Chemical Tests for Unknown Functional Groups

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Chemical T ests for Unknowns
Chemical tests have been developed as a means of identifying what functional groups are present in an unknown compound. Since IR and NMR spectroscopy has been developed, they are no longer critical to unknown analysis, but they can still be useful for confirming what you have determined by spectroscopy. Some of them look cool too!

In order for a chemical reaction to work as a chemical test, it must 1) create a visible result (a color change, a precipitate, etc) 2) work in a short amount of time (instantly to 5 minutes)
3) only work with one functional group
The following are chemical tests that we will use in this class. Some cover two related functional groups (such as the bromine test for alkenes and alkynes), while other only a specific subcategory of that functional group (such as the iodoform test for methyl ketones). In some cases there can be false positives (compounds which do not have the functional group still give a result test) or false negatives (compounds which have the functional group give a negative result). If your results do not match what you expected, use your judgment to determine if you should ignore the results or rethink your interpretation of the spectra. Summary of functional group tests

test

functional group

positive result
clear purple solution turns to
brown precipitate

Baeyer test

alkenes and alkynes

Bromine test

alkenes and alkynes

brown color disappears

Dinitrophenylhydrazine test

aldehydes and ketones

yellow to orange red precipitate

Ferrox test

any functional group
containing oxygen

reddish-purple color

Hydroxamate test

amides and esters

red-purple color appears

Iodoform test

methyl ketones

yellow precipitate

Iron hydroxide test

nitro groups

red-brown precipitate

Jones test

1o and 2o alcohols

orange reagent turns blue-green

Lucas test

2o, 3o, and benzylic alcohols

cloudy solution or separate layer

Tollen's test

aldehydes

silver mirror forms

Note: There are no chemical tests in this lab for amines, nitriles, or alkyl halides. If you have one of these functional groups, you must rely on spectral data to determine the functional groups.

Lucas test – alcohols
The Lucas reagent contains ZnCl2 in aqueous HCl, which reacts with some alcohols in an SN1 reaction to produce an alkyl chloride. Which it is first formed it appears cloudy because it is immiscible with water; eventually it separates to form an other layer. The test does not work well on solid alcohols because they do not easily dissolve in the first place, and multifunctional compounds may give unusual results. Phenols also do not react. The test is positive with secondary, tertiary or benzylic alcohols but negative with primary alcohols.

Procedure: Add about 2 drops of a liquid unknown to 0.5 ml of Lucas reagent in a small vial. Shake vigorously, then allow to stand at room temperature. If the alcohol is primary, it will simply dissolve in the reagent. Secondary alcohols will give a cloudy solution and/or separate layer after 2-5 minutes. Tertiary, benzylic, or allylic alcohols will give a cloudy solution and/or a separate layer almost immediately.

Jones test – alcohols
The Jones reagent contains CrO3 in sulfuric acid solution, which oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones. Chromium +6 in the reagent is orange, while the resulting chromium +3 is blue-green. This reaction is also the basis for the breathalyzer test for drunk drivers. Aldehydes will also be oxidized to carboxylic acids, and will also give a positive result. Phenols do not react (or give an unusual reaction) in this test. Procedure: Dissolve about 10 mg (if solid) or 2 drops (if liquid) of unknown in 0.8 ml of acetone in a small vial. Add one drop of Jones reagent and shake. Primary and secondary alcohols react within 10 seconds to form a blue-green precipitate; tertiary alcohols do not react...
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