Identifying unknowns A level
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5.3.2 Qualitative analysis
(a) qualitative analysis of ions on a test-tube scale: processes and techniques needed to identify the following ions in an unknown compound:
(i) Anions CO32–, Cl–, Br-, I–, SO42–
(ii) Cations NH4+, Cu2+, Fe2+, Fe3+, Mn2+, Cr3+
6.3.1 Chromatography and qualitative analysis
(a) interpretation of one-way TLC Chromatograms in terms of Rf values
(b) interpretation of gas chromatograms in terms of:
(i) retention times
(ii) the amounts and proportions of the components in a mixture
(c) qualitative analysis of organic functional groups on a test-tube scale; processes and techniques needed to identify the following functional groups in an unknown compound:
(i) alkenes by reaction with bromine (see also 4.1.3 f)
(ii) haloalkanes by reaction with aqueous silver nitrate in ethanol (see also 4.2.2 a)
(iii) phenols by weak acidity but no reaction with CO32–(see also 6.1.1 h)
(iv) carbonyl compounds by reaction with 2,4-DNP (see also 6.1.2 d)
(v) aldehydes by reaction with Tollens’ reagent (see also 6.1.2 e)
(vi) primary and secondary alcohols and aldehydes by reaction with acidified dichromate (see also 4.2.1 c, 6.1.2 a)
(vii) carboxylic acids by reaction with CO32–(see also 6.1.3 b)
(a) analysis of a carbon-13 NMR spectrum of an organic molecule to make predictions about:
(i) the number of carbon environments in the molecule
(ii) the different types of carbon environment present, from chemical shift values
(iii) possible structures for the molecule
(b) analysis of a high resolution proton NMR spectrum of an organic molecule to make predictions about:
(i) the number of proton environments in the molecule
(ii) the different types of proton environment present, from chemical shift values
(iii) the relative numbers of each type of proton present from relative peak areas, using integration traces or ratio numbers, when required
(iv) the number of non-equivalent protons adjacent to a given proton from the spin–spin splitting pattern, using the n + 1 rule
(v) possible structures for the molecule
(c) prediction of a carbon-13 or proton NMR spectrum for a given molecule
(d) (i) the use of tetramethylsilane, TMS, as the standard for chemical shift measurements
(ii) the need for deuterated solvents, e.g. CDCl3, when running an NMR spectrum
(iii) the identification of O–H and N–H protons by proton exchange using D2O
(e) deduction of the structures of organic compounds from different analytical data including:
(i) elemental analysis (see also 2.1.3 c)
(ii) mass spectra (see also 4.2.4 f–g)
(iii) IR spectra (see also 4.2.4 d–e)
(iv) NMR spectra
Qualitative analysis of inorganic ions
This topic allows learners to consider how anions and cations can be identified to prove the existence of them in solution. For example, when the learners study inorganic chemistry and complex formation they will be able to determine what they have in the reactions that they will carry out. Thus having the tools to qualitatively analyse the contents of their reactions will help them to see what is going on.
Qualitative analysis of organic functional groups
As above, these tests can be used while teaching about organic reactions and used as a tool to see what is going on in reactions. Qualitative tests can be used to monitor the outcome of experiments, to confirm the presence or absence of a functional group in the product. As with all practical work, learners will learn about these tests best by actually doing them.
Learners will have briefly come across chromatography as a separation technique in Key Stage 3. They are commonly told that this is a way of separating solids, but that is as far as it goes. It is sometimes used as a way of identifying colours such as those in sweets or in different coloured felt tip pens.
The use of TLC allows learners to be able to initially quantify the movement of the chemicals on a substrate. But in order to be able to explain gas chromatograms in terms of retention times and proportions of chemicals in a material, learners need a basic understanding of what is happening within the stationary and mobile phases of the system. In order to understand the processes, analogies have to be used to explain what is happening within the column, for example the ‘shopping centre’ analogy described in the activities.
Note that this qualification also requires understanding of the use of external calibration curves to confirm concentrations of components of a mixture. An important point is that a separate calibration curve has to be produced for each component.
It makes sense to introduce learners to carbon-13 NMR first, as this is more straightforward than proton NMR. Learners could be shown simple spectra along with the structures of the compound, and asked to consider the features of the spectrum and how it relates to the structure. They should pick up that the number of peaks is equivalent to the number of carbon environments: if the structure is symmetrical there will only be one peak for the carbon groups in the same environment.
From here, move on to giving learners spectra and asking them to determine the number of environments and possible fragments for each peak. Learner Resource 1 in this guide provides a structured worksheet learners can use.
A good tactic in spectral analysis is to take note of what isn’t there as well as what is there. For example, if there is no peak between 115 ppm and 140 ppm, then there is no C=C present in the molecule. Similarly, if there is a peak in the range 160–200 ppm but none in the range 50–90 ppm, the compound may be an aldehyde or ketone but not a carboxylic acid or ester.
Proton NMR is a different prospect due to the added complication of spin–spin splitting and the n + 1 rule. Each set of peaks relates to one proton environment, which is the same principle as for carbon-13 NMR, but identifying equivalent hydrogens is often more complex. The other added feature of proton spectra is that the integration number gives the relative number of protons in each environment, related to the peak areas. Learners can use a table similar to that provided in Learner Resource 1 for carbon spectra. Learner resource 2 provides a more extensive list of steps for learners to consider when analysing proton spectra, and a worksheet providing a sample spectrum to look at.
Important factors to stress in NMR are the use of TMS as a reference point, and the presence of solvent. The solvent of choice is generally CDCl3, often giving a visible peak in carbon NMR spectra but not influencing proton NMR spectra.
NMR must ultimately be combined with other spectroscopic data such as elemental analysis, mass spectrometry and IR spectra, which are all likely to have been taught at an earlier point in the course. It should be stressed to learners that no structure should be elucidated without agreement with data from at least three spectra. Remember that learners learn best by being given full sets of data to work out a structure, using a problem solving approach as per the research listed below.
Common misconceptions or difficulties learners may have:
Analytical methods can seem a bit dry if practical work is restricted to demonstrations, or prescriptive learner activities. Consider applying a problem-based approach, giving learners identification problems to solve, and allowing them to research useful information and devise their own approach. Research suggests that good learning results from such approaches, and that inquiry-based problems work well to engage learners of different abilities (Overton, T.L. and Randles, C.A., Chem. Educ. Res. Pract., 2015, 16, 251; Blonder, R., Mamlok-Naaman, R., Hofstein, A., Chem. Educ. Res. Pract., 2008, 9, 250). There are also indications that problem-based approaches to practical work encourage deeper engagement with the theory behind the methods used (Smith, C.J., Chem. Educ. Res. Pract., 13, 490).
Conceptual links to other areas of the specification:
Analytical methods link well into the organic chemistry topics as they show how mixtures are analysed in industry and at a research level. They can also be used as a tool in the laboratory practicals that the learners will carry out, allowing them to check the progress of simple organic reactions, for example using UV lights or suitable dyes such as ninhydrin.
The position of NMR at the end of the specification should not be taken as a requirement to teach this content at the end of the course. In fact, it could be taught usefully at the beginning of Year 13, following on from teaching IR spectroscopy. Analysis would then begin with compounds that have been covered up until that point in the course, such as hydrocarbons, alcohols and haloalkanes. As further functional groups are introduced, analysis questions can be provided alongside, establishing analysis as an integral part of organic chemistry. When synthetic routes are discussed, links can be made to the use of analytical techniques to monitor the progress of reactions.
Approaches to teaching the content:
Learners can have great difficulty when carrying out qualitative tests unless they have a specific context that they can use the tests within. For example, use the tests in a forensic type activity where they have to use the techniques to solve a problem. The Royal Society of Chemistry activity linked under Thinking Conceptually could be framed in such a context, for example suggesting that a factory has illegally released waste water. Learners have to match an ‘unknown sample’ to one of a selection of ‘factory samples’ by identifying the ions present in each. Learner Resource 3 in this guide provides a ‘prep room mix-up’ context for organic qualitative tests.
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