Overview
Lecture covers electrophilic addition of hydrogen halides to alkenes, regioselectivity (Markovnikov rule), carbocation intermediates, and carbocation rearrangements (hydride and methyl shifts) affecting major products.
Electrophilic Addition: Unsymmetrical Alkenes
- Unsymmetrical alkenes give two possible products; one is preferred.
- Example: Addition of HCl to propene yields a major and a minor product.
- Observed major product places Cl on the more substituted carbon.
Regioselectivity and Markovnikov Rule
- Regioselective reactions produce multiple possible products, but one dominates.
- Markovnikov rule: H adds to the carbon with more hydrogens; halogen to fewer.
- Equivalent statement: H to the less substituted carbon; X to the more substituted.
Regioselectivity Rationale: Carbocation Stability
- First step: ฯ electrons attack H of HX, breaking HโX; H adds to one alkene carbon.
- A carbocation forms on the other carbon; two locations are possible.
- Secondary carbocations are more stable than primary; more stable forms faster.
- Major product derives from the more stable carbocation intermediate.
- Halide ion (Clโ/Brโ) attacks the carbocation to give the product.
Worked Examples: Predicting Major Products
- Reaction conditions: Hydrogen halide in solvent (e.g., acetic acid as solvent).
Example Outcomes
- 1-Butene + HBr (acetic acid): H to terminal C; Br to internal C โ 2-bromobutane.
- 2-Methyl-1-propene + HBr: H to carbon with more H; Br to more substituted carbon.
- Methylcyclopentene + HCl: H to carbon with more H; Cl to carbon with fewer H.
Carbocation Rearrangement
- Carbocations can rearrange to more stable carbocations (driving force).
- Occurs via shifts from adjacent carbons: hydride shift or methyl shift.
- Start: Secondary carbocation adjacent to a tertiary or quaternary center.
- Rearrangement yields a tertiary carbocation, then halide capture gives product.
Hydride Shift
- A hydrogen with its electron pair migrates to the carbocation center.
- Original carbocation carbon becomes neutral; donor carbon becomes cationic.
- Resulting carbocation is more substituted (e.g., tertiary), thus more stable.
Methyl Shift
- A methyl group with the bonding electron pair migrates to the carbocation.
- Forms a new, more substituted carbocation at the donor carbon.
- Common when adjacent carbon is quaternary, enabling formation of tertiary cation.
Regioselectivity versus Rearrangement: Outcome
- Simple Markovnikov addition may predict product A; minor product B also possible.
- Observed major product C can arise after carbocation rearrangement.
- Path: Markovnikov protonation โ secondary carbocation โ rearrangement โ tertiary carbocation โ halide addition โ product C.
- Conclusion: When carbocations are intermediates, always consider rearrangements.
Structured Summary of Rules and Outcomes
| Concept | Rule/Definition | Consequence | Example Outcome |
|---|
| Regioselectivity | Reaction favors one direction/product among several | Major and minor products form | HCl to propene gives one major chlorinated isomer |
| Markovnikov rule | H adds to carbon with more H; X to fewer H | X ends on more substituted carbon | 1-butene + HBr โ 2-bromobutane |
| Carbocation stability | Tertiary > Secondary > Primary | More stable cation forms faster; controls major product | Secondary favored over primary during HX addition |
| Hydride shift | Hโ migrates with electron pair to cation center | Forms more substituted carbocation | Secondary โ Tertiary cation before Xโ attack |
| Methyl shift | CH3 migrates with electron pair to cation center | Forms more substituted carbocation | Secondary โ Tertiary cation prior to capture |
| Rearrangement impact | Occurs when more stable cation accessible | Rearranged product can be major | Product C after HX addition to rearranging alkene |
Key Terms & Definitions
- Regioselective: Reaction yields multiple isomers but favors one as major.
- Markovnikov rule: Guideline for HX addition to unsymmetrical alkenes; H to more H.
- Carbocation: Positively charged carbon intermediate; stability controls rate.
- Hydride: Hydrogen bearing two electrons (Hโ) that can shift in rearrangements.
- Hydrogen atom (radical): H with one electron; distinct behavior from hydride.
- Proton: H+; no electrons; electrophile in protonation steps.
- Hydride shift: Migration of Hโ with electrons to form a more stable carbocation.
- Methyl shift: Migration of CH3 with electrons to stabilize the carbocation.
Action Items / Next Steps
- Practice predicting major products for HX additions, including possible rearrangements.
- Draw mechanisms: Protonation, carbocation formation, rearrangement, halide capture.
- Prepare to show full mechanisms for cases where product C is major due to rearrangement.