Lab Tests

One very useful reaction of alcohols is a reaction with chromium in the +6 oxidation state. Chromium trioxide, CrO₃, is one such chemical. It is more properly called chromium(VI) oxide. Potassium dichromate, K₂Cr₂O₇, another form of chromium(VI) will also react in the same way. It was shown as the oxidizing agent in one of the earlier equations (for oxidation of 2-butanol, a secondary alcohol).

In aqueous sulfuric acid, chromium(VI) will oxidize primary and secondary alcohols. It will not oxidize tertiary alcohols. This reaction can (and will be) used as a test for identifying primary and secondary alcohols, but it cannot be used as a test to identify a tertiary alcohol as an unknown. That is one of the disadvantages of this particular test.

Equations for oxidation of alcohols using Cr(VI). [63rxn16.JPG]

Another disadvantage is that it also gives a positive result with aldehydes (top equation here), a group of chemicals we will be studying shortly. This is not a terrible disadvantage because aldehydes will react with other chemicals that do not react with alcohols. So it will be possible for you to distinguish between aldehydes and alcohols even though the CrO₃ test (or K₂Cr₂O₇ test) will not do so. You will work with that reaction later in this lesson.

Equations showing oxidation of aldehydes and alcohols using Cr(VI). [63rxn17.JPG]

The advantage of using one of these chemicals as an oxidizing agent is that they undergo an easily visible color change as they oxidize the alcohol. They start with a yellow to orange color (depending on pH and concentration) and change to a blue color (or green if some of the chromium(VI) remains in the mixture). You will get the opportunity to use this reaction when you do the lab work for this lesson.

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E-mail instructor: Eden Francis

Clackamas Community College
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