Most of the labs you will be performing this semester are synthesis experiments. On paper every experiment works, and these experiments have been tested by students. However, chemistry does not always work the way you expect. The primary goal of each experiment is to understand the data that you collect. Do not fixate on getting a high percent yield of the right product. There are some general guidelines to follow in any synthetic experiment.
1. Before starting the experiment, figure out which part of the reaction corresponds to which part of the procedure. Sometimes the reaction written out on paper does not correlate obviously with the steps in the experiment.
2. Identify the different species in the reaction: reagents, catalysts, solvents. Calculate which reagent is limiting. This practice will allow you to be judicious when measuring quantities.
3. Make sure you know where your product is at all times, especially in those experiments when you have multiple layers.
4. Try to understand the specific procedures within an experiment. For instance: Why are the reagents added in their particular order? or Why is the product isolated/purified in a certain manner and are there other ways that isolation or purification could be done?
5. If you do not have enough material for the next step in a synthesis, either scale the amounts accordingly or supplement your material with stock material. You may also borrow some from a coworker.
6. Try to have some idea of what to expect when you are analyzing your products. You should always know of some feature to look for in an NMR or IR. For instance, if you are synthesizing an alkene, you should see =C-H and C=C stretches in the IR at ~3100 and 1650 cm-1 and =C-H protons in the NMR at 5-6 ppm. Think about what you should see before you run the spectra. If your spectra show unexpected absorptions, think about what the starting materials would show since they are likely impurities. Remember, your samples must be free of solvents, otherwise the solvents will dominate the spectra.
Pre-Lab Requirements
Before you come to lab, the following things must be in your lab notebook
1. An introduction/purpose stating briefly what you are going to do, how you are doing it, and how you will be analyzing the results
2. Write the reaction using structures
3. A table of reagents listing the important physical properties of the chemicals. For instance, if a chemical is going to be used as a solvent, its boiling point and density are important, but its molecular weight is not.
4. Do NOT write out the procedure; you do that as you perform the experiment
General Report Requirements
For every report on a synthetic experiment, you will need to do the following:
1. Write out the mechanism for the reaction(s)
2. Calculate the overall percent yield based on the limiting reagent
3. Turn in labeled spectra
4. Discuss whether you actually obtained the expected product and its purity by citing specific features from spectra, chromatograms, melting points etc.
5. If a product is impure, discuss possible reasons and the likely identity of the impurity
6. Discuss possible ways of improving the experiment to get higher purity and greater percent yield. You may mention things like "Don’t tip the reaction over in the future" and "Find a better way to filter things", but you must have more substantial improvements.
Other specific directions are at the end of each experiment or will be given in lab.