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Simple primary and secondary alcohols can easily
be distinguished by their rate of oxidation with trichloroisocyanuric acid
(TCICA).1,2 The TCICA test is conducted by adding the unknown to a solution of TCICA in acetonitrile3 containing hydrochloric acid4 and measuring the time for a precipitate5 to form. Primary alcohols
react slowly and secondary alcohols react rapidly. To
generate comparison data for primary and secondary alcohols,
tests should be first carried out using ethanol and
2-propanol.6
TCICA Test Procedure
To a small test tube add 0.5 mL of the TCICA
solution in acetonitrile (30 mg/mL), one drop of 1 N HCl, and
one drop of the sample. After noting the time, flick the test
tube with your finger several times to mix the contents.
Watch the test tube until a precipitate forms and record
the elapsed time. The elapsed time could range from a few
seconds to a few minutes.
In order to destroy any unreacted TCICA before
disposal in the appropriate waste container, a few crystals
of sodium hydrogen sulfite (NaHSO3) and a few drops of
water should be added to the test tube, and the mixture
should be allowed to react for a few minutes.7
Caution: TCICA solution is a bleach and a
strong oxidizing agent and will discolor clothes. Do not
get the solution on your clothes, your skin, or any
lab surfaces. Spills can be cleaned up with sodium hydroxide sulfite solution.
Notes
1. Trichloroisocyanuric acid can be used to prepare
ketones from secondary alcohols; see Hiegel, G. A.; Nalbandy, M.
Synth. Commun. 1992, 22, 1589.
2. Other methods for distinguishing alcohols include the
Lucas testsee Roberts, R. M.; Gilbert, J. C.; Martin, S. F.
Experimental Organic Chemistry; Saunders: New York, 1994; p 693; and
1H NMR see McGreer, D. E.; Mocek, M. M. J. Chem.
Educ. 1963, 40, 358.
3. As other strong oxidizing agents, TCICA should be added
to the solvent, not the solvent to the TCICA. Solutions of TCICA
in acetonitrile are stable for years when stored in a brown
bottle. TCICA is used as a swimming pool disinfectant and is
widely available.
4. The reaction will proceed without the HCl, but the
reaction times are more reliable when it is used.
5. The precipitate is cyanuric acid, a solid used to inhibit
light-induced destruction of chlorine in swimming pools.
6. We have used this test in organic lab classes for
several years, and it gives reliable results in the hands of students.
Students are not provided with comparison reactions times;
therefore, each student runs the ethanol and 2-propanol tests to
get this information. Primary alcohols take about 7-30 min to give
a precipitate and secondary alcohols take about 0.1-1.2 min.
Tertiary alcohols, which cannot be oxidized directly, take 3 or
more hours before a precipitate begins to form; presumably this
occurs after dehydration to an alkene.
7. Iodide-starch test paper wetted with water can be used
to test for the presence of oxidizing power before the solution
is placed in the waste container
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