CHEM 2211L Experiment 1 - Recrystallization: Identification of a Impure Unknown

Five steps of recrystallization

1. Choose an appropriate solvent for recrystallization 2. Dissolve the impure mixture in the hot solvent 3. Remove the insoluble impurities from the solution via high temperature gravity filtration 4. Cool the solution to induce recrystallization of the pure compound 5. Isolate the pure crystals via suction filtration

Recrystallyzation

A laboratory technique used for the purification of organic solid compounds Goal: to dissolve and subsequently crystallize a pure substance from the solution Graphical representation of the recrystallization proces

1. Choosing an Appropriate Solvent

A suitable solvent will dissolve your target compound at high temperature but not at room temperature The impurities present in the mixture must either be soluble in the solvent at room temperature or insoluble at high temperature The solvent must be chemically inert (should not react with the target compound or impurities) The solvent must be volatile (relatively low boiling point and hence easy to remove

Mel-Temp Apparatus and Melting Point Determination

Contains a heating block, temperature control, thermometer well, and capillary holder Load a small amount of the dry product crystals into a melting point capillary tube Place the thermometer and capillary tube(s) into their appropriate wells Obtain a rough estimate of the melting point using a higher heat setting initially Allow the Mel -temp to cool roughly 20oC below the preliminary melting point Select a lower heat setting and obtain a more accurate melting point

3. Separation of impurities via hot filtration

High temperature gravity filtration

4. Recrystallization of the Pure Compound

Remove the flask from the hot plate and cool it to room temperature If crystals do not form, induce crystallization by 1. Scratching the inner walls of flask with a stirring rod 2. Introducing a seed crystal (of the pure compound) Cool the flask in an ice-bath If crystals do not form at this stage, you have most likely used an excessive amount of solvent. Simply boil away a portion of the remaining solvent and attempt to induce crystallization again

Eutectic Point

The lowest possible melting point of a mixture of two substances **impurities broaden and depress a pure compound's melting point range

2. Dissolving Your Unknown (Impure Solid)

Use a minimum amount of solvent to dissolve the impure solid Use of excess solvent will make it difficult to recover pure unknown

5. Isolating the Pure Compound via Suction Filtration Suction Filtration

Use the thick walled rubber hose for vacuum (black or red) Always clamp your filter apparatus at the neck of the flask Use a neoprene adaptor to achieve proper suction Wet the filter paper to adhere it to the bottom of the Buchner funnel prior to filtration

Mixed Melting Point Determination

Used to verify the identity of unknown compound Choose two compounds with comparable or higher melting points than your unknown from your table of reagents (say 1 and 2) Make two samples (a 50:50 mixture of the unknown and compound 1 and then a 50:50 mixture of the unknown and compound 2) Find melting points for both mixed samples using the Mel-temp apparatus If 1 is your unknown, you will not see a change in the melting point range of the mixture of the unknown and compound 1. If 1 is your unknown, you will see a marked change in the melting point range of the mixture of the unknown and compound 2 Remember that impurities broaden and depress apure compound's melting point range

Experimental Procedure

● Heat the water using a hot plate ● Add a minimum amount of hot water to the flask that contains your unknown compound and dissolve your unknown by heating on a hot plate ( do not heat the unknown on a hot plate without adding water ) ● Use hot gravity filtration to remove insoluble impurities from your unknown ( Keep the solution hot during filtration ) ● Cool the flask to room temperature ● If no crystals have formed, induce crystallization by scratching inner walls of the flask with a glass rod ● Cool flask in an ice-bath and collect the pure unknown via suction filtration (pull air through the crystals for approximately 10 - 15 minutes to remove excess solvent) ● Determine the melting point of the unknown ● Choose two compounds with which to perform mixed melting points (50:50 mixtures) ● Record your data in your lab note book ● Refer to the lab manual for a detailed procedure