Organic Molecules

Exercise A

The review on pages 41-42 is very informative and the illustrations help to convey the structure of mono/disaccharides. Table 5A-1 should look similar to the following:

 
Benedict's Test
Lugol's Test
Tube Original Color Before Boiling Color After Boiling Original Color Before Adding Lugol's Color After Adding Lugol's
1. Water
blue
blue
yellow
yellow
2. Starch
blue
blue
yellow
blue-black
3. Glucose
blue
red precipitate
yellow
yellow
4. Maltose
blue
red precipitate
yellow
yellow
5. Sucrose
blue
blue
yellow
yellow
6. Onion
blue
blue
yellow
blue-black
7. Potato
blue
blue
yellow
blue-black
8. Milk
blue
red precipitate
yellow
yellow

Some notes about the table:


Exercise B

The test for lipids involved two parts. I'm going to skip over the "grease spot" discussion because I'm sure most of us have spilt oil from some sort of food onto your clothes before. I know I have. Anyways, the Sudan IV test uses a red dye to help identify non-polar substances in a polar and non-polar solution. It's important to remember that there is no reaction occurring here! The dye is simply dissolving in non-polar liquids and allowing you to see them separate from polar solutions. Granted, it is sometimes hard to distinguish the presence of the dye if the mixture is dominently polar, but it still may form small "pockets" in non-polar liquids.

 

Exercise C

Testing for the presence of amino acids was also two part. The first part introduced the Biuret Reagent. This solution changes from blue to violet in the presence of proteins. While other colors may show up in some solutions, the only color you should be concerned about is violet. The results should look as follows:

Substance
Color After 2 Minutes
Protein Present (+) or Absent (-)
1. Distilled Water
blue
-
2. Egg albumin
violet
+
3. Potato Starch
blue
-
4. Glucose
blue
-
5. Amino Acids
blue
-

Since Biuret Reagent only tests for the presence of proteins and not free amino acids, we should not see a positive result for the last substance.

The last part of this exercise utilized Ninhydrin to test for the presence of free amino acids. With Ninhydrin, a purple or yellow color is indicative of a positive result (the presence of amino acids). However, and this is important to distinguish, yellow denotes the presence of an amino acid with a ring structure. So while either color tells you that there are amino acids, yellow should further inform you that a ring structure is present (such as in proline). Refer to page 49 for an illustration of an amino acid with a ring structure.

 

Exercise D

Omitted