Lecture 8: Addition Reactions Notes

Carbocation Rearrangements

• Electrophilic addition of a Brønsted acid across a C=C double bond is susceptible to carbocation rearrangements.
• Example: 2-Chloro-3-methylbutane is the product of a normal Markovnikov addition of HCl across the C=C double bond.

Carbocation Rearrangement Mechanism

• A carbocation rearrangement transforms a secondary (2°) carbocation into a more stable tertiary (3°) carbocation.
• The terminal carbon is protonated to produce a 2° instead of a 1° carbocation intermediate.
• A 1,2-hydride shift can account for the formation of 2-chloro-2-methylbutane.
• Mechanism steps:
  1. Electrophilic addition
  2. 1,2-Hydride shift
  3. Coordination

Stereochemistry in Addition of Brønsted Acid to Alkene

• Stereochemistry may or may not be an issue depending on the symmetry of the product.
• If a single tetrahedral stereocenter is formed, the products are chiral.

Producing Two Tetrahedral Stereocenters

• Two new chiral centers are produced in the reaction.
• Both R and S configurations are produced.
• Mechanism steps:
  1. Electrophilic addition
  2. Coordination

Addition of a Weak Acid: Acid Catalysis

• Water (H₂O) can add across a C=C double bond via acid catalysis.
• Example: But-1-ene to Butan-2-ol using H₂O and H₂SO₄ with a 90% yield.

Acid-Catalyzed Hydration Mechanism

• H+ adds to the terminal carbon to produce the more stable carbocation.
• Even though water is a weak nucleophile, the reactivity of the carbocation intermediate compensates.
• The adduct is stabilized by the removal of charge.
• H3O+ is regenerated, making it an acid catalyst.
• Mechanism steps:
  1. Electrophilic addition
  2. Coordination
  3. Proton transfer

HCl Is Not Used to Catalyze Hydration of an Alkene

• Example: Ethenylbenzene (Styrene) reacts with HCl and H₂O under reflux for 5 hours to produce (1-Chloroethyl)benzene with a 96% yield.

Electrophilic Addition to Conjugated Diene: 1,2-Addition & 1,4-Addition

• Buta-1,3-diene has conjugated double bonds because the two double bonds are separated by another bond.
• A conjugated diene such as buta-1,3-diene is electron-rich, so it undergoes electrophilic addition with Brønsted acids.

Mechanism for 1,2- & 1,4-Addition

• Both 1,2-addition and 1,4-addition products are produced from the same carbocation intermediate.
• Mechanism steps:
  1. Electrophilic addition
  2. Coordination
• Products:
  • 3-Chlorobut-1-ene (1,2-addition product)
  • 1-Chlorobut-2-ene (1,4-addition product)
• Resonance hybrid: Two resonance structures of the same carbocation intermediate.

Stability of the 1,2-adduct and the 1,4-adduct

• The thermodynamic product of electrophilic addition to a conjugated diene is generally the one in which the remaining C=C bond is the most highly alkyl-substituted.
• This could be either the 1,2-adduct or the 1,4-adduct.

Reaction Temperature and Product Distribution

• Room Temperature: If the electrophilic addition of HCl to buta-1,3-diene is carried out at room temperature, then the 1,4-adduct is the major product.
• Low Temperatures: If the electrophilic addition of HCl to buta-1,3-diene is carried out at cold temperatures, then the 1,2-adduct is the major product.

The 1,2 Adduct Is the Kinetic Product

• The 1,2-adduct is the kinetic product because of where Cl⁻ is located after completion of the first step of the mechanism.
• Upon addition of the H⁺ to the diene, Cl⁻ is closer to C-2 than it is to C-4.

Buta-1,3-diene Planarity and Bond Lengths

• Buta-1,3-diene prefers to be entirely planar.
• Bond lengths:
  • Central C-C bond: $135.4 \text{ pm}$, shorter than ethane's $154 \text{ pm}$.
  • C=C double bond: $133.8 \text{ pm}$, slightly longer than ethene's $132 \text{ pm}$.

Conjugation of p Orbitals in Buta-1,3-diene

• The four p orbitals are conjugated and undergo simultaneous $π$ overlap.
• Four $π$ electrons in this system are delocalized over all four carbons.
• Partial $π$ bond character.

Isolated π Systems

• Tetrahedral carbons disrupt conjugation, so $π$ systems appearing on either side of a tetrahedral carbon are effectively isolated from each other.

Electrophilic Addition via a Three-membered Ring: General Mechanism

• A carbocation has a C atom lacking an octet, so it is highly unstable.
• All atoms maintain their octets in the three-membered ring intermediate.
• An electrophile with a lone pair of electrons.
• Mechanism involves an electrophile (E) attacking the alkene, forming a three-membered ring.

Stereochemistry of Electrophilic Addition via a Three-Membered Ring

• The cis/trans relationship in the alkene is conserved for the groups attached to the C=C bond.
• If the cyclic product is chiral, then a mixture of stereoisomers is produced.

Electrophilic Addition Involving Molecular Halogens: Synthesis of 1,2-dihalides and Halohydrins

• Molecular bromine undergoes anti addition across a C=C double bond.

Electrophilic Nature of Molecular Halogen

• When isolated, Br₂ is not electron-poor.
• The electron-rich $π$ bond repels the electrons on one atom of bromine, temporarily generating an electron-poor site on the bromine atom closer to the alkene.

Mechanism for the Addition of Br₂: Bromonium Ion Intermediate

• The mechanism must not proceed through a carbocation intermediate; otherwise, both syn and anti addition would take place.
• The mechanism proceeds through a bromonium ion intermediate.

Chloronium Ion Intermediate

• Cl₂ also undergoes addition to alkenes to produce vicinal dichlorides.
• Example: Anti addition of Cl₂ to trans-But-2-ene yields (meso)-2,3-Dichlorobutane with a 73% yield.

Problems

• Problem 1: Draw the complete, detailed mechanism and predict the major product of the reaction shown.
• Problem 2: Show how to carry out the following synthesis.
• Problem 3: Predict the products of the reaction shown. Do you expect the product mixture to be optically active? Why or why not?