Spectroscopic identification of organic compounds

Nature of science:

Improvements in instrumentation—mass spectrometry, proton nuclear magnetic resonance and infrared spectroscopy have made identification and structural determination of compounds routine. (1.8)

Models are developed to explain certain phenomena that may not be observable—for example, spectra are based on the bond vibration model. (1.10)

• The degree of unsaturation or index of hydrogen deficiency (IHD) can be used to determine from a molecular formula the number of rings or multiple bonds in a molecule.
• Mass spectrometry (MS), proton nuclear magnetic resonance spectroscopy (1H NMR) and infrared spectroscopy (IR) are techniques that can be used to help identify compounds and to determine their structure.

Applications and skills:

• Determination of the IHD from a molecular formula.
• Deduction of information about the structural features of a compound from percentage composition data, MS, ¹H NMR or IR.

Guidance:

• The electromagnetic spectrum (EMS) is given in the data booklet in section 3. The regions employed for each technique should be understood.
• The operating principles are not required for any of these methods.
• The data booklet contains characteristic ranges for IR absorptions (section 26), ¹H NMR data (section 27) and specific MS fragments (section 28). For ¹H NMR, only the ability to deduce the number of different hydrogen (proton) environments and the relative numbers of hydrogen atoms in each environment is required. Integration traces should be covered but splitting patterns are not required.

• Monitoring and analysis of toxins and xenobiotics in the environment is a continuous endeavour that involves collaboration between scientists in different countries.

Theory of knowledge:

• Electromagnetic waves can transmit information beyond that of our sense perceptions. What are the limitations of sense perception as a way of knowing?

Utilization:

• IR spectroscopy is used in heat sensors and remote sensing in physics.
• Protons in water molecules within human cells can be detected by magnetic resonance imaging (MRI), giving a three-dimensional view of organs in the human body.

Syllabus and cross-curricular links:
Topic 1.2—determination of the empirical formula from percentage composition data or from other experimental data and determination of the molecular formula from both the empirical formula and experimental data.
Topic 2.1—the nuclear atom

Topic 5.3—bond enthalpies

• Aim 7: Spectral databases could be used here.
• Aim 8: The effects of the various greenhouse gases depend on their abundance and their ability to absorb heat radiation.