CHEMISTRY 332 FALL 08 EXAM I September 24-25, 2008

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1 First Three Letters of Last ame AME etwork ID EMISTRY 332 FALL 08 EXAM I September 24-25, 2008 The following materials are permissible during the exam: molecular model kits, course notes (printed, electronic, or hand written), textbook, calculator, Internet browser. You are not permitted to receive assistance, in any way, from another person during the exam. Receiving aid on an exam from any person, by any means of communication, is considered academic misconduct and can result in a failing grade for that exam. Answers must be submitted via AE rganic during the scheduled examination period. The paper copy of your exam will be collected, but not graded. Your goal is to answer every question correctly, in as few attempts as possible. Average Bond Energies (kcal mole -1 ) F Si S l Br I a 73 b 86 c 116 d e F a = 146, Si b = 147, 213 c = 176 (aldehydes) S d In F 4 58 l 46 Br e In nitrites and nitrates 36 I 179 (ketones)

2 EMISTRY 332 1) [4 pts. full credit; 0 pts. residual credit] orbornene-7-ylidene (A) is less reactive than (B). onsider the possible frontier orbitial interactions in (A) not present in (B). heck one statement that best explains the unusual stability of carbene (A) relative to (B). It may be helpful to examine a molecular model. (A) (B) 2) [2 pts. full credit; 0 pts. residual credit] According to the frontier orbital analysis used to answer QUESTI 1, which one statement best describes the reactivity of (A) relative to (B)? (A) (B) 3) [5 pts. full credit; 2 pts. residual credit] The transition state structure for the reaction of (B) with methanol is shown below. Finish drawing the expected product. It will help to consider the electron flow based on curved arrow notation leading to this transition state structure and the resulting product. 3 3 (B)? 4) [4 pts. full credit; 1 pts. residual credit] n the basis of the transition state structure shown in QUESTI 3, check two statements that best describe the frontier orbitals involved in this reaction. 1

3 EMISTRY 332 5) [5 pts. full credit; 2 pts. residual credit] The transition state structure for one step of a postulated mechanism that describes the reaction of (B) with methyl amine is shown below. n the basis of the transition state structure, finish drawing the intermediate resulting from this step. It will help to consider the electron flow based on curved arrow notation leading to this transition state structure and the resulting intermediate (Ref. J. rg. hem. 2008, 73, 6551). 3 2!" 3!? (B) 6) [5 pts. full credit; 2 pts. residual credit] The transition state structure of an alternative mechanism is shown below. n the basis of the transition state structure, finish drawing the two products resulting from this transition state. It will help to consider the electron flow based on curved arrow notation leading to this transition state structure and the resulting products.? (B) 3 7) [4 pts. full credit; 1 pt. residual credit] Suppose that the steps in QUESTIS 5 and 6 are rate determining for two different mechanisms; check all of the statements that are true. 8) [4 pts. full credit; 1 pt. residual credit] The Wikipedia entry for the mechanism of the benzoin condensation is shown below. Indicate the steps for which curved arrows are either missing or drawn backwards. A B D E 2

EMISTRY 332 9) [4 pts. full credit; 1 pt. residual credit] The image below outlines the mechanism of imine formation. Indicate the steps for which curved arrows are either missing or drawn backwards.

4 EMISTRY 332 9) [4 pts. full credit; 1 pt. residual credit] The image below outlines the mechanism of imine formation. Indicate the steps for which curved arrows are either missing or drawn backwards. A B D E The next three questions address the relative stabilities of cis- and trans-dichlorodiazenes whose structures are shown below (Ref. J. rg. hem. 2008, 73, 5429). l l cis-dichlorodiazene l l trans-dichlorodiazene In a recently published calculation, cis-dichlorodiazene was determined to be more stable than trans. onsider the hybrid orbitals of cis- and trans-dichlorodiazenes and the various pi-type frontier orbital interactions involved in resonance (a list of these interactions is provided in the AE rganic question). The alignment of the filled and empty orbitals involved in these interactions can vary for the cis and trans isomers. onsequently, energies associated with the various resonance interactions may contribute more to the stability of the cis isomer, contribute more to the stability of the trans isomer, or contribute approximately the same to both isomers. 10) [4 pts. full credit; 1 pt. residual credit] Which interaction(s) will help stabilize the cis isomer relative to the trans isomer? 11) [4 pts. full credit; 1 pt. residual credit] Which interaction(s) will contribute about the same to the stability of the cis and trans isomers? 12) [4 pts. Full credit; 0 pts. Residual credit] onsider the most important frontier orbitals involved in resonance for the cis and the trans isomers. Which one statement explains why the frontier orbitals involved in resonance have better overlap for the cis isomer than the trans? 3

5 EMISTRY ) [5 pts. full credit; 0 pts. residual credit] The aryl-substituted oxyindole substructure is present in natural products and in several potent therapeutic agents, such as anticancer drugs (1), cardiovascular drugs (2), and laxatives (3). l S 3 a a 3 S The following reaction was recently proposed as a new method for synthesizing aryl-substituted oxyindoles (Ref: J. rg. hem. 2008, 73, 6506). Balance the stoichiometry. Then use bond energy calculations to estimate the enthalpy change. T benzene / F 3 S 3 14) [5 pts. full credit; 2 pts. residual credit] The first equivalent of preferentially adds to one of the three heteroatoms of compound T. f the three heteroatoms, the preferred site of addition is the one that enables charge delocalization through a pair of resonance contributors (both contributors have an octet of electrons on all carbon, nitrogen, and oxygen atoms). Draw the best resonance contributor of T following the addition of a proton. (1 equiv) T draw the best resonance contributor following addition of a proton 4

6 EMISTRY 332 The next two questions involve the ionization state of isoniazid, an orally administered compound that is used in the treatment of tuberculosis. The structure of isoniazid is shown below along with estimated pka values of its functional groups. The percentages of the various ionized forms of isoniazid vs. p are shown in the plot. Ionized forms P, Q and R are present to a significant degree in the stomach (p = 2.0) upon ingestion of isoniazid and play a key role in the adsorption kinetics of this therapeutic agent. P S 2.4 (conjugate acid) Q 13.6 R Isoniazid 3.4 (conjugate acid) 15) [5 pts. full credit; 2 pts. residual credit] Draw the structure of P. 16) [5 pts. full credit; 2 pts. residual credit] Draw the structure of R. Questions ask you to draw missing structures in the mechanism shown at the top of the next page. This mechanism accounts for the formation of products G and J from aryldiazoacetate E and aldehyde F under acidic conditions. 2 2Me E F G 2Me J 2Me Study the mechanism. Then use the indicated frontier orbital interactions as a guide to determine the requested structures (Ref. J. Am. hem. Soc. 2008, 130, ). 5

7 EMISTRY E 2 Me F G 2 Me J 2 Me pi-type n!pi* resonance interaction QUESTI 17 E' resonance contributor of E revealing its carbanion character F' sigma-type n!pi* QUESTIS 18 & 19 M 2 pi-type sigma!sigma* 2 X Y 2 Me 2 Me G J 17) [6 pts. full credit; 3 pts. residual credit] Draw E, a resonance contributor of E that takes into consideration a n pi* pi-type frontier orbital interaction and which reveals the carbanion character of E. 18) [4 pts. full credit; 3 pts. residual credit] Draw M which results from the combination of E and F by an n pi* frontier orbital interaction. You must also draw M in a conformation that produces intermediate X by a pi-type, sigma sigma* frontier orbital interaction. Follow the steps below to draw this conformation. i. Do not delete or erase any atoms or bonds in the template. Do not move atoms in the template. You must replace the hydrogen atoms with the necessary bonds and atoms as described below. ii. lick on a atom in the structure to replace that atom with the atom or group at the tip of the cursor. 19) [4 pts. full credit; 3 pts. residual credit] Draw M which results from the combination of E and F by an n pi* frontier orbital interaction. You must also draw M in a conformation that produces intermediate Y by a pi-type, sigma sigma* frontier orbital interaction. Follow the steps below to draw this conformation. i. Do not delete or erase any atoms or bonds in the template. Do not move atoms in the template. You must replace the hydrogen atoms with the necessary bonds and atoms as described below. ii. lick on a atom in the structure to replace that atom with the atom or group at the tip of the cursor. 6

EMISTRY 332 20) [6 pts. full credit; 2 pts.

Your task is to interpret these pictures.

orientation of the orbitals with respect to the molecular plane). vinyl carbene 1 2 3 4 5 6 21) [6 pts. full credit; 3 pts.

It undergoes a unimolecular rearrangement that involves two pairs of frontier orbitals (a sigma a pi-type interaction and a n sigma* pi-type interaction).

8 EMISTRY ) [6 pts. full credit; 2 pts. residual credit] The Lewis structure of the simplest vinyl carbene 2 2, is shown below along with pictures of 6 different molecular orbitals. Your task is to interpret these pictures. Match the number in each picture to the name that indicates the kind of molecular orbital represented (int: Think about the hybrid Ms and pay attention to the orientation of the orbitals with respect to the molecular plane). vinyl carbene ) [6 pts. full credit; 3 pts. residual credit] Vinyl carbene is not stable. It undergoes a unimolecular rearrangement that involves two pairs of frontier orbitals (a sigma a pi-type interaction and a n sigma* pi-type interaction). Based on this description, predict the product that will result from the following vinyl carbene. (Ref. J. rg. hem. 2003, 68, 1339). pi-type sigma!a and pi-type n!sigma* draw the product resulting from the indicated frontier orbital interactions 22) [6 pts. full credit; 3 pts. residual credit] The neutral compound 3 2 has been detected in interstellar space. Such a structure is unusual and challenges our ideas of chemical bonding. Draw a Lewis structure of 3 2 that has no rings, no triple bonds, no () or (-) formal charges, and no radicals (i.e., no unpaired electrons). (Ref. J. Am. hem. Soc. 1997, 119, 5847). 7

CHEMISTRY 332 SUMMER 08 EXAM I June 26-28, 2008

First Three Letters of Last Name NAME Network ID CHEMISTRY 332 SUMMER 08 EXAM I June 26-28, 2008 The following materials are permissible during the exam: molecular model kits, course notes (printed, electronic,