BRAINCAKEDeducing Structures from Combined Data
No single technique identifies an unknown compound on its own. Combine the evidence step by step:
1. Molecular formula: from percentage composition (empirical formula) and the molecular ion peak, M+, in the mass spectrum, which gives the relative molecular mass. Fragment peaks give clues to parts of the structure.
2. Infrared: functional groups, e.g. C=O at about 1700 to 1750 cm-1, broad O-H in alcohols around 3200 to 3550 cm-1, very broad O-H in carboxylic acids 2500 to 3300 cm-1.
3. 13C NMR: number of carbon environments.
4. 1H NMR: H environments, chemical shift, integration ratio and splitting.
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Key terms in this lesson
- chemical shift
- The position of a signal in an NMR spectrum, measured in parts per million relative to TMS, determined by how shielded the nucleus is by electrons.
- carbon environment
- A set of carbon atoms in equivalent chemical surroundings, related by the symmetry of the molecule, that give a single peak in a 13C NMR spectrum.
- empirical formula
- The simplest whole-number ratio of atoms of each element in a compound.
- fragment peak
- A peak in a mass spectrum produced by a charged fragment formed when the molecular ion breaks apart, whose mass can help identify part of a molecule's structure.
More in Modern Analytical Techniques
- Retention Times
- Nuclear Magnetic Resonance
- Chemical Shift
- 13C NMR Spectroscopy
- 1H NMR Spectroscopy
- Integration Traces
- Spin-Spin Splitting
- The n+1 Rule
All 15 lessons in Modern Analytical Techniques · All Edexcel A-level Chemistry topics