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Esters, sulfuric acid, and gas chromatographic analyses

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1. In the preparation of the esters given in this experiment, the reaction product was extracted with 5% sodium bicarbonate solution (NaHCO3) in the isolation step. Why? What gas was evolved during this washing step? Write a balanced equation for the reaction that produced it.

2. Why is a large excess of acetic acid used in the preparation of isopentyl acetate?

3. Concentrated sulfuric acid is used as a catalyst for the esterification of acetic acid in the preparation of isopentyl acetate. Why is sulfuric acid needed if another acid, acetic acid, is already present?

4. Fatty acids are long-chain carboxylic acids, usually of 12 or more carbon atoms, isolated from saponification of fats and oils (esters of glycerol). Draw the structure of the two fatty acids named below and also determine their common name.

Hexadecanoic acid
Octadecanoic acid

5. Gas chromatographic analysis of a mixture of organic compounds gave the following peak areas (sq cm): hexane = 2.7; heptane = 1.6; hexanol = 1.8; toluene = 0.5.

(a) Calculate the mole percent composition of the mixture. Assume that the response of the detector (area per mole) is the same for each component.
(b) Calculate the weight percent composition of the mixture, using the same assumptions as in part (a).

6. Why is sulfuric acid, rather than hydrochloric acid, used to catalyze the dehydration of alcohols?

7. A sample of 150 mg of an organic compound is dissolved in 7.5 mL of water. The solution is placed in a 20-cm polarimeter tube and the rotation measured in a polarimeter. The rotation observed was +2.676 degrees. Distilled water, in the same tube, gave a reading of +0.016 degrees. Calculate the specific rotation for the compound.

8. Compound A is optically active and has the molecular formula CSH100. On catalytic hydrogenation (addition of hydrogen) of A, Compound 8 is obtained. Compound B has the molecular formula CSH120 and is optically inactive. Give the structure for Compounds A and B.

9. Which of the following compounds have a meso form?

2,3-Dibromopentane
2,4-Dibromopentane
2,3-Dibromobutane

10. Compounds such as naphthalene and 1,4-dichlorobenzene find use as mothballs since they sublime at a slow rate at atmospheric pressure. Explain this behavior in terms of the structure of the molecules.

11. How many peaks would you expect to find in the NMR spectrum of caffeine?

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Solution Preview

1. The reaction product was extracted with 5% sodium bicarbonate solution in the isolation step, so as to separate the excess acetic acid from the ester. This leads to the purification of the ester because the acetic acid is soluble in the sodium bicarbonate with which it reacted to produce carbon dioxide gas.

CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O

2. A large excess of acetic acid is used in the preparation of isopentyl acetate to ensure that that there is complete neutralization of the isopentyl alcohol.

3. Sulfuric acid acts as a catalyst as it is regenerated . Usually the reactions involving organic molecules are slow because they contain larger ions that have low velocity, so collision is rare. The H+ produced from the sulphuric acid ...

Solution Summary

Eleven answers provided regarding Esters, sulfuric acid, gas chromatographics, compounds, and peaks. Attachment included with formulas.

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