Purification of an Organic Compound

Question 1

How do we purify a mixture of organic compounds?

Answer 1

Cool the reaction mixture under reduced pressure using Buchner apparatus
Recrystallise the impure aspirin by dissolving in MINIMUM amount of HOT solvent, water.
Cool to allow crystallisation to occur then filter again under reduced pressure.
Wash purified aspirin using COLD water.
Dry

Question 2

How do we choose the solvent to use?

Answer 2

Needs to be good at dissolving the solute when hot and bad at dissolving when cold

Question 3

Why do we use the minimum amount of solvent?

Answer 3

If too much is used, some product may remain dissolved on cooling

Question 4

How do we check purity of dried aspirin/organic solvent?

Answer 4

Melting point - measure and compare to data value. Impurities lower melting point and widen melting point range (2 degrees)
TLC (Thin layer chromatography) - Run chromatogram of impure sample. Measure Rf value and compare to data value.
OR, run chromatogram of impure sample too and compare Rf values.