Synthesis of Aspirin, made unknown fluid

Question

When I tried synthesising aspirin, after quenching and cooling it I noticed a translucent, milky, white fluid floating in the solution.

The method used was:

1. Salicylic acid (2 ml) was added to acetic anhydride (2 ml) in a flask with 8 drops of concentrated sulfuric acid.

2. Warmed the solution in a water bath until all solid was dissolved.

3. Added 5 ml of cold water to the solution to quench.

4. Placed solution in water bath to dissolve the solids again. (I noticed that not all the solid dissolved)

5. Added 10 ml of water to solution and placed in an ice bath (This was when I noticed the fluid forming)

When the solution was filtered only that fluid remained as the residue. Another person also had the same problem but said they got rid of the fluid by scratching the side of the flask with a stirring rod after taking it out of the ice bath. Could someone please explain what it is?

Answer 1

Often, acetylizations are performed with acetic anhydride being deployed both as reagent, as well as solvent. Consequently, acetic anhydride is used in excess when referring to the number of $\ce{-OH}$ to react.

The addition of water you describe in step 3) intends to hydrolyse this excess of acetic anhydride to form acetic acid. Apart from acetic acid equally representing a good solvent for many organic products, it takes time to complete this hydrolysis of acetic anhydride. You may accelerate the hydrolysis of the acetic anhydride if the beaker is immersed into an ultrasound bath, or just continue stirring the mixture at room temperature as usual.

The second point is, albeit its high melting point (136 $\mathrm{^\circ{}C}$), the crystallization of aspirine equally takes some time, too. Crystallization of organic materials from water may demand some more attempts prior to success than from organic solvents, too. Cooling the mixture often will lower the solubility of your product in the solvent, however, if cooled too quickly to too low temperatures you may obtain either just a glassy gel / syrup instead of a solid easy to filter off, or a solid that has to be recrystallized to purify it further. Scratching the inner walls of the beaker with a glass rod intends to create fresh sites where crystallization may start.

For peracetylating glucose with acetic acid, for example, it is not too strange to stirr the quenched reaction mixture with an excess of water over night at room temperature to convert the initially "honey-like" product into a good solid.