1. Offer an explanation of why anthracene preferentially forms a Diels-Alder adduct at 9,10 positions.
2. There are four reasonable resonance structures for anthracene. Draw them.
3. A large number of polycyclic benzenoid aromatic hydrocarbons are known. One of these, benz/a/pyrene, is a powerful carcinogen found in tobacco smoke. From the literature, locate and then draw the structure of this hydrocarbon. Can you suggest other sources where this material might be expected to be present?
4. Outline a synthetic reaction scheme for the preparation of triphenylmethanol from:
a. Methyl benzoate
b. Diethyl carbonate
5. In the experiment, ligroin may be used as a solvent for the separation of the product from biphenyl.
a. What is ligroin?
b. Can you suggest an alternative solvent that might be used in this step?
6. Give the reaction scheme, showing the products formed (before hydrolysis), when one equivalent of ethylmagnesium bromide is treated with one equivalent of 5-hydroxy-2-pentanone. Does addition of two equivalents of the Grignard reagent to this yield a different product(s)? If so, give structure(s).
7. Explain why Grignard reagents cannot be prepared from an organic halide that also contains a hydroxyl (-OH), a carboxyl (-CO2H), a thiol (-SH), or an amino (-NH2) group.
8. What would be the final product of the reaction between methyl benzoate and two equivalents of ethylmagnesium bromide?
9. Consider the same reaction as in the previous question (#8) except that in this case it is carried out with ethyl benzoate. What product would be expected in this case?
10. Grignard reagents may be used to prepare other organometallic reagents, for example, ethylmagnesium bromide reacts with cadmium chloride to yield diethylcadmium.
2 CH3CH2MgCl + CdCl2 ----- (CH3CH2)2Cd + 2 MgCl2
Indicate the product of the following reactions and name the organometallic product.
4 CH3MgCl + SiCl4 -----
2 C6H5MgCl + HgCl2 -----
11. para-Nitrobenzaldehyde reacts at a faster rate than benzaldehyde in the Perkin reaction while p-N,N-dimethylaminobenzaldehyde is much less reactive toward the same nucleophile. Explain.
12. There are numerous condensations that are closely related to the Perkin reaction. Among these are the aldol (see Experiment #20), /3Badv/, and /A3a/, Knoevenagel, Claisen (Experiment /3Aadv/), and Dickmann condensations. What general class of compounds can be prepared using each of these well-known reactions?
13. Why it is important that any aldehyde used in Witting reaction be free of carboxylic acid impurities?
14. Explain why the C=C stretching mode gives rise to a rather weak IR band in 1-methylcyclohexane, while in its isomer, methylenecyclohexene, the band is of medium to strong intensity.
15. Predict the major organic product formed in the following reaction:
CH3CH2NO2 + CH2O NaOH
16. "Crossed" or "mixed" aldol condensations are practical for synthesis, if one of the aldehydes (or ketones) has no alpha-hydrogen atoms. Explain.
17. In the aldol condensation using the conditions of this experiment, why might it be essential that the benzaldehyde contain no benzoic acid?
18. It is likely that the amount of Grignard reagent your analysis indicates was formed is greater than the amount of Grignard reagent actually present just before you added water. Explain.
19. What hydrocarbon would you expect to obtain by the action of water on each of the Grignard reagents listed below?
a. Butylmagnesium bromide
b. sec-Butylmagnesium bromide
c. iso-Butylmagnesium bromide
d. tert-Butylmagnesium bromide
20. What product would each of the Grignard reagents in the previous question (#19) yield when treated with D2O?
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.
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. When tert-pentyl bromide is treated with 80% ethanol, the amounts of alkene products indicated in your lab textbook on pg. 218 (#6.57) are detected on analysis. Explain why Compound I is formed in far greater amount than the terminal alkene.
7. Why is sulfuric acid, rather than hydrochloric acid, used to catalyze the dehydration of alcohols?
8. 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.
9. Compound A is optically active and has the molecular formula C5H10O. On catalytic hydrogenation (addition of hydrogen) of A, Compound B is obtained. Compound B has the molecular formula C5H12O and is optically inactive. Give the structure for Compounds A and B.
10. Which of the following compounds have a meso form?
11. 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.
12. a. How many peaks would you expect to find in the NMR spectrum of caffeine?
b. What characteristic absorption bands would you expect to find in the infrared spectrum of caffeine?
13. The vapor pressures of 1,2-diphenylethane, p-dichlorobenzene, and 1,3,5-
trichlorobenzene are 0.06, 11.2, and 1.4 torr, respectively, at their melting point (52-54 degrees C). Which compound is likely to be sublimed most rapidly at a reduced pressure of 15 torr and a temperature of 40 degrees C?
14. To color spots on TLC plates for easier visualization after elution with solvent, the plates can be "developed" in a sealed chamber containing solid iodine. Explain how the solid-vapor equilibrium operates in this instance.
15. Caffeine is soluble in ethyl acetate. Do you think that the purity of your product could be checked by TLC using ethyl acetate as an elution solvent? Explain.
16. List several advantages and disadvantages of steam distillation as a method of purification.
17. Explain why the distillate collected from the steam distillation of cinnamon is cloudy.
18. Calculate the weight of water required to steam distill 500 mg of bromobenzene at 95 degrees C. The vapor pressure of water at this temperature is 640 torr; bromobenzene, 120 torr.
19. Steam distillation may be used to separate a mixture of p-nitrophenoln and o-nitrophenol. The ortho isomer distills at 93 degrees C; the para isomer does not. Explain.
20. The chiral carboxylic acid A (C5H6O2) reacts with 1 mol of hydrogen gas on catalytic hydrogenation. The product is an achiral carboxylic acid B (C5H8O2). What are the structures of Compounds A and B?
40 Organic Laboratory Questions are answered with clear explanations.