Does salicylaldehyde turn red in an alkaline environment?
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I am carrying out the synthesis of an organic intermediate using salicylaldehyde as a starting material. When pure soda water solution is added to salicylaldehyde and other raw materials in order to adjust the pH from acidic to neutral, the reaction mixture turns yellow-green in the area where the alkaline solution is added. As the pH approaches neutrality, the reaction mixture changes from light yellow-green to light red-brown. I’m not sure why salicylaldehyde turns yellow-green when it comes into contact with an alkaline solution, while the reaction mixture becomes red-brown when the pH is neutral or alkaline – what is the reason for this? The final reaction product is red-brown in color; if the pH value is alkaline, the color of the reaction product becomes even darker, turning brownish. Could it be that some reaction has taken place between salicylaldehyde and the base? How can this be avoided? Thank you!
Salicylaldehyde is an aldehyde that lacks an alpha-hydrogen; under alkaline conditions it undergoes a redox reaction in which one molecule is reduced to an alcohol while another molecule is oxidized to a carboxylic acid. This reaction is known as the Coni-Zarro reaction, or also as a disproportionation reaction! ! ! The change in color you observe is the result of the oxidation of salicylaldehyde; you might consider carrying out the reaction in an environment free from air! ! !