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Concept
Diazonium
Résumé
Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide.
According to X-ray crystallography the linkage is linear in typical diazonium salts. The bond distance in benzenediazonium tetrafluoroborate is 1.083(3) Å, which is almost identical to that for dinitrogen molecule (N≡N).
The linear free energy constants σm and σp indicate that the diazonium group is strongly electron-withdrawing. Thus, the diazonio-substituted phenols and benzoic acids have greatly reduced pKa values compared to their unsubstituted counterparts. The pKa of phenolic proton of 4-hydroxybenzenediazonium is 3.4, versus 9.9 for phenol itself. In other words, the diazonium group lowers the pKa (enhances the acidity) by a million-fold.
The stability of arenediazonium salts is highly sensitive to the counterion. Phenyldiazonium chloride is dangerously explosive, but benzenediazonium tetrafluoroborate is easily handled on the bench.
SN1 and SN2 reactions do not occur.
Arenediazonium salts are versatile reagents as described in the next sections After electrophilic aromatic substitution, diazonium chemistry is the most frequently applied strategy to prepare aromatic compounds.
Alkanediazonium salts are synthetically unimportant due to their extreme and uncontrolled reactivity toward SN2/SN1/E1 substitution. These cations are however of theoretical interest. Furthermore, methyldiazonium carboxylate is believed to be an intermediate in the methylation of carboxylic acids by diazomethane, a common transformation.
Loss of is both enthalpically and entropically favorable:
ΔH = −43 kcal/mol
ΔH = −11 kcal/mol
For secondary and tertiary alkanediazonium species, the enthalpic change is calculated to be close to zero or negative, with minimal activation barrier. Hence, secondary and (especially) tertiary alkanediazonium species are either unbound, nonexistent species or, at best, extremely fleeting intermediates.
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