What happen when salicylic acid heated with soda lime?
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You will not get a single product, and the primary product will depend both on what is meant by ‘high temperatures’ and the rate at which the temperature is reached, as well as other factors like the pressure and the presence or absence of air, and whether the salicylic acid or the soda lime is present in excess.
Anyway, you will first get a mixture of salicylate salts - calcium, sodium, and potassium salicylate (‘soda lime’ is not a single compound). Some carbon dioxide may be released if carbonates were present in the soda lime. This happens even at room temperature if the reagents are adequately mixed somehow.
As the temperature increases, the water created by the neutralization reaction is driven off. If the temperature is increased slowly, and in particular if it remains below the boiling point of phenol (182C) but above 150C or so, the salicylate will gradually react to form various complex oxybenzones and similar high molecular weight products, which in turn simply char if later subjected to higher temperatures. The basic mechanism here is the decarboxylation of the salicylic acid followed by condensation of the phenol residue with other molecules. Some CO2 may also be emitted - this will depend on whether the soda lime is present in excess, allowing for remaining hydroxides to form carbonates as they take up the CO2, or whether the salicylic acid is present in excess, in which case the CO2 will escape.
If the temperature is increased fairly rapidly to 225C or higher, then decarboxylation proceeds more rapidly than the various poorly characterized cross-linking reactions, and the primary product is phenol, which will boil off and can then be condensed. If phenol is the desired product, air should be excluded from the reaction vessel, as air oxidation of any kinds of phenolic products proceeds rapidly under basic conditions.
Even under these conditions however there will be significant organic char, which will combine with the alkali carbonates in the reaction vessel to make a difficult to clean mess.
There are catalysts that allow for decarboxylation of various organic compounds at significantly lower temperatures, and anyone who actually wants to prepare phenol by thermolysis of salicylate compounds would be wise to do further research on the topic.
Anyway, you will first get a mixture of salicylate salts - calcium, sodium, and potassium salicylate (‘soda lime’ is not a single compound). Some carbon dioxide may be released if carbonates were present in the soda lime. This happens even at room temperature if the reagents are adequately mixed somehow.
As the temperature increases, the water created by the neutralization reaction is driven off. If the temperature is increased slowly, and in particular if it remains below the boiling point of phenol (182C) but above 150C or so, the salicylate will gradually react to form various complex oxybenzones and similar high molecular weight products, which in turn simply char if later subjected to higher temperatures. The basic mechanism here is the decarboxylation of the salicylic acid followed by condensation of the phenol residue with other molecules. Some CO2 may also be emitted - this will depend on whether the soda lime is present in excess, allowing for remaining hydroxides to form carbonates as they take up the CO2, or whether the salicylic acid is present in excess, in which case the CO2 will escape.
If the temperature is increased fairly rapidly to 225C or higher, then decarboxylation proceeds more rapidly than the various poorly characterized cross-linking reactions, and the primary product is phenol, which will boil off and can then be condensed. If phenol is the desired product, air should be excluded from the reaction vessel, as air oxidation of any kinds of phenolic products proceeds rapidly under basic conditions.
Even under these conditions however there will be significant organic char, which will combine with the alkali carbonates in the reaction vessel to make a difficult to clean mess.
There are catalysts that allow for decarboxylation of various organic compounds at significantly lower temperatures, and anyone who actually wants to prepare phenol by thermolysis of salicylate compounds would be wise to do further research on the topic.
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