Overview
This lecture continues section 7.8, covering carbocation rearrangements in SN1 and E1 mechanisms, solvent effects, substrate reactivity, and stereochemical considerations for these reactions.
Carbocation Rearrangements
- Carbocations generated in SN1 and E1 mechanisms may rearrange to form more stable intermediates.
- 1,2-hydride shift: hydrogen moves to adjacent carbon, creating more stable carbocation.
- 1,2-methide shift: methyl group shifts to adjacent carbon, stabilizing carbocation.
- General rule: if carbocation rearrangement is possible, it will occur.
- Always look for opportunities to form more stable carbocations after leaving group departure.
- Both shifts generate tertiary carbocations from secondary or primary precursors.
- Products differ before and after rearrangements due to new carbocation position.
- Both substitution and elimination products result from rearranged carbocations.
Primary Substrates and Rearrangements
- Primary substrates under solvolysis conditions (ethanol, water) undergo rapid rearrangement.
- Primary alcohols from direct substitution are not observed as products.
- Only products from more stable carbocations appear in final mixture.
- Methide shift occurs as concerted process, generating tertiary carbocation.
- Water performs nucleophilic attack on rearranged tertiary carbocation.
- Subsequent proton transfer creates final alcohol product.
Solvent Effects on Reaction Rates
- SN1 and E1 reactions proceed faster in polar protic solvents.
- Polar protic solvents contain OH bonds (alcohols) with hydrogen-bonding capability.
- Polar aprotic solvents lack hydrogen-bonding protons despite polar bonds.
| Reaction Type | Preferred Solvent | Effect |
|---|
| SN2 | Polar aprotic | Raises nucleophile energy, lowers activation energy |
| SN1 | Polar protic | Stabilizes carbocation intermediates, lowers activation energy |
| E1 | Polar protic | Stabilizes carbocation intermediates, increases rate |
Leaving Group Quality and Substrate Reactivity
- Better leaving groups accelerate SN1 and E1 reactions.
- More stable halide ions result in faster ionization rates.
- Leaving group reactivity order: I⁻ > Br⁻ > Cl⁻ > F⁻ (iodine most reactive).
- Larger halogens form more stable halide ions upon departure.
Substrate Stability and Practical Reaction Rates
- Tertiary carbocations are stable enough for practical solvolysis reaction rates.
- Benzylic and allylic carbocations benefit from resonance stabilization.
- Primary and secondary alkyl halides react too slowly for practical solvolysis.
- Primary/secondary substrates undergo solvolysis only if rearrangement produces tertiary carbocation.
- Tertiary, allylic, or benzylic alkyl halides produce SN1/E1 mixtures without rearrangement.
- Benzylic substrates yield both alcohol (substitution) and alkene (elimination) products.
Regiochemistry in E1 Reactions
- E1 reactions can produce multiple elimination products from different beta carbons.
- Alpha carbon (carbocation site) connects to beta carbons bearing removable protons.
- Removal of different beta protons generates distinct alkene products.
- Most substituted alkene is major product (most stable, Zaitsev product).
- E1 reactions are regioselective but regioselectivity cannot be controlled.
- Unlike E2, E1 offers no control over which elimination product predominates.
Stereochemistry and Stereoselectivity
- E1 reactions producing stereoisomers favor least sterically hindered isomer as major product.
- Stereoselective reactions still yield mixtures of all possible products.
- SN1 reactions show slight preference for inversion of configuration at chiral centers.
- Nucleophile attacks backside more readily than frontside after leaving group departure.
- Inversion occurs because backside attack is less hindered than frontside.
- Mixture of retention and inversion products obtained, with inversion slightly exceeding retention.