Testing for functional groups
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Explainer Video
Summary
Testing for functional groups
In a nutshell
There are tests for functional groups including; alkenes, haloalkanes, phenols, carboxylic acids, carbonyl, aldehydes and primary and secondary alcohols. The tests listed can be seen with the naked eye.
Purpose of testing for functional groups
The purpose of testing for functional groups is crucial as it can tell you what your compound is as well as what properties it may have. A variety of tests can be used on a single compound as it may have more than one functional group.
Test for alkenes
Bromine water can be used to test for alkenes. When you are testing for alkenes you are testing for the presence of a double bond.
How to test for alkenes
Procedure
| 1. | Add of the substance that you want to test for in a tube. |
| 2. | Add of bromine water to the test tube (A) |
| 3. | Cover and shake the test tube. |
| 4. | A positive result will decolourise the solution from orange to colourless. (B) |
Test for haloalkanes
When testing for haloalkanes the presence of bromine, chlorine or iodine present in an organic compound is being tested for. Fluorine is not included because it bonds very strongly to alkanes and other tests are required to test for fluorine.
How to test for haloalkanes
Procedure
| 1. | Add five drops of the unknown compound to a test tube. |
| 2. | Add of ethanol and of aqueous silver nitrate to the test tube. |
| 3. | Heat the test tube lightly. If a haloalkane is present a precipitate will form. |
Haloalkane | Precipitate colour |
| Chloroalkane | White |
| Bromoalkane | Cream |
| Iodoalkane | Yellow |
Test for phenols
Sodium hydroxide can be used to test for phenols. When testing for phenols what is actually being tested is the fact that phenols are only slightly acidic in their nature.
How to test for phenols
The first test will only test for strong acids as sodium hydroxide is a strong base and will react with all acids, strong or weak. Now a test should be carried out with a carbonate, these are much weaker bases than sodium hydroxide and will only react with a strong acid, not a weak acid like phenol.
Procedure
| 1. | Add of an unknown substance to the test tube. |
| 2. | Add of sodium hydroxide . |
| 3. | If phenol is present a colourless solution of a sodium salt will form. |
| 4. | In a new test tube add of the unknown substance. |
| 5. | Add in of sodium carbonate. |
| 6. | If effervescence occurs it is likely that a strong acid is present and not a phenol. |
Test for carboxylic acids
Carboxylic acids are weak acids but they are more acidic than phenols. Therefore, carboxylic acids are able to react with metal carbonates to form a salt and carbon dioxide.
How to test for carboxylic acids
Procedure
| 1. | Add of the unknown substance. |
| 2. | Add of a carbonate (preferable sodium carbonate). |
| 3. | If the solution begins to fizz, bubble through limewater (saturated solution). |
| 4. | If the solution turns cloudy, this indicates the presence of carbon dioxide . |
Test for carbonyl
To test for carbonyls -dinitrophenylhydrazine . This is used to test for aldehydes and ketones.
How to test for carbonyls (aldehydes and ketones)
Procedure
| 1. | Dissolve of in of sulfuric acid, of water and of methanol. |
| 2. | In a different test tube, add of your solution and add a few drops of your unknown solution. |
| 3. | Shake the test tube and wait for a bright orange precipitate for a positive result. |
Test for aldehydes
Tollens reagents can be used to test for aldehydes. This test can be done after the test for carbonyls to identify whether an aldehyde or ketone is present in your solution.
How to test for aldehydes
Procedure
| 1. | Add of silver nitrate solution in a test tube. |
| 2. | Add five drops of of sodium hydroxide solution. |
| 3. | Add drops of dilute ammonia solution until the brown precipitate dissolves completely. |
| 4. | Place the test tube in a hot water bath and add drops of an unknown solution. |
| 5. | Wait a few minutes. |
| 6. | Aldehydes form a silver mirror on the walls of a test tube. |
| 7. | If no silver mirror is formed, ketone present. |
Test for primary and secondary alcohols
For primary alcohols, the orange solution turns green as an aldehyde forms. If you continue heating (vigorously), the aldehyde will be oxidised to a carboxylic acid. If secondary alcohols are present, the orange solution turns green as a ketone forms. If tertiary alcohols are present nothing happens.
How to test for primary and secondary alcohols.
Procedure
| 1. | Add an alcohol and of acidified potassium dichromate solution to a test tube. |
| 2. | Warm the mixture gently in a hot water bath. Observe for a colour change. |
FAQs - Frequently Asked Questions
How can carbonyls be tested for?
Carbonyls are tested for using (2,4 - DNP), sulfuric acid, water and methanol. A positive result gives a bright orange precipitate.
What colour precipitates form when a haloalkane is present?
When chlorine is present a white precipitate is formed. Bromine forms a cream precipitate. Iodine forms a yellow precipitate.
How do you test for alkenes?
Testing for alkenes involves adding bromine water to your unknown organic compound and shake. If the solution goes colourless then alkenes are present.