Carbocation Rearrangements and Addition Reactions

Carbocation Rearrangements

• Carbocations are reactive due to their positive charge and incomplete octet.
• In SN1 reactions, substitution can appear to occur at a carbon other than the one with the leaving group due to carbocation rearrangements.
• Example Reaction: 2-Iodo-3-methylbutane with $H_2O$ to form 2-Methylbutan-2-ol.

SN1 Mechanism with Carbocation Rearrangement

1. Heterolysis:
   • The leaving group departs, forming a secondary carbocation.
2. 1,2-Hydride Shift:
   • A hydrogen atom shifts from an adjacent carbon to the carbocation center.
   • This converts the secondary carbocation to a more stable tertiary carbocation.
3. Coordination:
   • Water (H2O) coordinates to the tertiary carbocation.
4. Proton Transfer:
   • A proton is transferred to another water molecule, yielding the alcohol product.

Favorable 1,2-Hydride & 1,2-Methyl Shifts

• 1,2-hydride or 1,2-methyl shifts are usually faster if they lead to a more stable carbocation.

Evaluating Different Possible Rearrangements

• Rearrangements occur only if they increase the stability of the carbocation.
  • Example of a favorable rearrangement: A 1,2-hydride shift from secondary to tertiary carbocation.
  • Example of an unfavorable rearrangement: A shift that does not increase carbocation stability.
• Resonance delocalization can drive carbocation rearrangement, providing significant stability.

Summarizing 1,2-Alkyl & 1,2-Hydride Shifts

• These shifts are also known as Wagner-Meerwein Rearrangements.
• Carbocations rearrange to form more stable tertiary carbocations when possible.
• Rearrangement of the carbon skeleton is a rapid, intramolecular process.
• 1,2-alkyl shift:
  • A methyl group shifts from an adjacent carbon to the carbocation center.
  • • $Me$ denotes methyl group.
  • Example: Conversion of a secondary carbocation to a tertiary carbocation via a 1,2-shift.
  • The sigma bond between carbon and carbon ($\sigma$C–C) shifts to the empty p orbital. This creates a more stable tertiary carbocation, starting from the less stable secondary carbocation
• 1,2-hydride shift:
  • A hydrogen atom shifts from an adjacent carbon to the carbocation center
  • Example: Conversion of a secondary carbocation to a tertiary carbocation via a 1,2-shift.
  • The sigma bond between carbon and hydrogen ($\sigma$C–H) shifts to the empty p orbital. This creates a more stable tertiary carbocation, starting from the less stable secondary carbocation

Resonance-Delocalized Intermediates in Mechanisms

• Substitution occurs at a carbon other than the one with the leaving group.
• This is an SN1 mechanism.
• Carbocation rearrangement may not be directly observed.

Resonance-Stabilized Intermediates

• SN1 reactions can proceed through carbocation intermediates with resonance structures.
• Mechanism:
  1. Heterolysis:
     • The leaving group departs, forming a carbocation.
  2. Proton Transfer:
     • Example with $H_2O$
  3. Coordination:
     • Water or another nucleophile coordinates to the carbocation.
• Resonance structures delocalize the positive charge, stabilizing the intermediate.

Addition Reactions

Reactivity of $π$ Bonds

• Alkenes and alkynes act as nucleophiles, reacting with electrophiles.
• $π$ bonds are easier to break than $σ$ bonds.

General Electrophilic Addition Mechanism

• Addition of a strong Bronsted acid to an alkene.
• Examples:
  • Cyclohexene + HCl → Chlorocyclohexane.
  • 2,3-Dimethylbut-2-ene + conc. $H<em>2SO</em>4$

Electrophilic Addition of a Strong Bronsted Acid to an Alkene

• Mechanism:
  1. Electrophilic Addition:
     • The electron-rich alkene attacks the electron-poor hydrogen of the acid.
     • Formation of a carbocation intermediate.
     • This is the rate-determining step.
  2. Coordination:
     • The electron-rich anion (e.g., chloride) attacks the electron-poor carbocation.

Rate-Determining Step

• Formation of the carbocation intermediate is the rate-determining step.

Driving Force for Electrophilic Addition

• Electrophilic addition reactions are energetically favorable.
• $π$ Bond lost + $σ$ Bond gained.

Benzene Rings and Electrophilic Addition

• Bronsted acids do not readily add across a C=C double bond in benzene.
• The $π$ electrons in benzene are heavily stabilized by delocalization.

Stability of Benzene

• Benzene's electron delocalization prevents typical addition reactions.
• Example:
  • 1,2-Diphenylethene + HBr → (1-Bromo-2-phenylethyl)benzene
  • Addition occurs because the double bond is not part of a benzene ring and lacks the stabilization from electron delocalization observed in benzene.

Regiochemistry and Markovnikov’s Rule

• In the addition of a Bronsted acid across a double bond, the hydrogen atom can bond to one of two possible carbon atoms.
• With an asymmetric alkene, two constitutional isomers can be produced.

Electrophilic Addition Mechanism Free Energy Diagram

• The more stable carbocation intermediate leads to the major product.
• The energy barrier is smaller when $H^+$ adds to the carbon that leads to the more stable carbocation.
• Example: Formation of 2-Chloropropane (major) vs. 1-Chloropropane (minor).

Markovnikov’s Rule

• Markovnikov's Rule: "The addition of a hydrogen halide to an alkene favors the product in which the proton adds to the alkene carbon that is initially bonded to the greater number of hydrogen atoms."
• This regioselectivity is called Markovnikov addition.

PROBLEM 1:

• Which substrate, A or B, would undergo an SN1 reaction faster?

PROBLEM 2:

• Which alkene, A or B, will react faster with HCl?

PROBLEM 3:

• Predict the major product when indene is treated with HCl.