6.6 Nuclear Magnetic Resonance Spectroscopy
- Nmr provides us with details about the nuclei of atoms. Nmr tells us about the different environments hydrogen nuclei are in, and how many protons there are in each.
- Nmr involves placing the substance in a very strong magnetic field. For every type of molecular arrangement, there is a slightly different magnetic field, as the protons align themselves differently.
- The H nuclei in different environments have different energy gaps between their high and low energy levels, so absorb different frequencies of radiation
- On an nmr spectrum, the spectrum trace is used to identify the type of H+ environment, and the integrated trace shows the relative number of protons
- TMS is also shown on the spectrum as a standard reference. The extent to which a signal differs from TMS is called its chemical shift.
- A sensitive nmr machine can produce a high resolution spectrum, where the spectrum trace peaks are split into smaller peaks. The number of 'small' peaks shows the number of H+ on the adjacent C atom + 1. This is the n+1 rule
- The interaction between protons on neighbouring C atoms is coupling, and causes the absorption peak to be split. Different interactions will cause different splitting patterns.
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14.1 Planning a Synthesis
- New compounds are made through synthesis, where complex structures are made from simpler starting materials
- When planning an organic synthesis, several steps may be needed and a number of intermediates prepared and purified before reaching the target molecule.
- In retrosynthesis, chemists look at the functional groups in the target molecule, and work backwards through a logical sequence of reactions until they reach the suitable starting materials.
- Target molecule- the desired compound
- Retrosynthetic analysis- the process of working backwards from the target molecule in order to devise a suitable synthetic route on paper
- Readily available starting materials- cheap, commercially available compounds that can be modified to produce the target molecule
- Disconnection- a paper operation involving an imagined breaking of a bond to suggest two fragments (or synthons) that could be reached to make the target molecule. The arrow symbol => shows a disconnection. When planning any synthesis, ensure that a good chemical reaction corresponds to the reverse of the disconnections
- Synthon- an idealised fragment resulting from a disconnection. May not actually exist
- Synthetic equivalent- the actual compounds used to function as synthons
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