高等有机化学-chapter03.pptVIP

3.5.5 The tunnelling effect Occasionally a PKIE is far in excess of the maximum value ( kH/kD = 7~8 ) at a given temperature. For example, Me2C–NO2 + + R = CH3 R = H kH/kD : 24 9 . . kH/kD : 13000 * Quantum-Mechanical Tunnel Effects ? de Broglie wavelength λ= h mv Particle: e- H D T C Br λ(pm) : 2690 63 45 36 18 7 Ea ? E x TS m ? Hesisenberg Uncertainty Principle DX ? DPx ≥ h for proton, DX ≈ h 4p ≈ 5 (pm) ? Low temperature ? narrow energy barrier ? solvation ? rare phenomenon. 3.5.6 Solvent isotope effects (SIE) The KIEs come from the change of the solvent, for example, from H2O to D2O. ? the possible cases of the SIEs The hydrolysis by solvents S + H+ SH+ product slow fast R2C=OMe + MeOH + R2C OH OMe R2C=O fast H2O H3O+ + R2C OMe OMe H2O + R2C OHMe OMe + kH/kD=1/3 kH/kD=1/0.69 D2O + t-Bu–C(CN)2 - kH/kD=3.7 Proton exchange phCOOD + ph2C=N2 ph2CN2 + phCOO D + - slow fast ph2C–O–COph + N2 D kH/kD= ~7 The solvation properties of a protic solvent, particularly, hydrogent-bonding capability, changed. phCOOH + D2O phCOOD + HOD 3.5.7 Isotopes in Labeling Experiments Using as labels for ascertaining the location of a given atom involved in a reaction in mechanistic studies. ? The nuclei most commonly used: 2H, 3H, 13C, 14C ? Detection: NMR: 1H, 2H; 12C, 13C MS: all IR: all radioactivity: 3H; 14C ? Example: NaNH2 ? + + ? ? ? NaNH2 ? + ? ? 3.6 Solvent Effects on Reactions 3.6.1 Solvent effects RI + NMe3 R―NMe3 I + - kr(MeCN) kr(CS2) = 440 ph–C–CN + MeO ph–C–CN + MeOH Me H Me - - kr(Me2SO) kr(MeOH) = 109 reaction rate = kr[A][B] Solvents are not included. The difference in rate constant is not from the concentration of solvents, but the reaction pathways or transition state structures. ？ ? Three main categories of solvents 1o. Protic: possessing a proton-donating fun