OrgChemLab

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Example

Qualitative Analysis

Goals

Utilize physical, chemical and spectroscopic data to identify structure of unknown compounds. After purifying the unknown solution, determine boiling point, measure RI, perform solubility and potassium permanganate test and obtain spectral data. This will provide sufficient data to identify the structure and identity of the unknown compound.

Procedure

Density:

Unknown example #1 in avail was obtained from the TA. The density of the liquid was measured by obtaining a graduated cylinder. The graduated cylinder was weighted before and after addition of the unknown sample. We calculate the density by using density equation [D=mass/volume]

RI:

Next, the RI was measured. A few drops of the unknown were added to the refrectometer. We adjusted it so field of view has well defined light and dark split. Then, the “read” button was pushed to record the RI and temperature.

Solubility Test:

A small vial was obtained and 20 drops of unknown sample were mixed with 25 drops of concentrated sulfuric acid. The contents of the test tube were mixed and the subsequent change in color to rustic red was noted.

Potassium Permanganate Test:

1 cm of unknown was mixed with 10 drops of KmNO4 and the change of color & phase separation was noted.

Boiling point:

The sand was heated and 0.4 mL of the unknown is placed on the top of the sand. The thermometer was placed on the top as well. The temperature was noted once the sample starts boiling.

Chemical Reactions

Observations

Solubility Test:

When sulfuric acid was mixed with unknown sample, the reaction color changed from clear to rustic red. The change in color indicated that our sample is soluble in sulfuric acid and thus, cannot be alkyl halide, which is no soluble in sulfuric acid.

Potassium Permanganate Test:

When 1 cm of unknown was mixed with 10 drops of KmNO4 there was a phase separation as well as color change. The top layer was dark purple and the bottom was pink. Also, there was a slight precipitate.

Results

Density:

The weight of vial = 11.4356 g

Vial+ sample = 12.053 g

12.053-11.4356 =0.6174 g

Density=mass/volume

0.6174 g/ 1 mL = 0.6537 g/mL

RI:

Index of refraction read: 1.4942 at 23.65 C

IR correction:

nobs + 0.00045 (t-20)

1.4942+0.00045(23.65-20)=1.4925

For proper identification of our unknown sample #1, we used the following experimental results:

Type of Test	Observed values
Boiling point	85 C
Density	0.6537 g/mL
RI (read)	1.4942 at 23.65 C
RI (corrected)	1.4925 at 20 C

Solubility test:

When Sample #1 mixed with sulfuric acid it produced to color change to rustic red.

Potassium Permanganate Test:

When the sample mixed with KmNO4, there was a phase separation with dark purple color in the top, pink at the bottom and slight precipitate.

IR:

H NMR

Discussion

Qualitative analysis is the way to identify unknown compound by obtaining physical and chemical properties for this compound. Some of these physical properties are boiling point, RI, and density. Boiling point is the temperature at which the vapor pressure of a liquid is the same as the atmospheric pressure. Refractive index is a measure of how much light is bent of reflected that inters the liquid. Also, density is mass per volume. We use these physical properties to identify our unknown sample. For instance, by comparing the observed boiling point with theoretical boiling point. Also, obtained RI should be compared with the theoretical RI of the original compound. Furthermore, we can compare observed density with theoretical one to make sure we have the correct compound.

For chemical test, we can obtain the structure and molecules in the compound. For NMR, we can figure out the compound formula. For IR, we can identify the type of bonds and functional groups presented in the compound.

Based on results we obtained, out compound shows that it is Ethyl Acetate. We figure this out by comparing the observed physical properties with the theoretical properties as illustrated in the table below:

Type of Test	Observed values	Theoretical Values
Boiling point	85 C	76-77 C
Density	0.6537 g/mL	0.902 g/mL
RI (read)	1.4942 at 23.65 C
RI (corrected)	1.4925 at 20 C	1.37

Also, solubility test shows that it is soluble, so this is an indication that it is alkene, alkyne or diene. By using potassium permanganate test, (by using 10 drops of KMnO4 and 1 cm-which equals to 10 drops- of sample), there were a phase separation which is an indication that is alkene only. Also, by using H NMR and IR, we can confirm our findings. In H NMR, since there are 3 types of hydrogens in Ethyl Acetate, one is singlet, one is quartet, and one is triplet, it confirms that there are 3 types of hydrogens. For IR, bands at 1300 that is C-O for Esters.

In this experiment, everything was clear since we did much of those techniques in Organic Lab I.

Conclusion

In this experiment, we use qualitative analysis to identify out unknown sample by using chemical and physical properties. W compared physical properties that we obtained with the theoretical properties for the original compound. Also, we use potassium permanganate test to identify if the compound or alkene, alkyne or diene as well as using solubility test. Also, we use chemical test such as NMR and IR to identify the structural formula and functional groups that are presented in the compound.

Post Lab Questions

1. KMnO4 molecular weight : 158.03 g/mol , 100 mL solution or 0.1 L

0.5 moles/L X 0.1 L= 0.05 moles of KMnO4 X 158.03 g/mol= 7.90 g KMnO4

18 M of H2SO4, we need 1M, 100 mL solution

(1M)(100 mL)/ 18M= 5.65 H2SO4

2. Since the unknown was not soluble in sulfuric acid, then, it must be an alkyl halide. Based on the given physical properties (BP: 150 C, Density: 1.50 g/mL) the compound must be 1-Ido-3-methylbutane, which has boiling point of 147 C and density of 1.521 g/mL.

3. The unknown liquid is soluble in sulfuric acid, hence, it is not alkyl halide. It must be an alkene, alkyne, or diene. Since the potassium permanganate test was positive and the unknown require half number of drops of reagent required for the known alkene, then the unknown must be an alkyne or a diene. Based on the given physical properties (BP: 116-120 C, density: 0.90 g/mL, the compound must be 1,3-cycloheptadiene, which has a boiling point of 120-121 C and density of 0.868 g/mL.

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