Chemistry 2030 “Survey of Organic Chemistry” Fall Semester 2017 Dr

Chemistry 2030 “Survey of Organic Chemistry” Fall Semester 2017 Dr. Rainer Glaser

Examination #2 “Reactions of Alkenes & Alkynes, Chemistry of Aromatic Compounds, and Stereochemistry”

Thursday, October 5, 2017, 8:25 – 9:15 am

Name:

Question 1. Reactions of Alkenes & Alkynes. 20 Question 2. Aromatic Compounds – Names, Bonding and Properties. 20

Question 3. Electrophilic Aromatic Substitution: SEAr Mechanism. 15 Question 4. Electrophilic Aromatic Substitution: Directing Effects. 25 Question 5. Stereoisomers – Chirality & Geometrical Isomers. 20 Total 100

ALLOWED: Periodic System of the Elements (printed, w/o handwriting on it). Molecular models (you can bring pre-made models). Simple, non-programmable calculator (not really needed).

NOT ALLOWED: Books. Notes. Electronic devices of any kind (other than a simple calculator).

— 1 — Question 1. Reactions of Alkenes and Alkynes. (20 points)

(a) Acid-catalyzed hydration of propene affords one alcohol, the Markovnikov product. Draw the structures of the initially formed protonated propene and the product alcohol. Name the product. (6 p.)

(b) Now consider the hydroboration-oxidation of propene. Show the structures of the intermediate borane compound (formed by addition of borane, BH3, to propene) and of the final product (formed by subsequent workup with an alkaline solution of H2O2). The alcohol formed is the ______(Markovnikov, anti-Markovnikov) product. Name the alcohol formed. (5 points)

(d) Consider the acid-catalyzed Primary Product Final Product hydration of propyne. This (formed by hydration) (formed by tautomerization) alkyne hydration also requires a strong Lewis acid as co-catalyst; usually one uses ______(give formula of co-catalyst). Draw the structures of the primary product and of the final product. (6 p.)

— 2 — Question 2. Aromatic Compounds – Names, Bonding and Properties. (20 points)

(a) Provide complete structures (all atoms, all bonds, all lone pairs) of aniline and biphenyl. (4 points) Aniline Biphenyl

(b) Consider the structure of vanillin (4-hydroxy-3-methoxybenzaldehyde). The formyl group is in position 1, the so-called ______position. The methoxy group in position 3 and the hydroxyl group in position 4 are ______(ortho, meta, para) relative to each other. The formyl group in position 1 and the hydroxyl group in position 4 are ______(ortho, meta, para) relative to each other. The formyl group and the methoxy group are ______(ortho, meta, para) relative to each other. (4 points)

(c) The space-filling model shows the polycyclic aromatic hydrocarbon pyrene. Draw one complete Lewis-Kekule structure of pyrene on the right (all atoms, all bonds, all lone pairs). No need to draw all resonance forms; just draw one resonance form. Below, show all possible products of monochlorination of pyrene. You can use abbreviated formulas for the chloropyrenes and, again, just draw one resonance form for each isomer. (6 points) Pyrene

Isomers of Chloropyrene. [Hint: How many different types of H occur in pyrene?]

(d) The heat of hydrogenation of cyclohexene is about 28.6 kcal/mol. The complete hydrogenation of benzene to ______(name of the C6H12 compound formed) releases about 49.8 kcal/mol. Based on the heat of hydrogenation of cyclohexene, one might have expected that the hydrogenation of benzene is much more exothermic, i.e., one might have expected a value of about ______kcal/mol. The difference between the expected and the measured values is the resonance energy of benzene, which is about ______kcal/mol. (6 points)

— 3 — Question 3. Electrophilic Aromatic Substitution: SEAr Mechanism. (15 points)

(a) Provide reagents and any catalyst needed to convert benzene into bromobenzene and draw the complete, unabbreviated structure of bromobenzene. (5 points)

(b) Provide reagents and any catalyst needed to convert benzene into acetophenone by Friedel-Crafts acylation and draw the complete, unabbreviated structure of acetophenone. (5 points)

(c) Consider the sigma-complex, which occurs as an intermediate in the Friedel-Crafts acylation of benzene to form acetophenone. Draw three resonance forms of this sigma complex that inform about the delocalization of the positive charge in the benzene ring. [Here, we are not interested in the resonance within the C=O group.] (6 points)

— 4 — Question 4. Electrophilic Aromatic Substitution: Directing Effects. (25 points)

(a) Consider the bromination of anisol. Write the reagent in the box on top of the reaction arrow and write the required catalyst in the box below the reaction arrow. For each one of the products, indicate whether it is a “minor” or a “major” product. (5 points)

(b) Outlines are shown of resonance forms of the sigma-complexes of the bromination of anisol in the ortho (top), meta (center), and para (bottom) positions. Complete the resonance forms: Add all double bonds and all lone pairs on the anisol O atom. Bromine will always have ______lone pairs and there is no need to draw the bromine lone pairs. (12 points)

— 5 — (c) Directing effects in the bromination of anisol. Explain your answer to part (a) considering the resonance forms you wrote in part (b). Note that the resonance forms in part (b) are labeled “A” – “L” for your convenience. Use these labels in your explanation. Be brief and concise. (4 points)

(d) Activation/Deactivation of the bromination of anisol. Select the correct answers for the following four questions. (4 points)

The methoxy group is ______(activating, deactivating) with regard to an electrophilic aromatic substitution.

Compared to the rate of reaction of the bromination of benzene, ortho-bromination of anisol is ______(much slower, slower, faster, much faster).

Compared to the rate of reaction of the bromination of benzene, meta-bromination of anisol is ______(much slower, slower, faster, much faster).

Compared to the rate of reaction of the bromination of benzene, para-bromination of anisol is ______(much slower, slower, faster, much faster).

— 6 — Question 5. Stereoisomers – Chirality & Geometrical Isomers. (20 points)

(a) Two models are shown of Model #1 or Thr Model #2 of Thr the amino acid threonine, H2N−CH(CH(CH3)−OH)−CO2H (O in red, N in blue). For each model, provide the “R” or “S” label for the alpha-C (the carbon to which the NH2 and COOH groups are attached). The stereoisomers shown are ______(the same, different). (6 points)

(b) The amino acid threonine has the structure H2N−CH(CH(CH3)−OH)−CO2H Priority of -H: ___ and contains a chiral alpha-C. Provide the Priority of -NH2: ___ CIP priorities of the four substituents. For Priority of -CH(CH3)-OH: ___ C(_ _ _) the two C-substituents, apply the sequence Priority of -COOH: ___ C(_ _ _) rule and provide their “lists”. (6 points)

(c) Provide a perspective drawing of the (R)-enantiomer of alanine. The amino acid alanine has the formula H2N−CH(CH3)−CO2H The perspective drawing should have two bonds in the paper plane, one bond that goes behind the paper plane, and one bond that goes in front of the paper plane. In addition, the C−H bond should be the bond that goes behind the paper plane. (4 points)

(d) Draw the structure of the (E)-isomer of 2-bromo-3-chloro-2-pentene. Show all atoms and all bonds. (4 points)

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