Carbocations, Organic Reaction Mechanisms I, Lecture notes of Organic Chemistry

Download Carbocations, Organic Reaction Mechanisms I and more Lecture notes Organic Chemistry in PDF only on Docsity! Topic 5: Carbocations Reading: I. Fleming Molecular Orbitals and Organic Chemical Reactions, 2.1.3 and 2.2 Professor David L. Van Vranken Chemistry 201: Organic Reaction Mechanisms I carbocation C+ EMO pC σ π n OH .. H carbocation substituent π π* + Carey&Sundberg Advanced Organic Chemistry, 3.4.1 Addition to Empty p Orbitals (and vice versa) ■ If all else is equal, the fastest reactions should involve addition of lone pairs to empty p orbitals, not σ* ■ Compare 2nd row atoms with empty p orbitals: ■ It is more insightful to look at the reverse reaction, ionization, because it should proceed through the same transition state. ■ Don’t worry about simple nitrenium ions, oxenium ions, or fluorenium ions. They are too unstable to form in typical organic reactions. ■ These cations aren’t equally reactive yet they all react at equal rates with water k ~109 M-1 s-1 (diffusion- controlled). EMO p π∗ σ∗ C+ pN+ pO+p F+Nu .. H C H H + N H H+ .. O H + .. .. F+ .. .. .. carbenium nitrenium oxenium fluorenium H C HHX X CH H H+ X N H H X O H X F>> >> >>: : : : :.. .. Need to PUSH OUT L.G. (usually via R migration) n p σ∗ EMO not so good ..N H H H H B H Cl ..N H H H H B H Cl H B HH2Nsubstitution The Best Carbocations are Trigonal Planar ■ Carbocations want to be planar because lower energy orbitals don't want to be empty. ■ Don’t generate sp2 carbocations through SN1 ionization L.E. Overman, et al. JACS 1988, 110, 612. ■ You CAN generate sp2 carbocations by adding electrophiles to alkynes H. Mayr, et al. J. Org. Chem. 1981, 46, 5336. EMO 2s 2p C sp C sp2 sp3 C super unstable! most stable Br + Bn N Bn N + + 30 equiv. NaN3 aq. MeOH 75 °C, 3 h Nu .. (note: Nu is involved in T.S.) Trigonal Planar Carbocations ■ Pyramidalization slows SN1 krel 1 0.001 0.0000000001 Br Br Br can't planarize + + unstable pyramidal cation Carbocation Stability: Donor Effects ■ Substituents perturb stability and reactivity. Perturbation theory allows us to predict these effects. We can use MO interaction diagrams to think about the impact of subsituents in the same way that we use MO diagrams to analyze. ■ Compare l.p. donors on carbenium (carbocation) reactivity A. M. El-Nehas JOC 1995, 8023 See C&S A Table 5.6 and Table 1.13 ■ Acylium ions are about as stable as t-butyl carbocations; both C and O have filled octets. ■ The best resonance structure satisfies octet rule. The second-best resonance structure predicts reactivity. O C + .. O C + .... about as stable as t-Bu OH HH +HH + R HH + > >carbocation reactivity: carbocation C+ EMO pC σ π n OH .. H carbocation substituent π π* + harder to add to a carbocation when it has a strong donor attached NH2CNH2C ..++ attack here NH2 H H+ .. OH H H+ .. F H + .. stability > > H H H+ > 80 51 6 0 H Stab. Energy (PM3) kcal/mol