IB Chemistry - Analytical Problems NAME: ______

IB Chemistry - Analytical Problems NAME: ______

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.

C3H7Br → C3H8O → C3H6O
A B C

(a) Using H2O as an example, describe what happens, at a molecular level, during the absorption of infrared radiation.

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(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.

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(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.

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(d) The evidence in (b) and (c) indicates that each compound (A, B and C) could have two possible structures. Draw the two possible structures of C.

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(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.

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(f) State the number of lines in the 1H NMR spectrum of each of the structures in (d).

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2. 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.

(a) The mass spectra of the two isomers A and B are as follows.

(i) Deduce, giving a reason, the molecular formula of the organic compound.

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(ii) Deduce the formula of the fragmentation ion responsible for the peak at each of these m/z values.

isomer A 29 ......

31 ......

isomer B 30 ......

45 ......

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(b) The 1H NMR spectra of isomers A and B are shown below.

(i) State the formula and function of the compound responsible for the peak at 0 ppm.

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(ii) Explain what information about the isomers can be obtained from the number of peaks and area under each peak.

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(iii) Deduce the structural formulas of isomers A and B and give the ratio of peak areas in each case.

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(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.

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