This is a set of more difficult long answer exam questions that require you to use data from mass spectra, IR spectra, 1H NMR and 13C NMR spectra to deduce the structure of a molecule.
These questions have been written to stretch your understanding and your analysis skills (there are no easy questions here!). The given answers showcase the well-structured and well-explained response expected of an A / A* candidate. Use them to develop ‘perfect’ answer statements or templates, remembering to double check your syllabus for preferred language and terminology.
They are notoriously poorly answered by students, so hopefully the model answers and exam tips here will help you power through!
The difference between one grade and another is almost always down to how well a candidate can communicate their knowledge on paper.
Questions
- A substituted phenol was found to have a percentage composition of 71% carbon, 6% hydrogen and 23% oxygen. The infrared spectrum confirms the presence of a carbonyl group and a hydroxyl group, and the mass spectrum gives the m/z for the molecular ion peak at 136. Use this information and the 13C NMR spectrum given below to determine a possible structure for the molecule.
- Determine the structure of the compound with the molecular formula, C4H8O2, with reference to both the spectra shown below and the data sheets.Â
- A student set out to make paracetamol using the reagents 4-aminophenol and ethanoic anhydride.Â
On completion of the reaction, a sample of the product was sent for analysis. The infrared, 13C NMR and 1H NMR spectra are shown below. Explain with reference to each of the spectra whether the student was successful.
- Hex-4-ynoic acid is an unsaturated fatty acid.
(a) Determine the percentage composition by mass of the constituent elements of the acid.
(b) Describe two distinctive features of the infrared spectrum of the acid.
(c) Chemical samples are commonly prepared for infrared spectroscopy by grinding up the solid and mixing with a mineral oil before sandwiching between a pair of sodium chloride discs. Suggest why this would not be a suitable method for recording an IR spectrum of aqueous hex-4-ynoic acid.
(d) The 13C NMR spectrum for an isomer of hex-4-ynoic acid is shown below. The IR spectrum shows the presence of a carboxylic acid functional group. Suggest a structure for this isomer.
- Butyrolactone is a prodrug synthesised from butane-1,4-diol. It is not biologically active in its own right but rapidly metabolised by enzymes in the blood to form 4-hydroxybutanoic acid, a neurotransmitter and depressant drug that acts as an anaesthetic.
Identify, with reasoning, which of these compounds is responsible for the infrared spectrum shown below. Describe the 1H NMR spectrum for this compound including the number of peaks expected, their relative chemical shift and their relative intensity.
- Ethanoic acid can be prepared in the laboratory by refluxing ethanal with acidified potassium dichromate(VI). Discuss the similarities and differences in the infrared and 13C NMR spectra of each compound.Â
- Ibuprofen is synthesised via a series of intermediates from benzene. Part of this reaction scheme is outlined below.Â
Deduce the structure of compound B by comparing the mass spectrum, infrared spectrum and 1H NMR spectrum for compounds A and B. Explain your reasoning and use evidence from each of the spectra in your answer.
Spectra for compound A:
Spectra for compound B:
- Compound P is a white, crystalline solid that has many applications as an intermediate in organic synthesis reactions and in the production of biofuels. It has a composition by mass of 51.7% carbon, 6.9% hydrogen and 41.4% oxygen. The infrared spectrum and 1H NMR spectrum for compound P are shown below.Â
Using all the information given deduce a structure for compound P and explain your reasoning fully.
Answers
