If an aldehyde is the organic unknown compound, a ‘silver mirror’ will form on the test-tube. However if no silver deposits are observed then a ketone will be identified as the unknown chemical.
Test 4: Carboxylic acids/alcohol
Test 4 involves adding the unknown sample to 0.5g of solid sodium. Both carboxylic acids and alcohols can react with sodium to form separate products with hydrogen gas.
The presence of the hydrogen gas can be tested using the classic ‘squeaky pop’ test where a lighted splint is put into the hydrogen filled test-tube. If this test is positive, an alcohol or carboxylic acid is present. Continue to test 5 to distinguish between carboxylic acids and alcohols.
Test 5: Carboxylic acid/alcohol
If test 4 is positive, an alcohol or a carboxylic acid is the unknown chemical. To determine which of the two compounds the unknown chemical is, a few drops of the unknown will be added to blue litmus paper. The change in colour of the litmus paper will indicate the identity of the unknown compound.
If the unknown compound is a carboxylic acid, the colour of blue litmus paper will turn red (indicating acidity). If the unknown compound is an alcohol, the colour of the litmus paper is expected to remain blue.
A chemical test to detect the presence of an ester has not been described above due to the absence of such a test. Therefore, if the five chemical tests described above produce negative results, this will indicate that a carboxylic acid, alcohol, aldehyde, ketone, phenol is not present. A process of elimination will be used to conclude the unknown compound is an ester.
Analysis
A negative result was observed in test 1 for a phenol as no decolourisation of bromine water occurred and no white precipitate formed. This indicated the absence of a phenol and after this, test 2 was carried out.
Test 2 using orange 2,4-dinitrophenylhydrazine solution was positive as an orange/red 2,4-dinitrophenylhydrazone precipitate was observed.
Following this, test 3 was also positive as adding of Tollens reagent (ammonical silver nitrate) to the unknown compound resulted in a ‘silver-mirror’ forming on the test-tube. A ‘silver-mirror’ formed on the test-tube as the silver ions (Ag+) in the ammoniacal silver nitrate is reduced to silver metal (Ag), which forms on the test-tube (oxidation number of silver fell from +1 → 0). Whilst the silver ions were undergoing reduction, the aldehyde was oxidised to a carboxylic acid. Oxidation and reduction processes occurring simultaneously indicate a redox reaction.
Ag+ → Ag (reduction)
−CHO + [O] → −COOH (oxidation)
No further chemical tests were carried out.
Due to observations during chemical tests, a simple conclusion can be drawn that Compound B is an aldehyde (a carbonyl compound).
Another chemical test that could be used to confirm the organic product would be Fehling’s solution (solution of copper (II) sulphate with an alkaline solution of a salt of 2,3dihydroxybutanoic acid). Fehling’s solution has a bright blue colour and when added to an aldehyde there is a colour change of the solution from blue → brick red precipitate. If a brick red precipitate forms, this chemical test is positive and this result would emphasise that unknown compound is an aldehyde.
A further test that could be carried out would involve recrystallisation of the orange/red 2,4-dinitrophenylhydrazone precipitate that was formed in test 2. Following recrystallisation, the melting point of this precipitate would be found. This extension of test 2 would show the exact aldehyde, compound B is (as melting points of aldehydes are unique).
An infrared, NMR and mass spectrum have been provided. Analysing these three spectra will help confirm the findings of the tests and help to deduce the exact structure of the aldehyde (compound B).
On the mass spectrum, the peak that has the largest mass to charge ratio, is the molecular ion peak (M+), which has a mass that is equal to the Mr of the compound. The molecular ion is formed when the molecule is ionised (removal of one electron). After observation of the mass spectrum provided it is clear that the molecular ion and hence the Mr of the aldehyde is 44. Knowing the Mr should help in deducing the structure of the aldehyde (types and number of atoms of each element present).
Infrared spectroscopy is a technique that identifies the types of functional groups present in an organic molecule. Different functional groups absorb at certain frequencies of infrared radiation. The absorption peaks on an infrared spectrum can help identify the functional groups present in a compound. After analysing the infrared spectrum provided, it is clear that there is a strong sharp absorption peak at approximately 1720 cm-1. This peak indicates the presence of a C=O functional group that is found in aldehydes and ketones. The finding from the infrared spectrum that a C=O group is present supports the results of the chemical tests however an infrared spectrum cannot distinguish between aldehydes and ketones.
Nuclear magnetic resonance spectroscopy (nmr) is used to determine the structure of a compound. Peaks on an NMR spectrum occur at different chemical shift values (ppm). The position of the peaks gives information about what proton-containing groups are present. The relative height of the peak provides information on the number of protons (hydrogen atoms) in the group. The splitting pattern of each peak shows the number of protons adjacent to each group. Overall an NMR spectrum provides information on the different groups and how these groups are attached/bonded together in a molecule.
The NMR spectrum provided gives the following information:
Chemical shift (ppm) Relative number of protons Splitting pattern
2.1 3 doublet
9.8 1 quartet
From the splitting pattern of the two peaks in the spectrum the following points can be deduced: To produce a quartet, the group at chemical shift 9.8 must be adjacent to a carbon atom with three protons. The peak at 2.1 is split into a doublet, which suggests it is adjacent to a carbon atom with 1 proton.
Chemical shift Type of proton
2.1 -CH3
9.8
Taking all the information from the NMR spectrum, I suggest that the displayed formula of compound B is:
Ethanal (aldehyde)
The Mr of ethanal is 44 and this is equal to the Mr taken from the mass spectrum. Mr of ethanal (C2H4O): (12 x 2) + (16 x 1) + (1x 4) = 44
Information from all three spectra support the original findings of the chemical tests and have given further details concerning the structure of the aldehyde allowing the exact identity of compound B to be established (ethanal).
Evaluation
The chemical tests were chosen, as they were sensible and easy to carry out in the laboratory and for this reason, the procedure can be regarded as suitable.
Anomalous Results
From following the flow chart compound B was shown to be an aldehyde. The outcomes of the chemical tests proved the unknown compound is an aldehyde. All available pieces of evidence support that compound B is an aldehyde and therefore, there are no anomalous results to comment upon.
The limitations of the experimental procedure are outlined below:
1. The results of chemical tests show the functional groups that are present and the functional groups that are absent in compound B, however do not provide any other details of the structure of the unknown compound for example, they do not provide:
* Details on the number of carbon atoms in the carbon chain
* Position of functional group in the molecule
* Whether the compound is aromatic or aliphatic.
The evidence from the chemical tests show that compound B is an aldehyde, for example, no decolourisation of bromine water with a white precipitate occurred in test one and this proved that a phenol was not present. Then the orange/red precipitate that formed when the 2,4-dinitrophenylhydrazine was added indicated that an aldehyde or ketone was present. The final experiment with tollens reagent (ammoniacal silver nitrate) proved that compound B was an aldehyde as a ‘silver-mirror’ formed on the test-tube.
2. A further limitation of the chemical tests is the proneness of human error and miss-judgement of the colour of the solutions. In test 3, a very faint silver-mirror was observed making observations very subjective and therefore this made it very difficult to make inferences.
All the evidence from the NMR, infrared and mass spectrum suggests that compound B is the aldehyde ethanal. Several limitations of the provided spectra are described below.
NMR:
Evidence from the NMR spectrum in particular show that compound B is ethanal. This is evident from the two peaks found at chemical shifts 2.1 and 9.8. The height of the peak at 2.1ppm suggests that the group has three protons and the chemical shift value supports this as it shows that it is a −CH3 group. The peak with a chemical shift of 9.8ppm has one proton and after checking the chemical shift value against the data sheet, it appears that a group is present. The −CH3 peak at 2.1ppm is split into a doublet, which indicates that this −CH3 group is adjacent to a carbon atom that is bonded to one proton ( group) The peak at 9.8ppm has a quartet splitting pattern that means that the group is attached to carbon atom that is bonded to three protons (the −CH3 group).
Therefore, the NMR has provided evidence on the number and types of proton-containing groups that are present as well as how these groups are bonded together.
The chemical shift values have provided information what types of proton-containing group are present. However, a limitation of NMR is that of these peaks at the chemical shift values are sensitive and are influenced by: type of solvents, other proton groups and concentrations.
Infrared spectroscopy is similar to the chemical tests, in that it only allows us to determine which functional groups are present in the organic compound. A limitation of an infrared spectroscopy is that it does not give any indication of the overall formula or structure of the compound (for example it does not give information on the number of carbon atoms, or whether the compound is aromatic or aliphatic).
The infrared spectrum provided has a sharp strong absorption peak at approximately 1700 cm-1 (shown by data sheet) and this peak indicates the presence of a C=O group. The absence of any other peaks in the spectrum lead to the conclusion that a carbonyl compound is present, either an aldehyde or a ketone. A second draw back of the infrared spectrum is that it cannot distinguish between aldehydes and ketones and so other techniques will have to be looked upon to make these conclusions.
Mass Spectrum:
The mass spectrum provides the Mr with no other details about the structure of compound B. The Mr was obtained, as it is equal to the molecular ion (M+) that is the peak on the mass spectrum that has the largest mass to charge ratio value. According to the provided mass spectrum, the M+ and hence the Mr for compound B is 44. Apart from the molecular ion and Mr the mass spectrum does not provide any additional information on compound B, for example, no information on the molecular formula or about the functional groups present in the molecule.
Improvements
1. The identity of compound B can be confirmed with another chemical test in future work. This test will involve recrystalising the orange precipitate collected in test 2 and determining the melting point of this precipitate. This technique is used to determine the exact aldehyde present.
2. A wider range of chemical tests will be used to confirm whether compound B is an aldehyde. For example, Fehlings solution can be used as it turns from blue to brick red precipitate in presence of an aldehyde.
3. An improvement would be to gain information of the percentage composition of elements contained in compound B, (by combustion analysis) which would allow calculation of the empirical and molecular formula.
Reliability
Reliability is a measure of how consistent the observations of the chemical tests are
Chemical tests 1 and 2 were only carried out once and therefore it is difficult to say how reliable the observations are. To check whether the outcomes of the test observed were reliable, each test would have to be replicated in future experiments.
Test 2 with tollens reagent only produced a faint silver-mirror on the test-tube and this made it difficult to draw inferences. This test was carried out on a second occasion to confirm the initial observation. A silver-mirror was also observed on the second attempt and since the same observation occurred during the first and second attempt, the outcome of this test can be considered reliable.
Bibliography
1. Eccles, H (2001) AS Revision Guide Heinneman (pages 64-65)
2. Earl, B & Wilford LDR (2001) Further Advanced Chemistry. John Murray Publishers (pages 136-141 & page 228)
3. Ratcliff, B (2000) Chemistry 1. Cambridge University Press (pages 133-134)
4. Ratcliff, B (2001) Chemistry 2. Cambridge University Press (pages 24-25 & 66-67)
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