RCOOH + NaHCO3 →RCOONa(aq) + CO2(g) +H2O(l)
The gas here is the carbon dioxide, which is then bubbled through the limewater in this reaction:
Ca(OH)2 + CO2 →CaCO3(s) + H2O
3rd Test:
This test shows if a carbonyl group is present in the unknown compound. For this test we use 2,4 dinitrophenylhydrazine (2,4 DNPH) and the organic unknown. Put around 4cm3 2,4 DNPH in a test tube and drip the unknown into it, just a few drops. If a carbonyl group is present then we should see a yellow/orange precipitate in the tube. However, this test is not always reliable and so we should heat the compound gently for a few minutes and then leave it for 3mins to see if a precipitate is formed then.
If there is still no precipitate formed then you have no carbonyl group, and this leaves only an alcohol and an ester group which may be present in the compound. If a precipitate is formed then go on to test 4 if not then go on to test 5.
4th test:
This test will determine which carbonyl compound we have, either an aldehyde or a ketone. Firstly we take some of the unknown compound (approx. 5cm3) and put it in a test tube. Then add the tollens reagent and place the test tube in a boiling water bath. Leave it to stand for around 5mins. If there is a sliver mirror formed on the side of the test tube then the compound was an aldehyde, if there is no silver mirror then the compound is a ketone. This works due to the fact that the hydrogen on the carbon with the C=O bond in an aldehyde is easily oxidised and so is a good reducing agent. The Ag2+ ions in the tollens solution are therefore reduced by an aldehyde into Ag atoms, which form a silver coating on the test tube.
RCH2O + H2O + 2Ag2+ → RCOOH + 2Ag + 2H+
5th test
This test involved two steps, firstly you oxidise your solution, and then do another test as test 3. You put 5cm3 of your unknown organic solution in a test tube with around 5cm3 NaCr2O7 (sodium dichromate) and an excess of H2SO4.Then place this reaction mixture in a hot water bath and when the dichromate changes colour remove the tube from the water bath and cool with water to prevent further oxidation.
If the compound is an alcohol then the reaction will do this:
RCH2OH + [O] → RCH2=O
Then you can drop a few drops of this solution into 2,4 DNPH and if a precipitate is formed then you have a ketone or aldehyde, which means that the compound that you oxidised was an alcohol. If this test is negative and has no precipitate then you had an ester in the tube, as all other possibilities have been eliminated. You can confirm that the compound is an ester by the fruity smell, or y adding sodium to the solution, if there is a reaction then the unknown compound is an alcohol, if not then it is an ester as all other possibilities have been ruled out.
Safety:
Wear goggles throughout as you do not know what the unknown compound is.
Take care when handling the concentrated H2SO4 and the NaCr2O7.
Be careful not to get any of the 2,4 DNPH on your hands.
Keep the Bunsen burner away from the organic compound as it may be flammable.
Bibliography
- Doc browns chemistry clinic
Chemistry in context pages: 475, 488,
