CHEM 222 Organic Chemistry: Chap 15 Electrophilic Addition

Kinetic vs Thermodynamic Products

• Two situations can arise when multiple products are formed
• Kinetic product forms faster; thermodynamic product is more stable at equilibrium
• Kinetic product: irreversible reaction, depends on reaction rate
• Thermodynamic product: reversible reaction, depends on product stability
• If a reaction forms faster at lower temperatures, it's the kinetic product
• If a reaction predominates at equilibrium, it's the thermodynamic product

Example: Kinetic vs Thermodynamic

• Electrophilic addition of HBr to isolated dienes (separate double bonds) follows Markovnikov's rule, yielding one product
• Electrophilic addition to conjugated dienes (connected double bonds) yields a mixture of 1,2- and 1,4-addition products

Mechanism 14.2: Electrophilic Addition of HBr to 1,3-Dienes

• Step 1: H⁺ from HBr adds to a terminal carbon of the diene, forming a resonance-stabilized allylic carbocation
• Step 2: Nucleophilic attack of Br⁻ can occur at either site of the allylic carbocation, forming 1,2- and 1,4-addition products

Reaction Conditions and Addition Products

• The ratio of 1,2- to 1,4-addition products depends on reaction conditions
• At low temperatures (e.g., -80°C), 1,2-addition is the major product
• At high temperatures (e.g., 40°C), 1,4-addition is the major product

Kinetic vs Thermodynamic Products (cont.)

• The isolated kinetic product can become the thermodynamic product when heated
• The 1,2-product is faster, so it predominants at lower temperatures
• The 1,4-product is more stable, so it predominates at equilibrium

Why Does This Occur?

• The 1,4-product is more stable than the 1,2-product because it has more alkyl groups bonded to the double bond
• Disubstituted alkenes are more stable than monosubstituted alkenes

Benzene

• Benzene is the simplest aromatic hydrocarbon (C₆H₆)
• It has four degrees of unsaturation: cyclic, with three double bonds
• It does not undergo addition reactions like other double bonds, as it's more stable in its current form

Structure of Benzene

• Benzene's structure must account for:
  • A six-membered ring
  • Three additional degrees of unsaturation
  • Equal bond lengths
• The Kekule description shows benzene as an equilibrium of two structures
• Benzene's structure is a hybrid of the resonance structures, meaning electrons are delocalized around the ring

Resonance Hybrid of Benzene

• The true structure of benzene is a resonance hybrid of two equivalent Lewis structures
• The dashed lines in the hybrid depict the position of bonds
• Delocalization of electrons makes the description of benzene easier

Representations of Benzene

• Benzene has 6 π electrons
• Actual bond length is intermediate between a single and double carbon-carbon bond (1.39 Å)

Each Carbon Atom in Benzene

• Each carbon atom in the benzene ring has three atoms bonded to it and no lone pairs of electrons
• Each carbon is sp² hybridized and trigonal planar with 120° bond angles
• Each carbon has a p orbital with one electron that lies above and below the plane of the molecule
• There's a high electron density above and below the ring plane

Naming Substituted Benzenes

• To name a monosubstituted benzene, name the substituent and add the word "benzene"
• Many monosubstituted benzenes have common names (e.g., toluene, phenol, aniline)

Nomenclature of Benzene Derivatives

• For naming two groups on a benzene ring use prefixes: ortho (1,2), meta (1,3), para (1,4)
• For clarity of the molecule, the numbers 1,2,3.4 are used.

Naming Benzene as a Substituent

• A benzene substituent is a phenyl group (Ph-)
• Formed by removing one hydrogen from benzene
• Benzene can be represented as PhH; phenol would be PhOH

Benzyl and Aryl Groups

• The benzyl group (C₆H₅CH₂-) is different from the phenyl group (C₆H₅-)
• Other substituted aromatic rings are collectively called aryl groups

Unusual Reactivity of Benzene

• Benzene exhibits lower than expected heat of hydrogenation compared to other highly unsaturated compounds
• It does not undergo typical addition reactions
• The reaction would involve a substitution without breaking or rearrangement of bonds within the ring.

What Accounts for Aromaticity

• Four criteria must be met for a molecule to be aromatic:
  • Cyclic
  • Planar
  • Completely conjugated
  • Satisfy Hückel's rule (4n + 2 π electrons)

Larger Aromatic Rings

• Rings larger than benzene that meet criteria for aromaticity are called annulenes
• Number of atoms in the ring is indicated in brackets e.g, [14]-annulene

Example of Not Aromatic Ring

• [10]-Annulene: has 10π electrons, but the ring is nonplanar to relieve strain, so it isn't aromatic

Fused Ring Aromatics

• Two or more six-membered rings fused together with alternating double and single bonds can be aromatic
• Examples include naphthalene, anthracene, phenanthrene

Resonance Structures of Fused Ring Aromatics

• Resonance structures increase as more fused rings exist in the compound
• For example, naphthalene is a hybrid of three resonance structures, similar to how benzene is a hybrid of two

Heterocycles

• Heterocycles containing oxygen, nitrogen, or sulfur can also be aromatic
• Lone pairs of electrons in the heterocycle can be localized on the heteroatom or part of the delocalized system
• Examples include pyridine and pyrrole

Effect of Delocalization

• In pyridine, the nonbonded electron pair on the nitrogen atom is localized in an sp² hybridized orbital
• In pyrrole, the nonbonded electron pair on the nitrogen atom is in a p orbital and delocalized over the entire ring

Ionic Aromatic Compounds

• Negatively and positively charged ions can be aromatic, provided the criteria for aromaticity are met, including satisfying Hückel's rule
• Examples include cyclopentadienyl anion and tropylium cation

Cyclopentadienyl examples

• Cyclopentadienyl structures: anion, cation, and radical
• Aromaticity is determined by electron count and if the molecule is flat, cyclic, and conjugated

Cyclopentadiene and Acidity

• Cyclopentadiene itself is not aromatic because it's not fully conjugated
• Loss of a proton makes the product and stabilize the product
• The conjugate base (cyclopentadienyl anion) is both fully conjugated and aromatic, hence, stable

Aromaticity in a cation

• The tropylium cation (seven-membered ring with a positive charge, having 3 double bonds) is aromatic due to satisfying Huckel's rule (having 6 π electrons, cyclic, planar, and conjugated

Molecular Orbital Theory

• Molecular Orbital Theory will be covered in a separate lecture

Remember

• Kinetic vs thermodynamic reactions (control)
• Aromaticity: definition
• Aromatic criteria: methodology of determining aromaticity