Basic Arrow-Pushing & Energy Concepts - Key Problem Set 1 Due Tuesday, 10/4/16

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1 asic ArrowPushing & Energy Concepts Key Problem Set 1 Due Tuesday, 10/4/16 1. For each compound, identify the atom most likely to be attacked first by a nucleophile and identify the unoccupied frontier orbital (e.g., p, π*, or σ*) associated with that atom. a. b. c. d. e. F Cl F C 3 C 3 a. b. c. d. e. u: σ* CF F u: Cl π* C u: π* u: F C 3 C p 3 σ* :u (:) 2. For each compound, identify the most reactive pair of electrons (either a bond or a lone pair) and specify the corresponding type of frontier orbital (i.e., n, p, or s). a. b. c. d. a. b. c. d. n Ȯ. n σ consider resonance : : : : n 3. Compare elementary reaction i with elementary reactions iiiv: i. ii. iii. iv. a. Which reactions do you expect to be slower than reaction i? b. Which reactions do you expect to be faster than reaction i? c. Draw M interaction diagrams for elementary reactions iiiv, clearly showing how the structural changes in the starting materials affect the energies of the filled π and empty p orbitals in the starting materials. Show qualitatively how the difference between π (filled M) and p (empty M) affects ΔE interaction. a. reactions iii and iv will be slower

2 b. reaction ii will be faster σ* CC σ* CC σ* CC p C p C p C σ* C pc E M ΔE interaction π C=C π C=C ΔE interaction σ CC σ CC ΔE interaction π C=C σ CC π = ΔE interaction σ C 4. Draw significant resonance structures for the following species: a. b. c... : not as significant as the other two (unfilled octet).. : 5. Using ess Law, decide whether each of the pericyclic reactions below is expected to be enthalpically favorable or enthalpically unfavorable. a. b. roken Formed σ CC 81 Δ = Σ(broken) Σ(formed) = 17 kcal/mol roken π C 94 Formed σ C 79 Δ = Σ(broken) Σ(formed) = 15 kcal/mol c. d.

3 roken Formed σ CC 81 σ CC 81 Δ = Σ(broken) Σ(formed) = 34 kcal/mol roken π C 94 Formed σ CC 81 σ C 79 Δ = Σ(broken) Σ(formed) = 2 kcal/mol 6. For each of the following reactions, ESTIMATE (without a calculator) the energy difference (or minimim energy difference) that accounts for the observed selectivity. a. 7dehydrocholesterol about 1.4 kcal/mol 25 C 8 : 92 25hydroxyvitamin D 3 b. anti T.S. is about 1.8 kcal/mol lower in energy than syn T.S. c. tu a 4 Et 25 C tu anti/syn d.r. 94:6 F 4 Et 2 S tu S 78 C tu syn T.S. is about 1.4 kcal/mol lower in energy than anti T.S. anti/syn d.r. 9/91 7. Suggest a plausible arrowpushing mechanism for the following reaction. FS 3 C 2 Cl 2 78 C, 1 h

4 A: A 2

5 Practice Problems The following problems demonstrate important concepts, but will not be graded. We will cover some of them in discussion section. 8. Suggest a plausible arrowpushing mechanism for the following reaction that is consistent with the stereochemistry of the product.. Mc 1:1 TF/ 2 rt, 20 min. 82% Mc = S Cl better L.G. than Ms. Mc : 2 u : A 9. Suggest a plausible arrowpushing mechanism for the following reaction. TsCl (= pc 3 C 6 4 S 2 Cl) C 3 pyridine 23 C, 1h Ts C 3 : R Tol Tol S Cl S Cl R Tol S Cl R : R Tol S 10. Suggest a plausible arrowpushing mechanism for the following reaction. 1) 9, TF 23 C, 14 h 2) 2 2, 2, a 23 C, 6 h racemic 9 = The pk a for hydrogen peroxide () is 11.6 R 2 R R R 2 R 2 : : R R R 2

6 11. Suggest a plausible arrowpushing mechanism for the following reaction. P C2 TF 23 C, 1 h C 2 P C2 P P 12. Suggest a plausible arrowpushing mechanism for the following reaction. Cl K 2 C C, 30 h : Cl Cl 2 C : C 2 : 13. ot all reactions are run at room temperature where free energy differences of 1.36 kcal/mol correspond to factors of ten in rates or ratios. a. What free energy difference corresponds to a factor of ten at 100 C? b. What free energy difference corresponds to a factor of ten at 78 C?

7 You get similar equations whether you are talking about differences in ground state energies or differences in transition state energies. ΔG = RTlnK eq = RTln k 1 ΔΔG k 2 ΔΔG ΔG at 100 C ΔG = (1.985 cal K 1 mol 1 )(373 K) ln 10 = 1.7 kcal mol 1 at 78 C ΔG = (1.985 cal K 1 mol 1 )(195 K) ln 10 = 0.89 kcal mol Provide a plausible arrow pushing mechanism for the following reaction. Anwer: : K : n 1) Cl DMF, 100 C, 24 h 2) 4 Cl (aq.) 100 C, 8 h A : Cl 2 n 3 Cl n n : A n : n : n : A : A C 2 A n n 2 : C 2 equivalent n : C 2 2 n C 2 C 2 n : A 2 C 2 n : A C 2 Whew! C 2 n A

8 15. Provide a plausible arrowpushing mechanism for the following reaction. 2 tu 1) C C, 15 min 2) CF 3 C 2 23 C, 12 h 2 nc C 2 tu Cn C 2 tu : Cn C 2 tu Cn C 2 tu Cn C 2 tu Cn C 2tu C 2 tu Cn : C 2 tu Cn : A Cn Cn n n A n : A n 16. Suggest a plausible arrowpushing mechanism for the following reaction. i Et 2 23 C, 3 d Et 2 Et 2 : Et 2 Et 2 Et 2 : Et 2 Et 2 i R. M. Albert, G.. utler Dual reactivity of 3,3dimethoxycyclopropene J. rg. Chem. 1977, 42,