Topic 5: Carbocations

Transcription

1 Professor David L. Van Vranken hemistry 201: rganic Reaction Mechanisms I Topic 5: arbocations carbocation * carbocation substituent p n E M σ Reading: I. Fleming Molecular rbitals and rganic hemical Reactions, and 2.2 arey&sundberg Advanced rganic hemistry, 3.4.1

2 Addition to Empty p rbitals (and vice versa) If all else is equal, the fastest reactions should involve addition of lone pairs to empty p orbitals, not σ* σ p N B l substitution 2 N B N B l E M n not so good σ ompare 2nd row atoms with empty p orbitals: Nu carbenium nitrenium oxenium fluorenium N F E M p p N p F These cations aren t equally reactive yet they all react at equal rates with water k ~10 9 M -1 s -1 (diffusioncontrolled). It is more insightful to look at the reverse reaction, ionization, because it should proceed through the same transition state. : : N : : F: Need to PUS UT L.G. (usually via R migration) Don t worry about simple nitrenium ions, oxenium ions, or fluorenium ions. They are too unstable to form in typical organic reactions.

3 haracteristics of S N 1 Reactions Two steps via an unstable, planar carbocation G Nu- - Nu- - Nu Nu - LiBr Ac Br configuration? Nu can attack either face of empty p orbital First step is slower than 2nd Nucleophile structure is usually NT important for the overall rate Added - should slow rate = ommon Ion Effect harged leaving groups F 3 S 3-2 N Tf - Ts- Ms- I - Br - l - PNB Neutral leaving groups SiMe 3 Ph 3 P

4 S N 1 Substrate Reactivity The ammond postulate ighly endothermic reactions involve late, product-like transition states ighly exothermic reactions involve early, starting material-like transition states G "LATE" T.S. l Me 2 δ Me 2 l δ - Me 2 l- Me 2 δ Me 2 R δ - R "EARLY" T.S. Nu R ow to use the ammond postulate: Me l is fast b/c Me stabilizes the carbocation Me & carbocation resembles the T.S. (ammond) Me l The same arguments for carbocation stabilization can also be applied to T.S. and S.M. l Me σ* -l is fast b/c Me donates into σ* in S.M. and T.S. Me l p/σ* -l similar to donation into p in the product Me p Best substrates for S N 1 = good donors next door: R 2 N 2 R 2 or benzyl Since S N 1 reactions involve formation of unstable carbocations, we should be able to predict S N 1 rates based on carbocation stability.

5 The Best arbocations are Trigonal Planar arbocations want to be planar because lower energy orbitals don't want to be empty. 2p most stable E M sp 3 sp 2 sp super unstable! 2s Don t generate sp2 carbocations through S N 1 ionization Br. Mayr, et al. J. rg. hem. 1981, 46, You AN generate sp 2 carbocations by adding electrophiles to alkynes 30 equiv. Bn NaN Bn N 3 N Nu aq. Me 75, 3 h (note: Nu is involved in T.S.) L.E. verman, et al. JAS 1988, 110, 612.

6 Trigonal Planar arbocations Pyramidalization slows S N 1 Br Br unstable Br pyramidal can't cation planarize k rel

7 arbocation Stability: Donor Effects carbocation reactivity: Substituents perturb stability and reactivity. Perturbation theory allows us to predict these effects. We can use M interaction diagrams to think about the impact of subsituents in the same way that we use M diagrams to analyze. R E M carbocation p * carbocation substituent n σ harder to add to a carbocation when it has a strong donor attached The best resonance structure satisfies octet rule. The second-best resonance structure predicts reactivity. N 2 N 2 attack here ompare l.p. donors on carbenium (carbocation) reactivity stability N 2 F Stab. Energy (PM3) kcal/mol A. M. El-Nehas J 1995, 8023 See &S A Table 5.6 and Table 1.13 Acylium ions are about as stable as t-butyl carbocations; both and have filled octets. about as stable as t-bu

8 arbocation Stability: Pi Donor Groups ompare: donor energy stability: N < < E M 2 N Br not good Note the discrepancy between allyl and benzyl cation stability depending on solvation Benzyl cation is more stable than unsubstituted allyl cation. Ph - N Stabilization Energy (PM3/ 2 ) Stabilization Energy (PM3 gas) kcal/mol Solvation stabilizes carbocations, but solvation of large, delocalized ions isn't always effective In the gas phase, ions with larger alkyl groups are more stable.

9 arbocation Stability: Sigma Donor Groups stability: Why is the t-butyl carbocation stable? Vicinal bonds stabilize carbocations by donation into the empty p orbital donation weak Question: Donation by bond or l.p.? Answer: Lone pairs. NR 2 -: NR 2 NR 2 NR 2 R YES N N Vicinal -R bond doesn t stabilize carbocations predicted S N 1 rates l R l l R E σ - σ - Winstein, S.; Lindergen,. R.; Ingraham, L. L. J. Am. hem. Soc. 1953, 75, 155.