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Testing for functional groups

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Explainer Video

Tutor: Alexander

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 $2cm^3$ of the substance that you want to test for in a tube.
2.	Add $2cm^3$ 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)

Chemistry; Analysis; KS5 Year 12; Testing for functional groups

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 $1cm^3$ of ethanol and $1cm^3$ of aqueous silver nitrate $(AgNO_3)$ 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 $(NaOH)$ 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 $2cm^3$ of an unknown substance to the test tube.
2.	Add $2cm^3$ of sodium hydroxide $(NaOH)$ .
3.	If phenol is present a colourless solution of a sodium salt will form.
4.	In a new test tube add $2cm^3$ of the unknown substance.
5.	Add in $2cm^3$ 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 $2cm^3$ of the unknown substance.
2.	Add $2cm^3$ of a carbonate (preferable sodium carbonate).
3.	If the solution begins to fizz, bubble through limewater (saturated $Ca(OH)_2$ solution).
4.	If the solution turns cloudy, this indicates the presence of carbon dioxide $(CO_2)$ .

Test for carbonyl

To test for carbonyls $2,4$ -dinitrophenylhydrazine $(2,4-DNP)$ . This is used to test for aldehydes and ketones.

How to test for carbonyls (aldehydes and ketones)

Procedure

1.	Dissolve $0.2g$ of $2,4-DNP$ in $1cm^3$ of sulfuric acid, $2cm^3$ of water and $5cm^3$ of methanol.
2.	In a different test tube, add $2cm^3$ 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 $2cm^3$ of $0.10mol\ dm^{-3}$ silver nitrate solution in a test tube.
2.	Add five drops of $0.80mol\ dm^{-3}$ 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 $10$ 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 $2cm^3$ of acidified potassium dichromate $(K_2Cr_2O_7)$ solution to a test tube.
2.	Warm the mixture gently in a hot water bath. Observe for a colour change.

Learn with Basics

Length:

Unit 1