Lecture Outline SN1 and E1 Reactions

Name ______________________________________ Text section 8.7 discusses carbocation stability. Localized carbocations are stabilized dramatically by alkyl groups attached to the cationic C. 1 Rank these isomeric carbocations in order of increasing stability (1 = least stable, 4 = most stable) 2 The positive Cs of the cations above all have the same geometry and hybridization. What are the geometry and hybridization state of each of these cationic Cs? 3 Keeping in mind the alkyl substitution effect as well as other factors that might be relevant, rank the following cations in order of increasing stability 4 The rate-determining first step of the SN1 reaction is "ionization" — loss of the leaving group to form a carbocation. The more stable the carbocation, the faster it is formed, and the faster the SN1 reaction proceeds. Rank the following organo-halides in order of increasing rate of SN1 reaction. Br Br Br Br 5 How many HW points do you think you have at this point? ________ (yes, that's a repeat question, but we don't know which of these will be turned in.) Lecture outline SN1 and E1 reactions — CH3 –OH (CH3 )3 C Br solvent — also functions as nucleophile (and base in final step of mechanism) rate = k[tBu–Br] (CH3 )3 C OCH 3 + (CH3 )2 C CH2 — consistent with a mechanism whose rate-determining (slow) step is unimolecular Here's the mechanism (add the curvy arrows and complete the missing bits) Step 1 is the same for both substitution and elimination — (CH3 )3 C Br ionization (rate-determining step) (CH3 )3 C + Br SN1 branch — carbocation undergoes addition of a nucleophile (if the nucleophile is solvent, this is a solvolysis rxn) (CH3 )3 C + CH 3 OH E1 branch — carbocation loses a proton (carbocations are strong acids) H (CH3 )2 C CH2 CH3 OH We don't normally write reaction mechanisms as a series of separate balanced equations. Here's the more conventional format for the SN1 pathway. Here's the reaction coordinate diagram for the SN1 reaction above Another example — draw the products H2O I — this is a hydrolysis (solvolysis with water) — more stable alkene predominates — E1 is non-stereoselective — any β-H can usually be aligned with the empty p-orbital and be lost. Note that we can't actually carry out that hydrolysis with the alkyl iodide in water alone — the alkyl iodide is not water-soluble! So we typically use a mixture of an unreactive organic co-solvent — that's often one of the polar aprotic solvents we learned works well for the SN2 reaction. This hydrolysis might be run in a mixture of acetone and water, for example. Rate of ionization depends on: 1. l.g. — better l.g. —> faster rxn 2. carbocation stability — more stable carbocation —> faster rxn Stabilities of carbocations — ———— increasing stability ————> H H C H H R methyl C H H R 1° C R R R 2° C R 3° Each alkyl ("R") group attached to the positive C stabilizes a carbocation by 10 - 15 kcal/mol (that's a lot!) — to a good approximation, only the number of R groups matters, not their structures or size — for example, a methyl is just as good as an ethyl or a pentyl or a tert-butyl. Alkyl groups stabilize carbocations by hyperconjugation — slightly antibonding combo p H !CH empty p H H E 2 e– s go down in E !CH slightly bonding combo Resonance description of hyperconjugation — H H H H H H 3 p-sp "!-bond" minor contributor — poor orbital overlap In general, via ionization of alkyl halides... — localized 3° cations are formed rapidly — localized 2° cations are formed more slowly — methyl and loc. 1° cations are too unstable to be formed at all (under SN1 conditions) that's important... Localized primary cations are NOT formed by solvolysis under ordinary SN1 conditions. But let's not go overboard with labels. Cation stability is the key — think stability, not labels — that's all the molecules care about. Think like a molecule... be the molecule... Ok, now explain this. Write the mechanism. Cl EtOH OEt OEt + 1-Bromonorbornane (1-bromobicyclo[2.2.1]heptane) ionizes 10–13 times as fast as tert-butyl bromide. They're both 3°, eh? So what's up with that? Br — vinylic cations are very high in energy (comparable to localized 1° alkyl), and they don't form via SN1 pathways. Aryl cations are even worse (why?) 3. Solvent polarity — more polar solvents are better at solvating and stabilizing ions than less polar solvents — the more stable the ions, the faster they form. e.g., tBu Cl tBu + Cl in the gas phase, ∆H° ≈ 155 kcal/mol (≈ ∆H° in a non-polar solvent like an alkane) in EtOH, ∆H° ≈ 27 kcal/mol — EtOH can solvate ions — polar, protic solvents are especially effective because they can hydrogen-bond to the l.g., further stabilizing it — some solvents that are polar enough to support ionization are: H2 O HCO2H CH3CO2H CH3OH EtOH DMSO — solvents that are not polar enough are: alkanes, Et2O, and alcohols larger than ethanol (e.g. tBu–OH) SN1 stereochemistry — cationic Cs are planar and can add nucleophiles on either face Et Et Et C Br CH3OH CH3O nPr Me Et –Br + nPr Me C nPr Me OCH3 – H+ CH3OH C nPr C Me (in reality, l.g. temporarily blocks one face of cation, so add'n of solvent to opposite face is faster, so slightly more inversion than retention of config.)