Wednesday, October 31, 2012

Procedure for recrystallization

Procedure for recrystallization-
1.    Place 1.5 to 2.0 g of impure unknown in a 250 mL Erlenmeyer flask and dissolve in the minimum amount of boiling water using hot plate. You should know the exact mass of the amount of material you are using. There may be impurities insoluble in water and dark oil many form in the hot solution. If oil forms this is called “oiling-out” and usually happens when a compound melts near the boiling point of the solvent being used. The oil will eventually go in to solution although a small amount of additional solvent may need to be added.
(Note: You should keep more water boiling in a separate flask than you think you will need- about 250 mL should do. Also, when performing the recrystallization, add the hot solvent.
(water) to the solute and not the solute to the solvent.)

2.    The boiling solution is allowed to cool slightly, and a small amount (about 0.25 – 0.5g) of charcoal s added. Estimate this amount by using the tip of scoopula. If the charcoal is added to the boiling solution, it usually causes the solution to boil over the top over the flask. Never adda solid substance to a solution at or near its boiling point.

3.    The solution is heated again it boiling, and is then filtered by gravity through a pre-warmed funnel and fluted filter paper while hot. This is called hot filtration. This is a critical step in recrystallization when there are insoluble impurities present such as charcoal. See below on how to flute filter paper. The filter paper doesn’t need to be perfect but it should be folded so as to increase the surface area and allow for rapid filtration. (Picture should be added)

4.    The clear filtrate is then allowed to cool slowly to room temperature. Continue cooling on an ice/water bath. After the cooled solution has stood in ice-water for about 30 minutes, crystals will form. If crystals don’t form, try scratching the side of the glass container below the solvent level with a stirring rod. This creates an uneven surface on which crystal formation may occur. You may also try adding a seed crystal (your TA may have one) to induce crystallization. If these fail you may have to concentrate your solution as you probably used more than a minimal amount of solvent.

5.    Collect the crystals by suction filtration. Be sure you use a clean filtration flask. Clamp the filtration flask and turn the water aspiration on full. Wet the filter paper with the solvent you are using for recrystallization. After the filtration is complete, break vacuum by taking the hose off the filtering flask BEFORE turning off the water. Your TA may demonstrate. If you believe your aspirator is not doing the job, tell your TA so that it may be replaced for next time.

6.    Dry the crystals on a watch glass or filter paper in your drawer until next week. Make sure they are stored safely but allow them to air dry. When they are dry, weigh carefully and record their melting point. From the melting point and mixed melting point determine the identity of your unknown. Please see the section on melting pint and mixed melting point. SAVE YOUR ENTIRE SAMPLE. You will run an Infrared Spectrum of this material. Proton NMR spectra will be provided. The filtrate is waste and should be placed in the aqueous acidic waste container.

Report the total weight of pure compound obtained and calculate the % recovery.

% recovery = amount obtained after recrystallization/amount of crude X100

Table 1 :  Unknown list. (Structures are provided on the next page.)

Compound name                                                        MP(in degrees of Celsius)
acetanilide                                                                                113-115
p-nitrobenzaldehyde                                                                106-108
benzoic acid                                                                             122-123
acetylsalicylic acid                                                                   138-140
4-dimethylaminobenzaldehyde                                                 73-75
4-hydroxy-3-methoxybenzaldehyde (also known as vanillin)    81-83
salicylic acid                                                                             159-160
p-aminoacetanilide                                                                   162-163
o-toluic acid                                                                             103-105
p-toluic acid                                                                             180-182
m-toluic acid                                                                            108-110
p-toluic acid                                                                             180-182
3-nitroaniline                                                                           112-114
o-aminobenzoic acid                                                               144-148
m-aminobenzoic acid                                                              178-180
t-cinnamic acid                                                                        134-135
o-nitrobenzoic acid                                                                  146-148
m-nitrobenzoic acid                                                                 139-141
p-nitrobenzoic acid                                                                  237-240


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