Kinetics

38. Steric effects can play a large role in the rate of SN1 reactions of tertiary alkyl halides and related R3C-X compounds. Use MMX to determine the steric energy difference between R-OH and R+ for the series R- = Me3C-, Me2(t-Bu)C-, Me(t-Bu)2C-, and (t- Bu)3C-. Then plot the experimental rate data (in logarithmic form) vs. deltaEst; see the cited article for details.

[Müller, P.; Mareda, J. J. Comp. Chem. 1989, 10, 863.]

39. Do the same (as suggested in Problem 38) for the series 1-R-1-OH-cycloalkane where R is CH3 or t-Bu and the cycloalkane varies from five-to six- to seven-membered. See the same reference for details and for the experimental rate data.

90. The kinetics of the solvolyses of some bridgehead allylic triflates (triflate = OSO2CF3, an excellent leaving group) have recently been reported. In order to account for the observed rate data and to see if "F-strain" in the reactant is a factor, MM2(87) calculations were performed on the corresponding bridgehead alcohols. Calculate the structures and heats of formation for the stereoisomeric [2.2.2] compounds: Z with R1 = CH3, R2 = H and E with R1 = H, R2 = CH3; do the same for the Z and E [3.2.2] compounds. Compare the heat differences for Z and E which you find with those in the article; discuss any differences.

[Takeuchi, K.; Ohga, Y.; Kitagawa, T. J. Org. Chem. 1991, 56, 5007; see also Takeuchi, K.; Kitagawa, T.; Ohga, Y.; Yoshida, M.; Akiyama, F.; Tsugeno, A. Ibid. 1992, 57, 280; Ohga, Y.; Munakata, M.; Kitagawa, T.; Konoshita, T.; Takeuchi, K.; Oishi, Y.; Fujimoto, H. ibid. 1994, 59, 4056.]

106. A recent article relates assesses steric effects on reaction rates. The following problems are quite similar to those described in Problems 38 and 39. Solving either Part A or Part B would constitute a satisfactory molecular mechanics project.

A. Compute the MMX energy for the series of 2-alkyl-2- bromoadamantanes shown to the right (where R = methyl, ethyl, isobutyl, tert-butyl, and neopentyl); also compute the MMX energy of the tertiary carbocation obtained by SN1 reaction of the halide. Plot the difference in energy between R-Br and R+ vs. the log of the solvolysis rates of such compounds (see the cited article) and discuss similarities/differences between your correlation and the published one.

B. Similarly, compute the MMX energy for a series of 1-bromo bridged bicyclic molecules and for the resulting cation. Do the calculations on the [3.3.3], [3.3.2], [3.3.1], [3.2.1], and [2.2.2] systems. Plot the difference in energies between R-Br and R+ vs. the log of the solvolysis rates of such compounds (see the cited article) and discuss similarities/differences between your correlation and the published one.

[Müller, P.; Millin, D. Helv. Chim. Acta 1991, 74, 1809.]

112. In a study reminiscent of that in Problem 90, the solvolysis of some bridgehead allylic adamantyl compounds has been performed. The observed rate differences between Z and E mesylates and chlorides have been rationalized in terms of "F-strain" (steric hindrance between an "inside" methyl and the leaving group) which will differ for the two leaving groups. You should do MM calculations on six molecules: the Z series (R1 = CH3, R2 = H) where X = Cl, OH, or +; and the E series (R1 = H, R2 = CH3) where X = Cl, OH, or +; [note: OH is being used as a model for mesylate]. By comparing the calculated deltaH for solvolysis of Z and E chlorides and alcohols, confirm or dispute the conclusions cited in the article.

[Takeuchi, K.; Ohga, Y.; Munakata, M.; Kitagawa, T. Chem. Lett. 1991, 2209; see also Takeuchi, K.; Ohga, Y.; Munakata, M.; Kitagawa, T.; Kinoshita, T. Tetrahedron Lett. 1992 33, 3335; Ohga, Y.; Munakata, M.; Kitagawa, T.; Konoshita, T.; Takeuchi, K.; Oishi, Y.; Fujimoto, H. ibid. 1994 59, 4056.]

117. In a recent article, MM2 calculations are performed to see if the experimental rates of epoxidation of unstrained and strained alkenes correlate with the calculated changes in steric energy from reactant to product. This problem entails doing MMX calculations on either: (a) a series of disubstituted alkenes (cis-cyclohexene, -octene, -nonene; trans- cyclooctene, -nonene; norbornene) and the derived epoxides; or (b) a series of trisubtituted alkenes (1-methylcyclopentene, -hexene; ethylidene cyclohexane; cis- and trans-1-methylcyclooctene; anti-Bredt compounds bicyclo[3.3.1]-1-nonene and bicyclo[4.3.1]-1(9)-decene) and the derived epoxides. Then, determine if the change in steric energy for the series you've done correlates with the log of the rate constant for epoxidation (see Table I and Figure 5 in the cited reference).

[Shea, K. J.; Kim, J.-S. J. Am. Chem. Soc. 1992, 114, 3044.]

125. The rate of cyclobutane cleavage for a series of meta,meta-cyclophanes has been determined. Use MMX to compute the strain energy of those cyclophanes which have n = 2, 3, 4, and 5; discuss whether strain-relief correlates well with the rate of reaction. Compare your results with those in the cited reference.

[Nishimura, J.; Wada, Y.; Sano, Y. Bull. Chem. Soc. Jpn. 1992, 65, 618.]

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