Organic observation exercise – II

In this exercise you will be assessed on your observations of a series of reactions and your ability to interpret these observations.

You are provided with two liquids, X and Y, which are ISOMERS of each other and contain only the elements carbon, hydrogen and oxygen.

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Place about 5cm3 of 2, 4 – dinitrophenylhydrazine reagent in each of two tubes. To one add several drops of the compound X, to the other add several drops of compound Y.

Compound X

OBSERVATIONS

INFERENCES

The colourless compound X forms orange-yellow crystals when combined with 2, 4-DNPH

2, 4-DNPH reacts with carbonyl groups to give orange coloured precipitates. The test gave a positive result when it produced orange-yellow crystals. This identifies the presence of the reactive C=O bond and that compound X is either an aldehyde or a ketone.

Compound Y

OBSERVATIONS

INFERENCES

The orange coloured 2, 4-DNPH when combined with compound Y forms a faint yellow crystalline precipitate.

2, 4-DNPH reacts with carbonyl groups to give orange coloured precipitates. The test gave a positive result when it produced yellow crystals. This identifies the presence of the reactive C=O bond and that compound Y is either an aldehyde or a ketone.

Place about 0.25cm3 of the compound in a test tube and add 4cm3 of iodine in potassium iodide solution. Add sodium hydroxide solution dropwise with shaking until the colour of the iodine has discharged.

Compound X

OBSERVATIONS

INFERENCES

The brown solution of iodine is combined with an excess of the colourless NaOH and compound X to produce a highly exothermic reaction with a slight smell of vinegar.

No visible colour change suggests that there is no CH3CO group in the compound. Compound X is likely to be an aldehyde other than ethanal, as it did not prove positive for this test.

Compound Y

OBSERVATIONS

INFERENCES

The brown solution of iodine is combined with 1 drop of NaOH and compound Y to produce a beige-yellow colour which after discharging smells of anti-septic. The precipitate appeared readily in a cold reaction.

The iodoform reaction is a useful test for the CH3CO group in carbonyl compounds. An alkaline solution of aqueous iodine when combined with compound Y is cold during the reaction. When the yellow precipitate of triiodomethane was produced, it showed that the presence CH3CO was highly likely. Compound Y could be propanone.

To about 5cm3 of silver nitrate add several drops of sodium hydroxide solution. Allow the precipitate to settle and then decant as much of the liquid as possible. Add ammonia solution dropwise to the remaining precipitate until it just dissolves. Divide this solution in half and use it in the following tests. To one sample of the reagent add two drops of compound X. To the other tube add two drops of compound Y. Place both tubes in a beaker of hot water and leave for a few minutes.

Compound X

OBSERVATIONS

INFERENCES

The combination of silver nitrate with NaOH produces a murky brown solution, which after decantation and on the addition of ammonia dissolves the precipitate. Adding compound X and heating in a water bath produces a silver mirror.

Tollens reagent provides yet another way of testing for aldehydes. This time the oxidising agent is a complex of silver(I) ions, which are reduced to silver in the test forming a ‘silver mirror’. This test shows that X is a good reducing agent and as aldehydes reduce an ammoniacal solution of silver nitrate to silver the compound is likely to be an aldehyde.

RCHO + 2Ag+ + H2O RCO2H + 2Ag(s) + 2H+

Compound Y

OBSERVATIONS

INFERENCES

The combination of silver nitrate with NaOH produces a murky brown solution, which after decantation and on the addition of ammonia dissolves the precipitate. Adding compound Y and heating in a water bath did not produce a visible colour change.

Ketones cannot normally be oxidised and they do not react with Tollens’ reagent. The reaction did not produce a visible colour change and so supports the theory that compound Y is the ketone, propanone.

Place about 1cm3 of potassium manganate (VII) solution into each of two test tubes. To one add a few drops of compound X to the other a few drops of compound Y. Shake both tubes.

Compound X

OBSERVATIONS

INFERENCES

The solution of KMnO4 changes colour from purple to brown when compound X is added and a sharp a smell is produced.

Compound X is a strong reducing agent it is capable of reducing the manganate ion from its +7 state to its +2 state. Aldehydes are strong reducing agents and I believe compound X is propanal.

Compound Y

OBSERVATIONS

INFERENCES

The reaction was very slow, the potassium manganate (VII) solution did not appear to change colour but after a long time the solution was slightly paler.

Compound Y is capable of reducing the potassium manganate (VII) but at a much smaller rate, adding a strong acid like H2SO4 would improve the result. I believe compound Y is a ketone, specifically propanone as this compound would produce results that correspond to my observations.

The diagrams on the next page show details for the infrared spectrum and the mass spectrum for compound Y:

A table of characteristic absorptions for some different bonds is given below:

Bond

Absorption / cm-1

Bond

Absorption / cm-1

C – Cl

600 – 800

C – O

1000 – 1300

C = C

1620 – 1680

C = O

1680 – 1750

C C

2100 – 2250

N – H

3320 – 3560

C – H

2850 – 3300

O – H 2500 – 3500

(Broad peak)

CN

2210 – 2260

By considering the infrared spectrum, explain why compound Y may be an aldehyde or a ketone but not a carboxylic acid.

For compound Y to be a carboxylic acid it would have to satisfy the criteria of peaks for the C-O, C=O and O-H bonds as shown on the graph at different absorption wavelengths corresponding to frequencies of infrared waves.

From the mass spectrum, deduce the relative molecular mass of Y and suggest a likely structure corresponding to the peak at m/e = 43

Devise a simple chemical test and carry it out to find out if a compound X is a carboxylic acid.

TEST

With carbonates or hydrogen carbonates, aqueous solutions of the acids generate sufficient ions to give off CO2.

CO32-(aq) + C2H5COOH(aq) C2H5COO-(aq) + H2O(l) + CO2(g)

If compound X is an acid then it will produce effervescence when it is reacted with sodium hydrogen carbonate and CO2 gas will be released. If it is an aldehyde then there should be no visible reaction. It is my intention to perform this devised to test to discover the possible identity of the compound.

OBSERVATION

Compound X produced a slight effervescence when it reacted with sodium bicarbonate. This produced a gas, which was colourless by nature and had no distinct smell.

INFERENCE

Although my results show the possible existence of a carboxylic acid it is known that after prolonged exposure to the air, the molecules of an aldehyde are oxidised to the carboxylic acid. This acid, which I suspect to be propanoic acid, would have sufficient strength to react with the NaHCO3 to produce CO2 gas and give a false indication as to the existence of the aldehyde. I still believe that compound X is an aldehyde. The following diagram shows the progression from a primary alcohol to aldehyde and finally to a carboxylic acid by oxidation:

R-CH2-OH R-CHO R-COOH

(primary alcohol) (aldehyde) (carboxylic acid)

Use the information you have obtained from the other parts of this question to deduce structural formulae for compounds X and Y and give their systematic names.

Structure of compound X:

Systematic name for compound X: Propanal

Structure for compound Y:

Systematic name for compound Y: Propanone

Chemistry Coursework Nick Curum

Organic observation exercise – II 1 27/04/2007