Chemistry 2050 Introduction to Organic Chemistry Fall Semester 2011 Dr. Rainer Glaser

Transcription

1 Chemistry 2050 Introduction to Organic Chemistry Fall Semester 2011 Dr. Rainer Glaser Examination #2 Practice Edition Arenes, Stereochemistry, and Organic Halogen Compounds, with Nucleophilic Substitution and Elimination. Wednesday, October 5, 2011, 10 10:50 am Name: Answer Key Question 1. Aromatic Compounds Names, Bonding and Properties 20 Question 2. Electrophilic Aromatic Substitution 20 Question 3. Stereoisomers Chirality & Geometrical Isomers 20 Question 4. Mechanisms of Nucl. Subst. and Elim. Reactions 20 Question 5. Reactions of Organic Halides with Nucleophiles 20 Total 100 1

2 Question 1. Aromatic Compounds Names, Bonding and Properties. (20 points) (a) Provide abbreviated structural formulas of acetophenone and styrene. (4 points) Acetophenone Styrene (b) Two structures are shown of C 6 H 4 Br 2. Are these compounds describing one compound or two different compounds? Explain. (6 points) These are resonance forms of one structure. Aromatic compounds contain cyclic delocalized pi-systems. (c) Draw the structure of 2,4,6-trinitrotoluene. The nitro groups are in meta (ortho, meta, para) positions relative to each other. The methyl group is ortho with regard to 2 (zero, one, two, three) nitro group(s), it is meta with regard to 0 (zero, one, two, three) nitro group(s), and it is para with regard to 1 (zero, one, two, three) nitro group(s). (6 points) (d) The model shows a small graphene sheet. Graphene sheets occur in (diamond, graphite, fullerenes). All carbons in graphene sheets are (sp, sp 2, sp 3 ) hydridized and there (are, are no) delocalized electrons. (4 points) 2

3 Question 2. Electrophilic Aromatic Substitution. (20 points) (a) Provide complete structural formulas for the formation reaction of the electrophile which is the reactice species in the nitration of benzene by way of aromatic electrophilic substitution. (6 points) (b) Draw the complete, unabbreviated structural formula of sigma-complex which occurs as an intermediate in the nitration of benzene. (4 points) (c) Consider the nitration of toluene. The substituent present in toluene is (activating, deactivating) and (o/p, meta) directing. In the space below, draw the three most relevant resonance forms of the sigma-complex which occurs as an intermediate in the formation of paranitrotoluene. [Do only consider this one sigma-complex here; do not consider the sigma-complexes for the respective formations of ortho- or meta-nitrotoluene.] Explain whether any of the resonance forms have any special advantage or disadvantage. State whether para-nitrotoluene will be formed. (10 pts.) Resonance B has a special advantage: The plus charge resides on the tertiary carbon. 3

4 Question 3. Stereoisomers Chirality & Geometrical Isomers. (20 points) (a) The amino acid alanine has the structure H 2 N CH(CH 3 ) COOH and contains one chiral C atom. Provide the CIP priorities of the four substituents. For the two C-atoms, apply the sequence rule and provide their lists. (6 points) (b) Two models are shown of the amino acid alanine. Do both models show the same molecule or do they show different enantiomers? Provide the R or S label for each model. (6 points) Priority of H: 4 Priority of NH 2 : 1 Priority of CH 3 : 3 C(H H H) Priority of COOH: 2 C(O O O) Model #1 Model #2 Both are S. (c) Perspective drawing of the (R)- enantiomer of alanine. 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) The line segment drawing is provided of one isomer of 1,4-hexadiene. This molecule can form (1, 2, 3, 4) geometrical isomers. The isomer shown is the (E; Z; E,E; Z,Z; E,Z) isomer. (4 points) 4

5 Question 4. Mechanisms of Nucl. Subst. and Elim. Reactions. (20 points) (a) The S N 2 mechanism is one of the most studied reaction mechanisms in organic chemistry. In the great majority of cases, the nucleophile X - attacks the carbon that carries the leaving group Y - from the backside and a Walden inversion occurs. This works really well for (primary, secondary, tertiary) substrates and the penta-coordinate transition state structure is readily accessible. In the transition state structure, the C X bond is partially formed and the C Y bond is partially broken. Using dashed lines for partial bonds, provide a perspective drawing of the transition state structure for the S N 2 reaction of iodide nucleophile with substrate ethyl chloride. Draw the C---X and C---Y bonds horizontally in the paper plane and draw the C CH 3 bond also in the paper plane. (8 points) (b) In the transition state structure of the S N 2 reaction of iodide nucleophile with substrate ethyl chloride (reaction in (a)) the (H C CH 3 ) angle is approximately (90, 109, 120 ), the (X C CH 3 ) angle is approximately (90, 109, 120 ), and the (Y C CH 3 ) angle is approximately (90, 109, 120 ). Now consider the replacement of the CH 3 group by an isopropyl group, CH(CH 3 ) 2. The new substrate (H 3 C) 2 CH CH 2 Cl (still, now) is a (primary, secondary, tertiary) chloride. Do you think the S N 2 reaction with iodide will be faster or slower for (H 3 C) 2 CH CH 2 Cl than for H 3 C CH 2 Cl? Briefly explain. (8 points) A little slower. The substrate still is a primary substrate, but the approach of the iodide will be somewhat hindered when two Hs are replaced by two CH 3 groups. (c) Reactions in which the solvent also is the nucleophilic reagent are called solvolysis reactions. The reaction of tertiary halides such as (H 3 C) 3 C Cl with methanol H 3 C OH is an example of such a solvolysis. This reaction involves a (weak, strong) nucleophile and will proceed via the (S N 1, S N 2) mechanism. The reaction proceeds by (homolysis, heterolysis), a (carbanion, radical, carbocation) is formed as intermediate, and the reaction will be faster in (more, less) polar solvents. (4 points) 5

6 Question 5. Reactions of Organic Halides with Nucleophiles. (20 points) (a) Using abbreviated structural formulas, show the substrate, the reagent, and the major product of the reaction of sodium ethoxide with 1-bromobutane. (5 points) (b) Using abbreviated structural formulas, show the substrate, the reagent, and the major product of the reaction of sodium cyanide with tertiary butylbromide. (5 points) (c) Using abbreviated structural formulas, show the substrate, the reagent, and the major product of the reaction of water with tertiary butylbromide. (5 points) (d) Using abbreviated structural formulas, show the substrate, the reagent, and the major product of the reaction of H 3 C CH 2 O - K + with 2-bromopropane. (5 points) 6