Chemical Tests for Unknown Functional Groups
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
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