RCHO + [O] RCOOH Aldehyde is oxidised
Ag+ + e- Ag Silver is reduced
If the carbonyl is an aldehyde, a layer of metallic silver mirror will form in the test tube. Ketones can’t be oxidised under normal conditions, thus there will be no reaction. Therefore, this test will distinguish whether the carbonyl is an aldehyde or a ketone.
To test for alcohols, I will oxidise the unknown compound with excess potassium dichromate (oxidising agent) and concentrated sulphuric acid. The mixture will also be refluxed. This process is the complete oxidation of alcohols. However, only primary and secondary alcohols can be completely oxidised and the positive results will be a colour change from orange to green. The colour change is the reduction of orange dichromate (IV) ions to green chromium (III) ions.
Therefore, the colour change will only indicate if the unknown compound a primary alcohol. The general equation for this reaction is shown below;
Primary alcohol (aq) + 2[O] RCOOH (aq) + H2O (l)
At this stage, there will only be two possibilities for the unknown compound. These are tertiary alcohol and an ester.
To test for a tertiary alcohol, I will dehydrate the unknown compound as it will remove an OH group from the alcohol. Overall a molecule of water, H2O will be removed from the compound, thus forming a double bond. To dehydrate the unknown compound, I will pass the vapours of the unknown compound through aluminium oxide and collect the gas in a different test tube. This gas should be an alkene if the unknown compound is a tertiary alcohol. Therefore, Al2O3 is a dehydrating agent. The equation for this test is shown below;
Al2O3
600K
Tertiary alcohol Alkene + Water
After dehydration, I can then perform the bromine water test to test for tertiary alcohol. Decolouring of bromine water will identify a tertiary alcohol is present. However, if it doesn’t decolourise than I can assume that the unknown compound is an ester by the process of elimination.
Risk Assessments
Whilst carrying out any of these tests, safety glasses, gloves and lab coats must be worn at all times.
- Bromine water is classified as harmful and it should be handled very carefully. Bromine vapours are released when left in open air, this can be harmful if you inhale the vapours and asthmatics should specifically be careful. Therefore, do not leave bromine water in open air. Also do not expose bromine water to skin as it may cause burns. Any contact with skin should be washed off with plenty of water immediately.
- Phenol is toxic when inhaled and highly corrosive. Therefore, it must be handled very carefully as it can cause severe burns when in contact with skin and can cause lung damage if inhaled. It must be used in a well ventilated laboratory. Any contact with skin should be washed off with plenty of water and soap. If inhaled, than seek medical help immediately.
- Sodium carbonate solution is irritant to eyes although it doesn’t pose a great danger. If contact is made with eyes, than wash it off with plenty of water.
- Brady’s reagent, 2,4-DNPH is classified as harmful and highly flammable. It is also explosive when dry; it should be treated with caution. Therefore, do not place it near flammable items and if contacted with skin, than it must be washed off with plenty of water.
- Potassium dichromate is highly toxic and harmful if in contact with skin or inhaled. It can also act as a carcinogen, thus may cause reproductive defects. This chemical is also an oxidizing agent and may react violently with reducing agents and flammable materials.
- Concentrated Sulphuric acid is harmful and extremely corrosive. Contact with eyes or skin may cause permanent damage and so should be washed off immediately with plenty of water if contact is made.
- Tollen’s reagent (silver nitrate dissolve in aqueous ammonia) is corrosive and harmful if contact is made with eyes. Wash it off with plenty of water immediately if in contact with eyes.
Apparatus for every test:
Unknown compound Bunsen burner
Bromine water Heat proof mat
Sodium carbonate solution Test tubes
Limewater Plastic pipettes
Brady’s reagent 50cm3 beaker
Tollen’s reagent 50cm3 measuring cylinder
Potassium Dichromate 5ml syringe
Sulphuric acid 250cm3 beaker
Aluminium Oxide Double neck pear shaped flask
Anti-bumping granules
Dropping funnel
Cotton wool
Test tube bung
Method of Each Test
-
Test for alkenes: add 5 drops of bromine water using plastic pipette into 10cm3 of the unknown compound in a 50cm3 beaker and shake the mixture.
Alkene (aq) + Br2 (aq) Dibromoalkane
The unknown compound can be indentified as an alkene if bromine water is decolourised.
-
Test for Phenols: add 5 drops of bromine water, one drop at a time, using a plastic pipette into 10cm3 of the unknown compound and shake the mixture.
Phenols (aq) + 3Br2 (aq) 2,4,6 – tribromophenol (s) + 3HBr (aq)
After shaking, formation of white precipitant will show that the unknown compound is a phenol.
-
Test for carboxylic acid: before this reaction takes place, set the apparatus such that CO2 gas can be collected into a separated vessel.
Then add 25cm3 of the unknown compound using a 50cm3 measuring cylinder and add it to a 50cm3 beaker. Then add 5cm3 of Na2CO3 using a 5ml syringe into the beaker.
I will then add 5cm3 of limewater into CO2 from my collected gas; a white cloudy precipitate should form indicating the presence of CaCO3. This shows that the unknown compound is a carboxylic acid.
Reaction between RCOOH Na2CO3 is shown below;
RCOOH (aq) + Na2CO3 (aq) Sodium carboxylates (aq) + H2O (l) + CO2 (g)
The following equation shows the reaction between limewater and CO2;
Ca(OH)2 (aq) + CO2(g) CaCO3 (s) + H2O (l)
-
Brady’s reagent: using a 50cm3 measuring cylinder, add 25cm3 of the unknown compound into a 50cm3 beaker. Than add 5 drops of Brady’s reagent using a plastic pipette into the unknown compound. Orange crystals should form if the unknown compound is a carbonyl.
-
Tollen’s reagent: using a 50cm3 measuring cylinder, add 20cm3 of Tollen’s reagent into a test tube. Than add 5 drops of the carbonyl found in step 4 using a plastic pipette into the test tube.
Place the mixture in a water bath with a constant temperature of 60◦C and wait for 10 minutes. After, if a silver mirror appears in the test tube, than a conclusion can be drawn that the unknown compound is an aldehyde. If the silver mirror does not appear, than the unknown compound is a ketone. The following reaction would occur if the carbonyl is an aldehyde:
RCHO + [O] RCOOH
This reaction shows that the aldehyde is oxidised.
- Oxidation:
-
Put 5cm3 of water into a 25cm3 beaker and gradually add 8cm3 of concentrated sulphuric acid. Swirl the mixture to ensure complete dilution.
- Add 10g of potassium dichromate to the solution ensuring it is fully dissolved.
- Put the mixture into a double neck pear-shaped flask and add some anti-bumping granules.
-
Then add 5cm3 of the unknown compound and 15cm3 of water into the dropping funnel. Make sure the dropping funnel valve is closed at this point.
- Pass cold water through the condenser into the dichromate mixture and slowly open the dropping funnel valve to allow the mixture of unknown compound and water to drip into the flask.
- Reflux the flask for 10 minutes.
- Observe the reaction.
As the mixture of the unknown compound and water is mixed with the potassium dichromate solution, the colour of potassium dichromate will change from orange to green. This observation will identify that a primary alcohol is present.
- Test for Dehydration:
Set the apparatus as shown below.
Soak 2cm3 of the unknown compound with cotton wool and place it at the bottom of the test tube. Than align the test tube horizontally as shown below and place 1cm3 of aluminium oxide in the middle of the test tube.
Once the apparatus is placed, heat the aluminium oxide granules with a gentle flame. This will cause vapours from the unknown compound to pass through and collect into the other test tube.
As the alkene gas is collected, bung the test tube for the next part of the experiment (addition of Bromine water).
Picture taken from
The reaction of dehydration of a tertiary alcohol is shown below.
Al2O3
600K
Tertiary alcohol Alkene + Water
- Bromine water test:
After the alkene gas has been collected, carefully open the bung and immediately add 5 drops of bromine water. Put the bung back in place and shake the test tube.
If the unknown compound is a tertiary alcohol, than bromine water should decolourise. If it hasn’t decolourised, than through the process of elimination, we can conclude that the unknown compound is an ester.
Bibliography:
-
Alkenes and its reactions, date accessed 13th October 2008,
Nelson Thornes, Essential AS Chemistry, Reactions of Alkenes, page 98. Date accessed 13th October 2008
-
Phenols and its reactions, date accessed 13th October 2008,
Nelson Thornes, Essential A2 Chemistry, Reactions of Phenol, pages 15 – 18. Date accessed 13th October 2008.
Michael Clugston, Advanced Chemistry, Halogenation of Arenes, page 424. Date accessed 13th October 2008.
-
Carboxylic acids and its reactions, date accessed 13th October 2008,
Nelson Thornes, Essential A2 Chemistry, Acid reactions, pages 26 – 27. Date accessed 14th October 2008.
Nelson Thornes, Essential AS Chemistry, Limewater test, page 63. Date accessed 14th October 2008.
-
Carbonyls and its reactions with Brady’s reagent and Tollen’s reagent, date accessed 14th October 2008,
Nelson Thornes, Essential A2 Chemistry, Identifying a carbonyl compound. Date accessed 14th October 2008.
Michael Clugston, Advanced Chemistry, Oxidation of Aldehydes, page 466. Date accessed 14th October 2008.
-
Oxidation of Alcohols, date accessed 15th October 2008.
-
- backed up by the link below;
Nelson Thornes, Essential AS Chemistry, Selective Oxidation, pages 110 – 111. Date accessed 15th October 2008.
-
Dehydration of Alcohols, date accessed 16th October 2008.
Nelson Thornes, Essential AS Chemistry, Dehydration, page 109. Date accessed 16th October 2008.
-
Esters, date accessed 16th October 2008.
Nelson Thornes, Essential AS Chemistry, Esters, pages 27-28. Date accessed 16th October 2008.
-
Risk Assessments, date accessed 16th October 2008.
