17.1 Stretch and challenge Identifying an unknown molecule using spectroscopy PDF

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OCR Chemistry A

17 Investigating the molecule Stretch and challenge

Identifying an unknown molecule using spectroscopy Specification reference 

4.2.4 c) (i) (ii) (iii) d) f) g)

Introduction  In this activity you will use two different types of spectroscopy, which you have learnt about in ‘Chapter 17: Spectroscopy’ to help you identify the structure of an unknown compound. Gradually piecing together the information is a little like detective work; you will also need to be able to describe and explain how you are approaching the problem.  You will need to be confident with ideas about the structure of alkenes from Chapter 13: Alkenes as well as alcohols, carboxylic acids and esters from Chapter 14: Alcohols.

Learning outcomes After completing the worksheet you should be able to:  use an infrared spectrum to identify familiar functional groups in a molecule from the peaks present in the spectrum  interpret the infrared spectrum of an unfamiliar substance, using supplied data  use a mass spectrum of an organic compound to identify the molecular ion peak and hence determine molecular mass  identify parts of a structure by analysing fragmentation peaks in a mass spectrum.

Background When chemists synthesise a new molecule, its structure can be identified rapidly by using the infrared and mass spectrum of the molecule. A research group of polymer chemists, trying to produce new monomers for polymerisation reactions, obtained a liquid product, X, which was found to have an empirical formula C5H8O2. They obtained the infrared spectrum and mass spectrum of the product. These provided enough evidence for them to deduce the structure of the product and to decide whether it would be a suitable monomer to use in addition polymerisation. The spectra are shown in Figures 1 and 2.

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17 Investigating the molecule Stretch and challenge

OCR Chemistry A

Figure 1 Infrared spectrum of product X

Figure 2 Mass spectrum of product X Some details of the wavenumbers at which different bonds absorb infrared radiation are given in the data booklet for this course. Some data tables for infrared spectroscopy give slightly more detail about the exact range at which bonds in different functional groups absorb, as shown in Table 1. Table1 Wavenumber range for IR absorption by bonds in different environments Bond

Location

Wavenumber range / cm–1

C–H

alkanes and alkyl groups

2850–2950

C–H

alkenes

3000–3100

O–H

alcohols (hydrogen bonded)

3200–3600 broad

O–H

carboxylic acids (hydrogen bonded)

2500–3200 very broad

CC

alkenes

1620-1680

CO

carboxylic acids

1700–1725

CO

esters

1735–1750

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OCR Chemistry A

17 Investigating the molecule Stretch and challenge

Questions 1 The molecular formula of the product X was deduced to be the same as the empirical formula, using just one piece of data from the spectra. a State which piece of data is used. (1 mark) b Explain how the molecular formula was deduced from this piece of data. (1 mark) 2 The chemists analysing the spectra suggested that a CC bond is present in molecule X. a Explain which type of spectrum is most helpful in deducing the bonds present in a molecule.(2 marks) b Give two pieces of evidence from the spectrum that supports the suggestion made by the chemists. (2 marks) 3 Knowing that a CC bond is present in the molecule, chemists were able to suggest several possible structures for the molecule. Some of these were esters and some were carboxylic acids. a Draw out two possible structures of X, one containing each of the following i an ester group and ii a carboxylic acid group. Remember that a CC bond is also present. (2 marks) b Deduce whether molecule X contains an ester group or a carboxylic acid group, using evidence from the infrared spectrum to inform your answer. (4 marks) 4 Once the types of functional group present in molecule X were known, chemists were able to use the mass spectrum of X to deduce the exact structure of the molecule. To do this, chemists identified the fragments that were responsible for certain peaks in the mass spectrum. a Suggest likely fragments that might be responsible for the peaks observed at i 29 ii 45 iii 27 iv 55 v 71 (4 marks) b Discuss how the mass spectrum can be used to deduce a complete structure for molecule X, and suggest what this structure is. In your answer, you should also comment on whether the molecule would be suitable as a possible monomer for an addition polymerisation reaction. (6 marks)

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