[sharbeldam]

After we react salicylic acid with acetic anhydride we get aspirin and acetic acid then we start the recrystlization process by using hot ethanol first and then hot water, so our solvents are ethanol and water.

As I understood the main impurity in aspirin is salicylic acid, we add hot water to the ethanol so we can decrease the solubility of aspirin and it can form crystals, but as I understood that salicylic acid is poorly soluble in water at 25C, so why wouldn't it form crystal as well?

shouldnt we use two solvents, 1 that is aspirin and impurity are both soluble in at room temp and in heat and another 1 that our impurity soluble in at room temp and in heat while aspirin is only soluble in heat?

thanks

[AWK]

In the synthesis of drugs, crystallization is the cheapest method of purifying the final product and in most syntheses optimized by trial and error. This was the case with aspirin. Gerhardt received it in 1853, but it was not until Hoffmann purified it by crystallization in 1898.
The solubility of aspirin and salicylic acid in water at room temperature is similar - a few grams per liter. The solubility of aspirin in ethanol is much higher and salicylic acid dissolves much better. However, crystallization from pure ethanol does not give a sufficiently pure product.
Such a procedure was found experimentally: the crude product is dissolved hot in the least amount of ethanol, then hot water is added slightly below the temperature of the ethanol solution to turbidity, then some ethanol is added to the water-ethanol solution until the turbidity disappears and allowed to crystallize at RT, then at 0°C.

[sharbeldam]

Thank you for a great answer, but if the solubility of them both is kinda equal in water ( I suppose salicylic acid is a bit more soluble because of the hydroxy group ) and aspirin is way more soluble in ethanol than SA, then why does aspirin crystalize? what is the solubility logic behind it? we have aspirin and SA in water/ethanol and since aspirin is more soluble in ethanol, shouldnt SA crystalize? since it is less soluble in ethanol?

[AWK]

Crystallization is accompanied by other phenomena, e.g. occlusion, co-crystallization. These phenomena can be minimized or used (in this case even a Nobel Prize can be obtained) during the optimal procedure for the crystallization.

[sharbeldam]

So basically there is no solubility logic behind aspirin crystalization because of other phenomena that I don't get

[AWK]

Some time is still needed to understand all phenomena that accompany crystallization.

[clarkstill]

Thank you for a great answer, but if the solubility of them both is kinda equal in water ( I suppose salicylic acid is a bit more soluble because of the hydroxy group ) and aspirin is way more soluble in ethanol than SA, then why does aspirin crystalize? what is the solubility logic behind it? we have aspirin and SA in water/ethanol and since aspirin is more soluble in ethanol, shouldnt SA crystalize? since it is less soluble in ethanol?

I presume when you've run your reaction the majority of material is the product, with the residual starting material as a minor impurity. If (for example), the solubility of both salicylic acid and aspirin are both 1 g / L, and you dissolve a sample containing 99 g of aspirin and 1 g of salicylic acid in 1L of solvent, then you will successfully dissolve all of the impurity, and (theoretically) get back 98 g of pure aspirin, having sacrificed 1 g of aspirin to the crystallization gods.

[sharbeldam]

Appreciate it clark! got it.