Page 265 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg
P. 265

245

                                                                                           PROBLEMS









                         1
            Fig. 2.16Pa. Partial H NMR spectra of cis and trans isomers of 1-benzyl-2,6-dimethylpiperidine.
            Reproduced by permission of Elsevier.




















                        1
           Fig. 2.16Pb. Partial H NMR spectra of three stereoisomeric benzyl ethers of 2,6-dimethylcyclohexanol.
           Reproduced by permission of the American Chemical Society.
          2.17. The trans:cis ratio of equilibrium for 4-t-butylcyclohexanol has been determined
               in several solvents near 80 C. From the data, calculate the conformational free

               energy, − G , for the hydroxy group in each solvent. What correlation do find
                          c
               between the observed conformational equilibria and properties of the solvent?


                            Solvent            trans (%)      cis (%)
                       Cyclohexane              70.0           30.0
                       Benzene                  72.5           27.5
                       1,2-Dimethoxyethane      71.0           29.0
                       Tetrahydrofuran          72.5           27.5
                       t-Butyl alcohol          77.5           22.5
                       i-Propyl alcohol         79.0           21.0



          2.18. Trans-3-alkyl-2-chlorocyclohexanones (alkyl=methyl, ethyl, isopropyl) exist
               with the substituents in the diequatorial conformation. In contrast, the corre-
               sponding E-O-methyloximes exist in the diaxial conformation. Explain the
               preference for the diaxial conformation of the oxime ethers.
          2.19. The two stereoisomers (19-A and 19-B) of the structure shown below have
               distinctly different NMR spectra. Isomer 19-A shows single signals for the methyl
   260   261   262   263   264   265   266   267   268   269   270