In: Chemistry
Why is not possible to recrystallize ASA using a single-solvent recrystallization from water, as is done with acetanilide?
Acetyl salicylic acid is soluble in ethanol but very poor soluble in water , this is due to fact
Compared to water, ethanol has a hydrophobic carbon chain. In this sense, water can at best dissolve the carboxylic acid and polar portion of aspirin - and other polar compounds even the methylester may not be adequately dissolved in water. ... The ethyl side chain, here in ethanol is a hydrocarbon side chain, can form Vander waal (VDW) forces with hydrophobic benzene
aspirin, a sylicylate derivative:
has:
A bulky benzene ring (hydrophobic)
A benzoic acid group (hydrophilic)
An methylester group (polar)
Compared to water, ethanol has a hydrophobic carbon chain. In this sense, water can at best dissolve the carboxylic acid and polar portion of aspirin - and even the methylester may not be adequately dissolved.
In ethanol, the alcohol group can form favourable hydrogen bonds with carboxylic acid and dipole interactions with the ester group. The ethyl side chain, which is a hydrocarbon side chain, can form VDW forces with hydrophobic benzene.
Also due to intramolecular H bonding bitweeb two adjecead group reduces water solubility of ASA further
Thuse due to this solubility difference
Now coming to our question
Ethanol is added to completely dissolve everything. Then cold water is added to precipitate out the aspirin only but not other impuritiea
What you're altering is the temperature/composition of the solvent, that is, the quality of the solvent. You have to look at the solvent as a whole with properties intermediary to that of the pure components, not as two separate solvents.
For above reason we mentioned at start, the impurities in aspirin are more soluble in cold water+ethanol than in ethanol alone, because acetylsalicylic acid is more apolar than the impurities, so acetylsalicylic acid precipitates selectively in cold water. The impurities can be unreacted reagents, like salicylic acid, which is more soluble in cold water than acetylsalicylic acid due to being more polar.
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