Ultraviolet absorption of flavonoids. V. The structure of 3- and 5-hydroxyflavones
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1. The peculiarities of the properties of 3-hydroxyflavone are due to the fact that the introduction of the proton-donating hydroxy group directly into the heterocycle leads to an increase in the aromaticity of the pyrone fragment through the stabilization of a pyrylium structure.
2. The causes of the decreased acidity and basicity of 5-hydroxyflavone are the formation of an intramolecular hydrogen bond of the chelate type and a disturbance of the coplanarity of the molecule. The combination of these factors explains the anomalously high position of the band of the stretching vibrations of the carbonyl group of this compound.
KeywordsCarbonyl Group Flavone Hydroxy Group Intramolecular Hydrogen Bond Methyl Ether
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