EXPERIMENT #2 Purification Of An Unknown Solid By Recrystallization And Identification By Melting Point Recrystallization is a procedure that is often used to 'purify' organic compounds which are obtained in an impure state from a chemical reaction or extraction process. In recrystallization the compound to be purified, a solid at room temperature, is dissolved in a hot solvent, filtered (usually gravity) and allowed to cool. The purified compound will then crystallize out of the solution. Separation of the purified compound from the 'mother liquor' is achieved by filtration (usually vacuum) followed by air drying. Typically purity is 90% after one recrystallization step (99%, 99.9% after two and three recrystallizations respectively). For recrystallization to occur the following requirements must be met: 1.
The solvent must not react with the desired compound 2.
The purified compound must be easily removed from the solvent (usually by filtration) 3.
The purified compound must be very soluble in hot and almost insoluble in cold solvent 4.
The impurities must either be very soluble in cold solvent or almost insoluble in hot solvent When recrystallizing mixtures contain small quantities of colored impurities, small amounts of selective absorbents such as charcoal are often added to the hot solution before filtration, to remove the colored impurities. It is important to avoid excess amounts of absorbents since they have a tendency to also absorb appreciable amounts of the substance being purified. To ensure good recovery it is also necessary to avoid excessive amounts of solvent. Usually 10% more than the minimum amount of solvent is used (The minimum amount of solvent required may be determined by using tabulated solubility data). During the filtration process it is necessary that both the solution to be filtered and the apparatus are kept hot. If appreciable cooling occurs, crystals may form prematurely (in the filter) which results in significant yield reduction. In many instances crystallization does not occur spontaneously. To help induce crystallization it may be necessary to scratch the side of the container with a stirring rod (both in and out of the solution). Alternatively, crystallization may be induced by the 'seed' method, this involves adding a few very small crystals of pure compound to the cooled sample.
Step 1: Recrystallization
You will receive a small sample of material of an impure solid from TABLE 1. Each of these materials can be successfully recrystallized from hot water (so you will not be going through the process of determination of the proper recrystallization solvent). Heat your solvent close to the boiling point and add your impure unknown to the water and allow the water to continue heating for 3 to 5 minutes. Try no to add more than 20 mL of solvent. At this point you will perform a “hot filtration” over a steamed fluted piece of filter paper. If you need to, this is the point where you will boil off excess solvent. This material needs to be removed from the hot plate and allowed to cool slowly. Once this flask has cooled off, you should see crystals form. This material gets chilled in an ice bath and the contents of the Erlenmeyer flask is filtered with a Büchner funnel. You should allow yourself enough time to recrystallize the material and allow it at least 30-40 minutes to dry in the oven before you attempt to perform a melting point/mixed melting point. The oven should be at least 80
Step 2: Melting Point Determination
O C to drive off as much water vapor as possible (at least 20 minutes). Determine the melting point of your unknown, heating the unknown at no more than 4 degrees per minute near the suspected melting point. Please be aware that there are only 6-7 Meltemps per lab room so you will need to share. However, a Meltemp can have three melts going on simultaneously (in parallel tracks) so it can be useful to do pure and mixed samples side by side.
TABLE OF POSSIBLE UNKNOWNS: Name When you look through the eyepiece of the Meltemp, this is what you will see: three tubes with material side by side. When you attempt to ascertain the identity of your unknown, you want to mix your material 1:1 with materials that you suspect of being your material. If you chose correctly your material and the known will melt at the same point. If your material is not the same as the chosen known, you will see a melting point depression below the melting points of both of the materials (the eutectic point). Also, allow the Meltemp to cool down to below 70 degrees before you attempt to perform another melt attempt. Melting Point( o C) Benzamide Benzoic Acid 128 122 Acetanilide Succinimide Cyanoacetamide Glucose Pentaacetate Adipic Acid Benzilic Acid Salicylic Acid Cinnamic Acid 114 126 120 131 152 150 157 133