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1 1. Organic compounds are often identified by using more than one analytical technique. Some of these techniques were used to identify the compounds in the following reactions. C 3 H 7 Br C 3 H 8 O C 3 H 6 O A B C Using H 2 O as an example, describe what happens, at a molecular level, during the absorption of infrared radiation. (b) The infrared spectrum of B showed a broad absorption at 3350 cm 1. The infrared spectrum of C did not show this absorption, but instead showed an absorption at 1720 cm 1. Explain what these results indicate about the structures of B and C. The mass spectrum of A showed two lines of approximately equal height, one of which was at m/z = 122. State the m/z value of the other line and explain these observations. (d) The evidence in (b) and indicates that each compound (A, B and C) could have two possible structures. Draw the two possible structures of C. (e) Fragmentation of C in a mass spectrometer produced lines with m/z values of 15 and 28, but none at values of 14 or 29. Identify C and explain how you used this information to do so. 1

2 (f) State the number of lines in the 1 H NMR spectrum of each of the structures in (d). 2. A student used the technique of ascending paper chromatography in an experiment to investigate some permitted food dyes (labelled P1 P5). The result is shown below. 6 Distance / cm P1 P2 P3 P4 P5 By reference to the diagram above, describe how the experiment would be carried out and explain the meaning of the terms stationary phase, mobile phase, partition, solvent front and R f value. (8) (b) Calculate the R f value of P1.... State, giving a reason, whether P4 is a single substance or a mixture

3 3. Explain the following observation: Hydrogen iodide is infrared active whereas iodine is infrared inactive All chromatographic techniques involve the phenomena of adsorption or partition. They all use a stationary phase and a mobile phase, but these phases can include solids, liquids or gases. Complete the following table to show which states of matter are used in the two phenomena. Adsorption Partition Stationary phase Mobile phase (b) Explain the term R f value used in some chromatographic techniques Outline how the technique of column chromatography could be used to separate a mixture of two coloured substances in solution (4) 5. Aqueous solutions containing complexes of transition metals are usually coloured. This is due to the absorption of part of the spectrum of white light passing through the solution. Three factors help to determine the colour absorbed. For each of the following pairs, state the difference between the two complexes that is responsible for the difference in colour. [Co(NH 3 ) 6 ] 2+ and [Ni(NH 3 ) 6 ] [Fe(H 2 O) 6 ] 2+ and [Fe(H 2 O) 6 ] [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ and [Cu(H 2 O) 6 ]

4 (b) The wavelength of colour absorbed by the complex can be explained in terms of the splitting of the d orbitals in the metal ion. The arrangement of electrons in the d orbitals of the Cu 2+ ion is shown in the following diagram. Draw a diagram to show how the electrons are arranged in Cu 2+ when it is present in the [Cu (H 2 O) 6 ] 2+ ion. Predict whether the splitting of the d orbitals in [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ and [CuCl 4 ] 2 would be less than or greater than the splitting in [Cu(H 2 O) 6 ] 2+. splitting in [Cu(NH 3 ) 4 (H 2 O) 2 ] splitting in [CuCl 4 ] 2... (Total 5 marks) 6. The mass spectra of halogenoalkanes show more than one line corresponding to the molecular ion. This is due to the presence of isotopes such as 35 Cl, 37 Cl, 79 Br and 81 Br. Analyse the following spectra of halogenoalkanes P and Q and deduce the formula of all the molecular ion species. 100 Spectrum of P Relative abundance / % m/z 100 Spectrum of Q Relative abundance / % m/z [Source: NIST Mass Spec Data Center, S E Stein, director, IR and Mass Spectra in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P J Linstrom and W G Mallard, July 2001, National Institute of Standards and Technology, Gaithersburg MD, ( species in P... species in Q... 4

5 (b) Predict the m/z values of the molecular ions for the compounds C 2 H 5 Br and C 2 H 4 Cl 2 C 2 H 5 Br... C 2 H 4 Cl This question is about the three organic compounds involved in the following reaction. C 2 H 4 O 2 + C 2 H 6 O C 4 H 8 O 2 + H 2 O W X Y The infrared spectra of all three compounds showed several absorptions. Describe what happens on a molecular level when molecules absorb infrared radiation (b) Use the following information about their infrared spectra to deduce which bonds are present in the three compounds. All three compounds showed an absorption close to 1200 cm 1. There were broad absorptions in both W and X. The one in W was centred around 3000 cm 1, and in X around 3400 cm 1. Compounds W and Y showed absorptions close to 1700 cm 1. bonds in W.. bonds in X... bonds in Y... The 1 H NMR spectra of the three compounds were available. State what can be deduced from each of the following. The presence of two peaks in the spectrum of W..... The presence of a triplet and a quartet, with areas in the ratio 3:2, respectively, in the spectra of both X and Y..... (d) Use your answers to (b) and to deduce the structures of the three compounds. W.... X... 5

6 visible Y... (e) The infrared spectrum of compound Z, an isomer of Y, is shown below Transmittance / % Wavenumber / cm 1 Estimate the wavenumber values of the three most prominent absorptions in this spectrum and suggest which bonds are responsible for them. absorption 1... absorption 2... absorption 3... Deduce the structure of Z The figure below depicts the visible region of the electromagnetic spectrum and the two regions nearest to it. A B Increasing wavelength Name the regions labelled A and B, identify the atomic or molecular processes associated with each region and compare the energies of the photons involved in these processes. (5) (b) State, giving a reason, which region (A or B) could be used to test for metal ions. 6

7 .... obtain information about the strengths of bonds Draw structures for two possible isomers of a compound with the formula C 2 H 4 O 2. (b) The infrared spectrum of this compound shows the following absorptions: 2920, 2765 and 1710 cm 1. Use the information in Table 18 of the Data Booklet to assign each absorption to a particular vibration cm cm cm 1... Deduce the structure from which would demonstrate the infrared absorptions above. Explain your answer H NMR spectroscopy can be used to obtain information about the structure of molecules. State the information that can be obtained from the number of peaks..... chemical shift..... (iii) ratio of peak areas..... (iv) splitting pattern. 7

8 Relative Intensity.... (b) The 1 H NMR spectrum of a compound with the formula C 4 H 8 O 2 exhibits three major peaks with chemical shifts, areas and splitting patterns given below. chemical shift / ppm peak area splitting pattern triplet quartet singlet Using information from Table 19 in the Data Booklet, determine the types of proton present in the molecule. Deduce a structure consistent with the information indicated in (b). Explain your answer. (5) 11. An organic compound with three carbon atoms has two structural isomers A and B with the same functional group. The infrared spectrum of the compound shows a broad absorption at about 3350 cm 1. The mass spectra of the two isomers A and B are as follows m/z Isomer A

9 Relative Intensity m/z Isomer B Deduce, giving a reason, the molecular formula of the organic compound Deduce the formula of the fragmentation ion responsible for the peak at each of these m/z values. isomer A isomer B (4) (b) The 1 H NMR spectra of isomers A and B are shown below. Isomer A chemical shift / ppm

10 Isomer B chemical shift / ppm State the formula and function of the compound responsible for the peak at 0 ppm Explain what information about the isomers can be obtained from the number of peaks and area under each peak (iii) Deduce the structural formulas of isomers A and B and give the ratio of peak areas in each case (4) (iv) Draw the structural formula of a third isomer C that has the same molecular formula as isomers A and B but a different functional group. Give the ratio of peak areas in isomer C Describe a chromatographic technique used to identify the amino acids formed when a protein is hydrolysed Suggest a chromatographic technique that could be used to detect the alcohol concentration in a 10

11 sample of blood. Outline the essential features of this technique Identify one analytical technique, different in each case, which can be used to obtain the following information about a molecule. Information Number of different hydrogen environments Types of functional group Analytical technique Molecular mass 15. The infrared spectra of propanoic acid and methyl ethanoate contain absorptions in characteristic wavenumber ranges. Using Table 18 in the Data Booklet, identify: two wavenumber ranges common to both compounds (b) one wavenumber range found only in the spectrum of one compound Identify one analytical technique, different in each case, that can be used to obtain the following information: Information Analytical technique Isotopic composition of an element Functional groups present in an organic compound Concentration of Fe 3+ ions in industrial waste waters 17. The diagram below represents the principal parts of a double beam infrared spectrometer. S M A B C D Name the parts labelled A, B, and C. 11

12 A:... B:... C:... Describe the function of the monochromator, M. (iii) Explain how the detector, D, works. (b) State and explain what happens to a molecule when it absorbs infrared radiation Each of the infrared absorptions A, B and C is produced by one of the compounds I, II and III. Deduce which compound is responsible for each absorption, and identify the bond causing the absorption. I OH II H III CH NH 2 2 C C CH 3 H wavenumbers in cm A B C Absorption Compound Bond A B C (5) 12

13 (b) Identify which of the absorptions, A, B or C, indicates the greatest amount of energy, giving a reason for you choice Chromatography is usually coupled with another analytical technique. Identify the analytical technique that is most commonly coupled with gas-liquid chromatography (GLC) (b) Chromatographic techniques include the following. LC (column chromatography) GLC (gas-liquid chromatography) HPLC (high performance liquid chromatography) Explain why LC is used in preference to GLC or HPLC to separate a pharmaceutical drug from its production batch. Identify, giving a reason, which of the techniques GLC or HPLC is more suitable for determining the composition of a mixture of sugars. Two samples of unknown alkaloids (X and Y) are analysed using paper chromatography. They are compared against the results obtained with samples of four known alkaloids A, B, C and D. The results are shown in the table below: A B C D X Y Sample R f value A 0.1 B 0.5 C 0.7 D 0.8 X 0.1 and 0.3 Y 0.5 Explain how an R f value is calculated. 13

14 Transmittance Relative abundance Use the results above to discuss the composition of samples X and Y. 20. An organic compound X contains 48.6 of carbon, 8.2 of hydrogen and 43.2 of oxygen by mass. The mass spectrum and infrared spectrum of this compound are as follows: 100 Mass spectrum of compound X m/z Infared spectrum of compound X A wavenumber/cm 1 B C Calculate the empirical formula of this compound (b) Deduce, stating a reason for your answer, the molecular formula of this compound Suggest the formula of the fragmentation ion that could be responsible for the peak at each of the following m/z values

15 (d) Identify the bond responsible for each of the absorptions labelled A, B and C. A... B... C... (e) The compound X exists as three structural isomers. An 1 H NMR spectrum was obtained for each isomer. Draw the structural formula of the three isomers and deduce the splitting pattern in each case (6) (f) Suggest with a reason the identity of compound X Electronic transitions occur when a molecule absorbs energy from the visible or UV regions of the electromagnetic spectrum. Predict the region (visible or UV) in which benzene and nitrobenzene would absorb energy.... Explain why CH 2 =CH CH=CH 2 shows an absorption band at a longer wavelength than CH 2 =CH 2. (iii) Explain why compounds of Zn 2+ and Cu + are colourless. 15

16 (b) An aqueous solution of copper(ii) sulfate is light blue, but when excess ammonia is added the colour changes to dark blue. State the formula of the copper(ii) complex ions in each of the two solutions. Explain why the colour of copper complex solutions is different when different ligands are present Compound A, with the molecular formula C 3 H 6 O has this infrared spectrum. 1 Wavenumbers (cm ) Use Table 18 in the Data Booklet to list four classes of compounds that have absorptions near 1700 cm 1. Identify which of the classes listed in part could not have the molecular formula C 3 H 6 O. (b) The mass spectrum of compound A has prominent peaks at m/z = 15 and 29. Deduce the formula of the species responsible for each peak. m/z = m/z = Deduce the structure of compound A. 16

17 23. Explain, with reference to the d orbitals in a transition metal, why complex ions are coloured Outline why different ligands produce different colours with the same transition metal The structures of four hydrocarbons are shown below. I II III IV Identify the compounds that most strongly absorb ultraviolet radiation.... Identify the compound that absorbs ultraviolet radiation of the longest wavelength, and explain your choice Describe what occurs at a molecular level when molecules such as carbon dioxide absorb infrared radiation (b) Spectra A and B (see below), represent the infrared spectra of two of these compounds: CH 3 CH 2 COOH CH 3 COOCH 3 HCOOCH 2 CH 3 Use Table 18 in the Data Booklet to identify the groups responsible for the absorption near 1050 cm cm 1... Deduce which one of the three compounds produced spectrum A, giving a reason for 17

18 your choice. (iii) Explain why the other two compounds have similar infrared spectra. Figures C and D represent the splitting patterns and the chemical shifts shown in the 1 H NMR spectra of the same two compounds used to obtain spectra A and B. 18

19 Figure C Chemical shift / ppm Figure D Chemical shift / ppm State what general information can be deduced from the three different types of splitting patterns in these spectra. Explain how the chemical shift values close to 4.1 in figure C and close to 11.5 in figure D can be used to identify the two compounds. Refer to information from Table 19 in the Data Booklet in answering this part. 26. Paper chromatography and column chromatography can be used as examples to explain the difference between adsorption and partition. For each of these chromatographic techniques, identify the stationary and mobile phases how the mobile phase moves. Identify, with a reason, which of the two techniques is more suitable for collecting samples of a mixture for further analysis (Total 5 marks) 27. There are four structural isomers that are alcohols with the formula C 4 H 9 OH. Explain why the infrared spectra of all four alcohols show very similar absorptions around 3350 cm 1 and 2900 cm

20 (b) Describe how these alcohols can be distinguished using their infrared spectra Explain why the mass spectra of all four alcohols show a peak at m/z = (d) Suggest the formulas of the fragments formed from C 4 H 9 OH with the following m/z values: m/z = m/z = (e) The numbers of peaks, and the areas under them, in the 1 H NMR spectra of these alcohols can be used to identify them. Explain why the 1 H NMR spectrum of (CH 3 ) 2 CHCH 2 OH has four peaks. Predict the ratio of the areas under the peaks. Deduce the structure of the alcohol whose 1 H NMR spectrum has two peaks with areas in the ratio 9: Identify two effects of the absorption of infrared radiation on the bonds in a molecule of carbon dioxide. Explain why an oxygen molecule does not absorb infrared radiation (Total 3 marks) 29. Nuclear magnetic resonance spectroscopy ( 1 H NMR) can be used to identify organic compounds. Several features of 1 H NMR spectra are helpful in obtaining information about the structure of a molecule. Outline what can be learned from each of the following: the number of peaks in the spectrum the ratio of areas under each peak chemical shift values ( ) of 1.3 and 9.7 ppm

21 (4) (b) There are four esters with the molecular formula C 4 H 8 O 2. Two of them are shown below. ethyl ethanoate CH 3 COO CH 2 CH 3 methyl propanoate CH 3 CH 2 COO CH 3 The 1 H NMR spectra of these two esters, not necessarily in the correct order, are given below. Spectrum Chemical shift / ppm Spectrum Chemical shift / ppm State and explain, in terms of the structures of both esters, the origin of the triplet and the quartet.... Identify the origin of the singlet peak for either ester, and explain why this peak is not split.... (iii) By referring to the singlet peak, and using information from Table 19 of the Data Booklet, deduce which spectrum corresponds to which ester, giving a reason for your choice in each case. ethyl ethanoate... 21

22 .. methyl propanoate..... Another ester with the formula C 4 H 8 O 2 has the structure. H COO CH CH CH 3 Predict the number of peaks, and the ratio of the areas under the peaks, in its 1 H NMR spectrum. number of peaks... ratio of areas Explain why the mass spectrum of 2-chloro-2-methylpropane shows molecular ion peaks at m/z values of 92 and 94 in the ratio 3: Suggest the formulas of the species with the following m/z values in this spectrum: m/z = 77 m/z = 57 (b) Predict the m/z values of the three main molecular ion peaks in dichloromethane Compounds A and B are alcohols with the molecular formula C 3 H 8 O. The following information was obtained from a mass spectrum of each alcohol. A: peaks at m/z = 29, 31, 60 B: peaks at m/z = 45, 60 Deduce the formula of the species responsible for the peak at m/z = (b) Deduce the formula of the species with m/z = Deduce the structure of each alcohol. Structure of A Structure of B 22

23 Absorption visible 32. The figure below shows the visible region of the electromagnetic spectrum and the two regions nearest to it. A B increasing wavelength Name the regions labelled A and B, identify the atomic or molecular processes associated with each region and compare the energies of the radiation involved in these processes Region A Region B (5) (b) State, giving a reason, which region (A or B) could be used to test for metal ions State the main use of atomic absorption spectroscopy (AAS) (b) Ore samples may be analysed for iron using AAS. An ore sample was prepared in acid and diluted to 1 part in 10. The diluted solution gave an absorbance reading of Determine the concentration of iron in the sample in mg cm Concentration / g cm Describe the use of each of the following components of the AA spectrophotometer. Atomizer Monochromatic light source

24 34. The colours of transition metal complexes depend on several factors. Use [Mn(H 2 O) 6 ] 2+ and [Fe(H 2 O) 6 ] 2+ as examples to outline why the colours depend on the identity of the transition metal (b) Outline why the colour depends on the oxidation state of the transition metal An analgesic tablet contains 400 mg of aspirin and 80 mg of caffeine. The molecular formula of aspirin is C 9 H 8 O 4 and that of caffeine is C 9 H 10 N 4 O 2. State and explain which method, gas-liquid chromatography (GLC) or high performance liquid chromatography (HPLC), would be best for the separation and mass determination of aspirin and caffeine in the tablet (b) State and explain which of the two components would have the shorter retention time Sketch a chromatograph for the separation of the aspirin and caffeine in the analgesic tablet. time 24