Chapter 4: Recrystallization & Melting Point Recrystallization A purification technique for impure solid compounds A several-step process Can be on on a microscale or macroscale Melting Point Verifies the purity of a compound Aids in the identification of an unknown Typically reported as a range An impure solid will have a lower melting point than the pure solid - less attractive forces within the solid, thus less energy to break up those forces, thus lower mp
The Experiment You will be given an impure Recrystallization Unknown. 1. Recrystallize/purify this solid 2. Take a melting point of the pure & impure solid 3. Calculate the percent recovery from the recrystallization You will be given a Spectral Unknown (Ch 12) - take its melting point. Once you have the mp, go to the course website & click on the Spectral Unknown link. List all compounds that have melting points from +5 C and -5 C of your observed melting point. One of these compounds will be your spectral unknown.
Possible Recrystallization Unknowns CH 3 CH 3 CO2H o-toluic Acid m-toluic Acid CO 2 H Fluorene O NH 2 E-Stilbene O Benzamide OH E-Cinnamic Acid Use your solubility data to help explain the identification of your unknown! Remember the like dissolves like concept.
Recrystallization The Experimental Process: 1. Find a suitable recrystallization solvent for your solid (solute) - solubility; refer to Table 4.1 First try hexanes (nonpolar), if needed try water (polar) Trial and error Choice of solvent will give a clue as to the type of compound you have - either nonpolar or polar Ideal solvent: solute insoluble at rt, solute soluble with heat 2. Dissolve the compound in a minimal amount of the chosen solvent 3. Remove insoluble impurities (may skip) Pipet Filtration, hot filtration 4. Crystallize your compound - slow cooling of crystals; may need to scratch tube to induce nucleation 5. Collect and wash the crystals Pipet filtration or vacuum filtration 6. Dry the crystals
Recrystallization Example: Recrystallize an impure sample of benzamide: O NH 2 Benzamide mp 127-130 C
1. Find a recrystallizing solvent: Recrystallizing Benzamide Consider the polarity of benzamide Like dissolves like concept O NH 2 Polar H-bonding Possible solvents (Table 4.1): Hexanes Least Polar Toluene Benzene Ether Chloroform Dichloromethane Acetone Ethanol Methanol Acetic acid Water Most Polar Test polar solvents Good solvent: Solute should be insoluble at room temp and soluble at high temps; impurities should be soluble at all temps. Water is a good solvent for recrystallizing benzamide.
Recrystallizing Benzamide 2. Crystallize a larger amount of benzamide in water. 3. Remove impurities by hot filtration or with Norit (macroscale). 4. Allow the hot filtrate to cool slowly! Slow cooling allows better crystal growth - better crystal growth means higher purity of crystals. Slow cool by allowing the filtrate to cool to room temperature then cool in an ice-water bath. 5. Collect and wash crystals. Collect crystals by filtration. Be sure to wash crystals with cold solvent. Example: For benzamide, wash with cold water. 6. Dry crystals: air dry or pat crystals with filter paper.
Four main principles: Solubility Purification by Recrystallization Saturation Level: The concentration of the desired solute is significantly higher than the concentration of the impurity. When solution cools, the impurities will remain in solution, and the desired solute will crystallize out of solution. Exclusion: Every solid has a defined crystal structure/lattice. As the solution cools, crystals form into their well-defined lattice. Impurities cannot fit inside these lattices. The desired crystal solute will be pure since impurities stay in solution. Nucleation
Recrystallization What if a suitable solvent isn t found? You can try a two-solvent system; two solvents that are miscible with each other. Table 4.2 lists the miscibility of common organic solvents. Examples of two-solvent systems: Water & acetone Hexanes & acetone Dichloromethane & ether Methanol & acetic acid
Melting Points Load a small amount of sample into a capillary tube. Use a Mel-Temp or Thomas-Hoover and to obtain a melting point. These are located throughout the lab; limited number. Soluble impurities in a compound will cause the compound s melting point to be lowered. Insoluble impurities have no effect on a compound s melting point. Report melting point as a range. Record the temperature at which you start to see the compound melt (even if it s a drop at first) then record the temperature at which all of the sample becomes liquid.
Determining the Unknown Mixed Melting Point: To verify the proposed identification of unknown Mix your purified unknown with a known sample If the two compounds are the same, the melting point will match the melting point of your purified unknown. If the two compounds are not the same, the melting point will be lower than the melting point of your purified unknown. Consider and discuss in your final report: Does the choice in recrystallization solvent make sense in terms of like dissolves like? Compare the experimental mp to the actual, reported mp - do they match?
Recrystallization Lab Next Week in Lab: PreLab for Chapter 4 is due. Quiz 1 on Chapter 4