Heat the unknown organic compound gently (reflux) with acidified potassium dichromate (VI), which is an oxidising agent, and concentrated H SO in distilling apparatus.
Indication of a positive test
No formation of an aldehyde or a ketone, as tertiary alcohols cannot be oxidised.
4) Test for the presence of a carbonyl group
2,4-dinitrophenylhydrazine (DNPH) test shows a positive test for aldehydes and ketones
It reacts with the carbonyl group of the aldehydes and ketones. Condensation reaction occurs immediately followed by an elimination process.
Add 5 drops of DNPH to 2cm of the unknown compound. Shake the test tube.
A positive test is indicated by:
An orange/red precipitate forms.
5) Test to distinguish between a ketone and an aldehyde
Tollens’ reagent
Shows a positive test for aldehydes. An oxidation reaction takes place. Only the aldehyde will produce a silver mirror on the side of the test tube because, aldehydes are stronger reducing agents and reduce silver (I) ion to silver metal. No reaction with ketones.
Put 1cm of 0.05 moles AgNO3 into a test tube and add 3 drops of sodium hydroxide solution. Drop by drop add ammonia solution until the precipitate of silver oxide dissolves. Add one or two drops of the unknown compound, shake the tube gently and place in a beaker of warm water.
A positive test is indicated by:
A silver mirror is produced on the sides of the test tube from the colourless solution.
6) Test for an alcohol (-OH) group
Add three drops of the compound to be tested to 10drops of an acidified cerric ammonium nitrate solution (a yellow solution)
The alcohol complexes with the cerric nitrate ion
A positive test is indicated by:
The immediate formation of red or red-brown colour indicates a positive test. If the unknown compound is not soluble in water, two layers will be present. A red colour in either layer will indicate a positive test.
7) Test for the presence of a carboxylic acid:
Sodium bicarbonate test
An acid/base reaction occurs. Sodium bicarbonate reacts with the carboxylic acid to give off carbon dioxide gas.
Add a few drops of the unknown compound to 1cm of methanol and slowly add to 1cm of a saturated solution of sodium bicarbonate.
A positive test is indicated by:
Effervescence occurs and the gas evolved is carbon dioxide (test by bubbling into limewater) and a carboxylic acid group is present.
8) Test for the presence of ester
Using phenolphthalein indicator and then refluxing with dilute acid or alkali can identify esters. There should be a colour change from colourless to pink and after warming in water if ester is present the colour should change from pink back to colourless.
Add a few drops of the unknown substance to 2cm of water. Add a drop f phenolphthalein indicator, shake, then add sodium hydroxide solution drop by drop, until the indicator just changes colour. Allow solution to stand in a beaker of hot water for a few minutes.
A positive test is indicated by:
After leaving the solution to stand the solution turns colourless, this indicates the presence of an ester.
9) Test for the presence of a phenol
The iron chloride test.
Phenols are compounds, which contain a hydroxyl group attached to an aromatic ring. Phenols form a complex with the iron ion.
Add 5 drops of FeCl solution (a yellow solution) to a test tube containing 2 drops of the unknown compound.
A positive test is indicated by:
The solution changes colour to a purple.
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Risk Assessment
- Phenolphthalein can cause skin contamination, always wear rubber gloves when handling.
- Sodium Hydroxide is corrosive and an irritant. Handle carefully by wearing gloves and goggles
- Potassium dichromate (VI) is toxic if inhaled, handle carefully and do not leave bottles uncapped.
- Iron (III) chloride is irritating to eyes and skin so wear gloves and protective goggles.
- 2,4 DNPH is toxic by skin contact.
- Organic substances are flammable so all heating should be carried out in a water bath.
- Clean up all chemical spills immediately, and dispose of all waste as directed.
Apparatus
- 10cm measuring cylinder
- Teat pipette
- Test tube rack
- Water bath
- Small beaker
- Distilled water
- Large beakers
Reagents
- 2,4-dinitrophenylhydrazine (DNPH) (aq)
- 20 drops of 2M ammonia solution
- 20 drops of 2M sodium hydroxide solution
- Limewater
- 5 drops Iron Chloride (5% FeCl )
- 5 drops Phenolphthalein indicator
- 5 drops Bromine solution
- Potassium dichromate (VI) (aq)
- Concentrated sulphuric acid (aq)
- 1cm 0.05 moles Silver nitrate solution
- 10 drops of acidified cerric ammonium nitrate solution
- 1cm methanol (aq)
- 1cm saturated solution of sodium bicarbonate
Bibliography
chemistry2- by Brian Ratcliff and Helen Eccles
Advanced practical chemistry- by Alec Thompson and Lambros Atteshils
