LAB #1. RECRYSTALLIZATION OF BENZOIC ACID AND AN UNKNOWN
*Before lab begins, read the following:
Zubrick, Chapters 1-3 (p 1-44). These three chapters provide some basic background information on safety and keeping a notebook.
Zubrick, Chapter on Recrystallization (Ch 13). This is the procedure you will be doing in lab.
Zubrick, Chapter on Melting Point Determination (Ch 12). This is how you will characterize your purified product.
*Make a table of physical constants including compound name, molecular weight, melting point, boiling point, density if a liquid, solubility in different solvents, and the safety information for each chemical used in the experiment. See the Organic Chemistry Homepage for MSDS (Material Safety Data Sheets) links. Solubility data is easy to find in the CRC Handbook of Chemistry and Physics in the lab.
*Draw a flow chart outlining your activities in lab.
*In place of a balanced equation, draw the structures of benzoic acid, urea, naphthalene, anthracene and sulfanilamide.
Completion 5 pts
(includes lab and a report turned in on time)
Prelab 5 pts
Experimental 2 pts
Results 3 pts
Conclusions 5 pts
Give an empty sample vial labeled with your
name and drawer number to your TA. This will be used in Part 2.
You will first recrystallize benzoic acid from a minimum volume of water using the crystallization procedure listed in Zubrick (General Guidelines for...). The crude sample will really be about 95% benzoic acid and 5% impurity. You will use about 150 mg of this crude sample (it doesnt need to be exactly 150 mg; just make sure you record how much you actually use. Remember not to put excess reagent back in the bottle; put it in the waste container, instead). Since this is a small amount of material, use a small test tube, rather than an Erlenmeyer flask, for your benzoic acid. You probably wont need more than 1 mL of water. Remember to add a boiling chip.
Often in the organic lab we heat things up in a hot water bath in a 100 mL beaker on a hot plate, and this would be a good idea when dissolving your sample today. It takes a while to warm up so start it ahead of time. Clamp your test tube so it doesnt fall into the water bath.
You will probably not need to do the hot filtration described in Zubrick. A diagram of a Hirsch filtration apparatus (similar to the Büchner set-up in Zubrick) is shown on the next page.
After you have recrystallized benzoic acid, you will recrystallize an unknown (also 95% purity) given to you by your TA and discover its identity through melting point analysis. You will need to weigh it before you begin. Follow the directions in Zubrick for determining a solvent and performing an effective recrystallization. You will have less to work with, so do not use 100 mg portions to determine solubility; scale this whole procedure down by a factor of 4. Use all of unknown sample in your final recrystallization. The solvents available to you will be ethanol, acetone, methanol, and water. The unknown may be one of the following: urea, naphthalene, anthracene, or sulfanilamide.
You will need to find a melting point of both of your recrystallized compounds. A discussion of melting points is found in Chapter 12 of Zubrick. Also, pictures of the Mel-temp apparatus used in our lab are shown on pages 76-78.
Find the melting point and the percent recovery of the recrystallized benzoic acid. Determine the melting point, the percent recovery, and the identity of the unknown compound. Discuss the relative purity of both recrystallized compounds. Include a few sentences on these procedures and their effectiveness.
Labeled waste containers will be available in the middle hood on the left side of the lab as you face the blackboard. This hood will often be used for waste collection as well as for dispensing material that the whole lab needs to use.