Chem 150 Final Exam 14 Dec. 1998

Transcription

1 Chem 150 Final Exam 14 Dec Total Points: 300 ote: Relax. Don t panic. There is plenty of time. The test looks long but you have two hours! Survey entire exam before answering. Q1. [15 points] (a) 2-Vinylmethylenecyclopropane rearranges in the gas phase to 3-methylenecyclopentene (J. Am. Chem. Soc. 1973, 95, 8096). Two possible reaction mechanisms are: (1) or (2) (i) (ii) Sketch a reaction energy diagram for each process. ow might an isotopic labelling experiment distinguish between these mechanisms?

2 Q2. [30 points] Write careful arrow-pushing mechanisms for the following. Make sure you proceed logically and pay close attention to the pattern recognition rules that you have learnt over the semester. ote that in (a), you cannot planarize the bridgehead carbon marked (*) (Bredt s Rule). (a) Wender J. Am. Chem. Soc. 1982, 104, * K + C 3 I * (b) C 3 3 C C 2 (1) excess LiAl 4 Tos (2) +, 2

3 Q3. [30 points] The rate of acid catalyzed lactonization of hydroxyvaleric acid (VA) is given by the rate law: Cl + 2 Rate = d[va]/dt = k [VA][Cl] where k = 4.0 M 1 min 1 at 25 ºC. (a) Assuming negligible back reaction, calculate the time required for half the VA to react, if the initial concentrations of both VA and Cl are 0.01 M (25 ºC). (b) For the same initial concentrations as in (a), at what temperature should the half life be halved if the activation energy is 20 kcal/mol?

4 Q 4. [25 points] A simple reaction for a DA molecule is the exchange of a DA proton with a water proton (in this case, deuterium). In the DA base pair shown below the imino proton (in red) exchanges with heavy water (D 2 ) at a rate measurable by MR spectroscopy. C 1 ' T A C 1 ' The stoichiometry of the reaction is T A + DD TD A + D A mechanism for it s rate of exchange is as follows. (T A) closed k o (T A) (1) open k c k t (T A) open + Base + DD TD A + Base + D (2) where (T A) closed represents a closed base pair; (T A) open represents a open base pair ready for exchange; the Base is used as a catalyst Use the steady state approximation for [(T A) open ] to obtain an expression for the initial rate {assuming that the reverse rate constant in eqn. (2) is negligible} of exchange, d[td A]/dt.

5 Q5. [25 points] (a) n standing, cyclopentadiene spontaneously forms dicyclopentadiene (A), from which it can be regenerated by "cracking". A (i) What reaction has taken place in the formation of dicyclopentadiene? (ii) What reaction would regenerate cyclopentadiene? (iii) Draw a transition state picture and give a physical basis for production of stereoisomer A only. (b) Predict the product for the following reaction. Pay attention to both regio and stereo chemistry. + Et C 3??

6 Q6. [30 points] Although "Dewar Benzene", I is less stable by 60 kcal/mol than it s isomer, benzene, it s conversion to benzene is surprisingly slow [E a = 37 kcal/mol; t 1/2 = 2 days (25 ºC)]. The high activation barrier has been attributed to the "symmetry-forbidden" nature of this transformation. Explain. (J. Am. Chem. Soc. 1977, 99, 8112) I Q7. [25 points] The alkaline polymerization of caprolactam gives the household polymer nylon-6. Caprolactam itself, can be made by the Beckmann rearrangement (to refresh your memories, Beckmann rearrangements convert oximes to amides under acidic conditions). What ketone would you start with to get to caprolactam? Write a mechanism for this reaction sequence.?? (1) 2 (2) + caprolactam

7 Q8. [20 points] Meanwhile, at the laboratory, our naïve first year graduate student (still hiding from Chem 150) has encountered serious frustration with the solvolysis of the diketone benzil (with the apparent formation of benzilic acid). This time around, having learned from previous mistakes, a Chem 150 veteran was consulted immediately. The Chem 150 student, in the usual cavalier fashion, cursorily glanced at the results and immediately knew the cause of panic and frustration; benzil had undergone the benzilic acid rearrangement. Write a mechanism for this transformation. int: When ac 3 was used as the base, the methyl ester of the carboxylic acid was formed. (1) a (ac 3 ) R (2) + benzil R = C (I), CC 3 (II) I = benzilic acid

8 Q 9. [20 points] The pk a of benzoic acid is 11.0 in DMS. Calculate the pk a of p nitrobenzoic acid in DMS. [ρ ArC 2 (DMS) = 2.48; σ (p 2 ) = 0.78] Q 10. [30 points] Predict the stereochemical outcome of the following reactions. (a) rg. Synth. 1984, 63, 89. (1) LDA, 78 C TMS (2) (3) 4 Cl

9 (b) Top. Curr. Chem. 1980, 88, 145. Ph C 3 (1) C 3 MgI (2) +, 2 (c) J. rg. Chem. 1986, 51, C 3 [P 3 R Ir(CD)pyr] 2 3 C

10 Q 11. [25 points] agemann s ester, a widely utilized intermediate in organic synthesis, is usually prepared using the following sequence (aldol, Michael, intra-aldol and decarboxylation). Show the steps in the synthesis of this ester. + ac 2 5, Et 3 + C 3 + C 2 + C 2 5 C 2 C 2 5 agemann's ester

11 Q 12. [25 points] ow would you prepare the following compounds from cyclohexanone? More than one step may be required. (a) (b) (c) C (d) C