8. Organic compound identification

Question 116. At three test tubes there are gelatin, glucose and starch solutions (there is one substance at one test tube). In what way is it possible to identify substances?

Answer. Identification is stating equality of some unknown substance with other substance which is known. Usually substance properties are correlated using qualitative reactions typical of this class. Gelatin is protein, glucose is mono-saccharide and starch is homo-poly-saccharide.

Qualitative reaction on starch is blue colour at iodine solution addition. From each test tube it is necessary to pour some part of solution in to new test tubes (pour 1 ml in to clean test tubes). We should add 1-2 drops of iodine solution in to each test tube. At one test tube solution turns blue, it means that starch is in the test tube from which this sample was taken.

Let’s take samples from two left test tubes, add silver oxide ammonia solution to them and heat them on water bath. In the sample with glucose there is silver precipitate – “silver mirror” – it is qualitative reaction on glucose.

So, we identified two substances and we can conclude that in the third test tube there is gelatin. To check our conclusion we should add sodium hydroxide solution to the third test tube and a few drops of copper sulfate (biuret reaction), in the test tube there is violet colour.

You should remember that you should not have qualitative reactions with all initial amount of substances, as after the first reaction there is no substance left for the next qualitative reaction or repeating some reaction if it was not correctly performed.

Question 117. At three test tubes there are oil, glycerol and maltose solutions (in one test tube there is one substance). In what way is it possible to identify these substances?

Answer. We should use qualitative reactions to identify these compounds. Oil is tri-acyl-glycerol, containing higher unsaturated fatty acid residues, maltose is reducing di-saccharide and glycerol is tri-alcohol.

Qualitative reaction on multiple bonds is bromine water discoloration or potassium permanganate solution discoloration. We do not know what is the test tube with oil, that is why, we should pour some part of solution from each test tube into new clean test tubes (i.e. we should take samples to perform reactions). To each sample we add 1 ml of bromine water (or potassium permanganate solution) and mix it. At one of the test tubes there is bromine water or potassium permanganate solution discoloration, it means that this sample is oil.

Let’s take sample once again from two test tubes left, to both of them add silver oxide ammonia solution and heat at the water bath. At the sample with maltose there is silver precipitate – “silver mirror” – it is qualitative reaction on compounds with aldehyde group.

We identified two substances and can come to conclusion that in the third test tube there is glycerol. To check we should mix sodium hydroxide solution and copper sulfate solution at the test tube, as a result copper hydroxide (II) precipitate is formed. After adding few drops of glycerol precipitate is dissolved and there is bright blue colour of the solution, it is glycerol copper complex.

Question 118. At three test tubes there are ethanol, sodium oleate and tartaric acid solutions (in one test tube there is one substance). In what way is it possible to identify these compounds?

Answer. For identification we should use qualitative reactions. Ethanol is saturated mono-alcohol, sodium oleate is higher fatty acid salt, tartaric acid is hetero-functional compound containing two carboxylic groups and two hydroxyl groups at viscenal position (at neighboring carbon atoms).

Qualitative reaction on multiple bonds is bromine water or potassium permanganate discoloration. We do not know what is the test tube with sodium oleate, that is why, from each test tube we should take some part of solution in to three other clean test tubes (i.e. we take samples). To each sample we should add 1 ml of bromine water or potassium permanganate solution and mix. At one test tube there is bromine water or potassium permanganate discoloration, it means that in this test tube there is sodium oleate.

СН₃(СН₂)₇СН=СН(СН₂)₇СОONa + Br₂  СН₃(СН₂)₇СНBr-CHBr(СН₂)₇СОONa

Oleic acid sodium salt 9,10-di-bromo-stearic acid sodium salt

Once again let’s take samples of two left substances and add 2-3 ml of chrome mixture to them. In the sample with ethanol as a result of redox reaction and chrome oxidation degree change there is orange color transformation into green, reaction mass is becoming warmer and there is ethanol oxidation product specific smell. Acetaldehyde has green apple smell.

3С₂Н₅ОН + K₂Cr₂O₇ + 4H₂SO₄  3СН₃CHO + K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O

We identified two substances and can conclude that in the third test tube there is tartaric acid. To check our conclusion we add several drops of potassium hydroxide to the sample from this test tube, shake it intensively and could see gradual formation of white crystal precipitate of potassium hydro-tartrate. To this precipitate we add sodium hydroxide solution in drops until precipitate is completely dissolved. Double tartaric acid potassium-sodium salt solution is formed (signet salt):

After that we should put several drops of copper sulfate solution in to clean test tube, add sodium hydroxide solution till copper sulfate (II) blue precipitate formation, add received precipitate from tartaric acid (signet salt). Precipitate is dissolved and violet colour of copper complex appears in the test tube:

CuSO₄ + 2NaOH  Cu(OH)₂↓+ Na₂SO₄

Question 119. At three test tubes there is acetone, fructose solution and cysteine solution. In what way is it possible to identify substances?

Answer. For identification we use qualitative reactions on these substances. Acetone is ketone, fructose is ketohexose, and cysteine is sulfur-containing amino acid.

Qualitative reaction on ketohexoses is red colour formation at heating with Selivanov’s reagent.

We do not know what test tube is with fructose, so, we should pour some part of solution from each test tube in to new clean test tubes (we take samples to perform reactions). To each sample we should add 1 ml of Selivanov’s reagent, heat the mixture and after heating let the samples stay for 5 minutes. At one of the test tubes there is red colour, it means that test tube with red colour has fructose solution.

Once again we should take samples from two left test tubes and add to each of them 0,5 ml of 2,4-di-nitro-phenyl-hydrasine saturated solution. In the sample with acetone there is yellow precipitate formation of acetone 2,4-di-nitro-phenyl-hydrason.

We identified two substances, so, we can conclude that the third test tube contains cysteine solution. To check our conclusion we add twice as much sodium hydroxide solution, mix the solution and boil it for 1-2 minutes. To the alkaline solution we add several drops of lead acetate (II) and boil it again. There is black precipitate formation.

Question 120. In three test tubes there are solutions of albumin, tyrosine and salicylic acid. In what way is it possible to identify these compounds?

Answer. For identification we use qualitative reactions on these substances. Albumin is a protein, tyrosine is amino acid with aromatic hydrocarbon radical, salicylic acid is aromatic hydroxyl acid containing phenol fragment.

It is well-known that phenols form coloured complexes with iron chloride (III). We do not know what test tube has salicylic acid solution, so, we take samples from each test tube and pour them to new clean test tubes. To each test tube we should add 1 ml of iron chloride (III) and mix. At one test tube solution gets intensively violet, it means that in this test tube there is salicylic acid solution.

Qualitative reaction on amino acids with aromatic hydrocarbon radical is xanto-protein reaction. Once again we take samples of two left substances, add 2-3 drops of concentrated nitric acid, heat it constantly shaking. At one test tube there is yellow colour. Cool the test tube and add 2-3 drops of 1 aqueous solution of sodium hydroxide. Bright orange colour proves tyrosine presence at the solution.

We identified two substances and it means that in the third test tube there is albumin solution (egg white). To prove our conclusion we should add sodium hydroxide solution to the third test tube and we should add several drops of copper sulfate solution (biuret reaction on proteins), in the test tube there is violet colour.

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