page is under the heading "What the triiodomethane (iodoform) reaction shows".

The full equation for the reaction isn't given on that page. For a general alcohol which undergoes the reaction, and where R is any hydrocarbon group or a hydrogen atom, the equation is:

At the time of writing, this equation had never been asked in an exam. CIE teacher support material does, however, say that it should be taught.

My advice would be that this is such a bother to learn that you would probably be safe to ignore it until you have learnt everything else on the syllabus.

You will, however, be expected to be able to write the structures of both of the organic products of the reaction - the triiodomethane and the sodium salt, RCOONa. CIE expects you to give this in the form of the carboxylate ion as shown in the syllabus statement.

Note:  A carboxylate ion is the negative ion from a carboxylic acid such as ethanoic acid, CH3COOH. A general carboxylic acid is written as RCOOH.

So, sodium ethanoate has the formula CH3COONa, and contains the ethanoate ion, CH3COO- (actually preferable to the CH3CO2- given in the syllabus in my view!).

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© Jim Clark 2020