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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