Chemistry 216 First Exam (November 3, 2010)

(1 hr 50 min, 100 points) Dr. Kyoung Moo Koh

Lab section_______________________ GSI name________________________ Name ___________________________ Please print

Signature________________________ Student ID#______________________

GSI initial

I 14

II 8

III 15

IV 15

V 12

VI 8

VII 6

VIII 10

IX 12

Total 100

216 F10-Exam #1 Page 2 of 13. Lab section _______________ Name____________________ I. (14 points) The scheme below represents the experimental procedure of the acetylation of an aniline compound (Experiment 1) and the structural change from a starting material to a product in each of the steps. 1) Fill in the answer boxes in the scheme using the key structures in the box below the scheme. (3 pts)

2) When you used p-nitroxy aniline, you needed to add more HCl to the mixture of starting materials and water for converting from biphasic solution to the clear solution. Explain why. (4 pts)

(2pts)

3) If you used 1 g of aniline and 5 g of acetic anhydride to make 1g of product, what is the theoretical yield and the percentage yield. (4pts)

The limiting reagent is aniline. Therefore, the theoretical yield is (1 (g) /93.13 (g/mole))*(135.16) = 1.45 g. (2pts) The percent yield of the reaction is (1/1.45)*100 = 68.96 % (2pts)

Because of the electrostatical destabilization in the resonance structure, pKa of nitroxy aniline is lower than the pKa of aniline. Therefore, for generating anilinium, we need to make very low pH solution (2pts).

216 W10-Exam #1 Page 3 of 13. Lab section _______________ Name____________________ II. (8 points) The Jones’ reagents, which are chromium(VI) based, are well-known as strong oxidants,but they are largely viewed as being undesirable due to the generation of toxic residues. To solve the issue of the toxic contamination, one research group recently developed a new oxidation strategy using immobilization of Jones’ reagents onto silica column. The oxidized product can react with 2,4-dinitrophenylhydrazine (2,4-DNP) to make a DNP derivative. The following scheme shows the process of oxidation and the synthesis of the DNP derivative. 1) Draw the structure of the product for each step. (6pts)

2) Describe the change of IR spectrum after the oxidation of the starting materials. (2 pts)

A new peak derived from the VC=O of the ketone will appear at the 1650 – 1700 cm-1 region.

216 W10-Exam #1 Page 4 of 13. Lab section _______________ Name____________________ III. (15 points) In experiment 3, you used an alcohol compound as a starting material and oxidized it to make a ketone. The following scheme illustrates the experimental procedure.

1) Describe the possible reaction that occured, when you added sodium bisulfite (NaHSO3) to the mixture. (5 pts)

2) Explain why you added thymol blue and NaOH before the steam distillation step. (6 pts)

3) Which layer will your products move to, during the extraction process (ether or water)?(2 pts) For the washing process, you used a brine solution. Explain what brine is (2 pts).

The product moves to the ether layer.

Brine is a saturated NaCl aqueous solution.

Thymol blue is a pH indicator. When the solution is neutral, the color of thymol blue changes into yellow from

red.

Acetic acid can be evaporated during the steam distillation process, so we need to convert it to salt form by

reaction with NaOH. Acetic acid + NaOH -> Sodium acetate + H2O

NaOCl + NaHSO3 -> NaCl + NaHSO4

The oxidants (NaOCl) will be reduced to convert into sodium chloride (NaCl)

216 F10-Exam #1 Page 5 of 13. Lab section _______________ Name____________________ IV. (15 points)

1) A protection step is essential for some organic synthetic processes. For the following reaction, carboxylic acids are protected prior to the reduction process. Provide in the boxes below the structure of the appropriate intermediate compounds or products. (10 points)

+ Ethanol

2) Explain the possible changes of the yield of the first reaction, when we used conc. HCl (33 wt%) for the acid catalyst of the first step of reaction, instead of conc. H2SO4 (98 wt%).(5pts)

The first reaction is the esterification reaction which is reversible, and water contents cause the yield of

esterified products to decrease. (3pts)

The conc HCl contains a lot of water contents. (67 wt% of the solution is water).

Therefore we cannot expect high yield of the reaction.(2pts)

216 W10-Exam #1 Page 6 of 13. Lab section _______________ Name____________________ V. (12 points) In experiment 4, you used unknown compounds and reduced them. The following ketone compounds are some of your unknown compounds. 1) Draw the structure of the products after reduction of the following starting materials. (4pts)

2) The melting points of both starting materials above are very similar and we cannot find a huge difference between the melting points of the products. Therefore, we should use some other characterization methods for identifying each structure. Using TLC and IR results before and after reduction, describe the difference between benzophenone and 2-acetonaphthalene. (8 pts)

TLC (4 pts) After the reduction of benzophenon, the spot of the product on a TLC plate cannot be observed under UV light, so the TLC plate needs to be stained by a vanillin solution for the observation. On the other hand, the product of the reduction of acetonaphthalene can be viewed under UV light. IR (4 pts) Acetonaphthalene has methyl group, so we can observe the weak peak at 2900 cm-1 derived from VC-H of alkane groups

216 W10-Exam #1 Page 7 of 13. Lab section _______________ Name____________________ VI. (8 points) When liquids are immiscible, such as toluene and water, they can be distilled by steam distillation. How many grams of water must be distilled to steam distill 3.889 g of toluene (molecular weight = 93 g/mole) from an aqueous solution? (4 potints) What will be the composition (wt % of toluene and water) of the distillate? (4 potints) The mixture distills at 85.0 oC (Vapor pressur of water at 85.0 oC = 433.6 mm Hg).

VII. (6 points) For each of the following synthetic reactions, draw the structure of an organic compound in each of the boxes provided.

The partial pressure of toluene is as follows: Po

toluene = P total - Powater

= (760 – 433.6) = 326.4 mm Hg The composition of the distillate is as follows: Weight of toluene/weight of water =(326.4 (mm Hg) * 93 (g/mole)) / (433.6 (mm Hg) * 18 (g/mole)) = 3.889 gtoluene/gwater

From the weight ratio, the weight of water is 1 g The weigh percentage is as follows: Toluene = 3.889 g / (3.889 g +1 g) *100 = 79.5 wt % Water = 1 g / (3.889 g +1 g) *100 = 20.5 wt %

216 W10-Exam #1 Page 8 of 13. Lab section _______________ Name____________________ VIII. (10 points) You find four unlabeled solvent bottles. From a solvent list, you know that they contain either acetone, ethyl acetate, N, N-dimethylacetamide (DMA), or acetic acid. Identify each unlableled solvent (A, B, C, and D) using their IR spectra. (4pts) Explain the peak shift of VC=O in IR spectra of ethyl acetate and DMF. (6pts)

Solvent A

Solvent B

Acetone

Acetic acid

216 W10-Exam #1 Page 9 of 13. Lab section _______________ Name____________________

Solvent C

Solvent D

Ethyl acetate

DMA

V C=O in the IR spectrum of ethyl acetate The peak is shifted to higher wave number region than 1715 cm-1 (Inductive effect, 3pt)

V C=O in the IR spectrum of DMA The peak is shifted to lower wave number region than 1715cm-1 (Resonance effect, 3pt)

216 W10-Exam #1 Page 10 of 13. Lab section _______________ Name____________________ IX. (12 points) After a certain reaction, you ran a TLC of the starting material and product, and got the following result.

1) When you used equal volumes of ethyl acetate and hexane as the eluent for TLC, you did not get very resolved spots. After changing the solvent composition of ethyl acetate and hexane, you were able to achieve greater separation of the spots. Explain the change of solvent composition. (4 pts)

2) The type of reaction can be inferred from the TLC result. From the choice in the keywork box below, choose the mostly likely reaction (from examination of the TLC) and explain you reasoning. (4 pts)

.

3) Predict the change of IR spectrum before and after the reaction. (4 pts)

Starting material: vC=O peak (ca. 1700 cm-1) can be observed. After the reaction, vO-H peak (3200- 3600 cm-1) can be observed.

Reduction.

The starting material has higher Rf than that of the product. This implies that starting material is less polar than the product. In the keyword box, we can obtain such product by reduction.

Hexane is non polar solvent. For the better resolution of TLC, we need to add more hexane.

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