Alkanes, Alkenes, and Conjugated Dienes - Complete Study Guide

ALKANES

SP³ Hybridization in Alkanes

• Hybridization: Carbon atoms in alkanes undergo sp³ hybridization
• Geometry: Tetrahedral geometry with bond angles of 109.5°
• Bonding: Four equivalent sp³ hybrid orbitals form σ bonds
• Characteristics:
  • Single C-C and C-H bonds only
  • Free rotation around C-C bonds
  • Saturated hydrocarbons

Halogenation of Alkanes

Free Radical Mechanism:

1. Initiation: X₂ → 2X• (homolytic cleavage)
2. Propagation:
   • R-H + X• → R• + HX
   • R• + X₂ → R-X + X•
3. Termination: Radical combination reactions

Selectivity Order:

• Tertiary (3°) > Secondary (2°) > Primary (1°) hydrogen abstraction
• Relative rates: 3° : 2° : 1° = 5.0 : 3.8 : 1.0 (for chlorination at 25°C)

Uses of Paraffins

• Fuel: Natural gas, gasoline, kerosene, diesel
• Lubricants: Engine oils, greases
• Raw materials: Petrochemical industry feedstock
• Solvents: Non-polar solvents for organic reactions
• Waxes: Candles, polishes, coatings

ALKENES

Stabilities of Alkenes

Stability Order: More substituted > Less substituted

• Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted > Ethylene

Factors Affecting Stability:

• Hyperconjugation: Increased alkyl substitution provides more hyperconjugation
• Steric effects: Bulky groups can cause destabilization
• Resonance: In conjugated systems

SP² Hybridization in Alkenes

• Hybridization: Carbon atoms undergo sp² hybridization
• Geometry: Trigonal planar with 120° bond angles
• Bonding:
  • Three sp² hybrid orbitals form σ bonds
  • Unhybridized p orbital forms π bond
• Characteristics:
  • Planar structure around double bond
  • Restricted rotation due to π bond
  • Higher electron density in π bond region

ELIMINATION REACTIONS (E1 AND E2)

E1 Reactions

Mechanism:

1. Step 1: Slow ionization of alkyl halide (rate-determining)
   • R-X → R⁺ + X⁻
2. Step 2: Fast proton elimination by base
   • R⁺ + B⁻ → Alkene + BH

Kinetics:

• Order: First-order kinetics
• Rate equation: Rate = k[RX]
• Rate-determining step: Carbocation formation

E2 Reactions

Mechanism:

• Concerted process: Single step, simultaneous bond breaking and forming
• Anti-periplanar geometry: H and X must be anti to each other
• Kinetics: Second-order kinetics
• Rate equation: Rate = k[RX][Base]

Order of Reactivity of Alkyl Halides

E1 Reactions: Tertiary > Secondary >> Primary (Primary doesn't undergo E1)

E2 Reactions: Tertiary > Secondary > Primary

Explanation: Stability of carbocation (E1) or transition state (E2)

Rearrangement of Carbocations

Types:

1. 1,2-Hydride shift: H⁻ migration to more stable carbocation
2. 1,2-Alkyl shift: R⁻ migration to more stable carbocation

Driving force: Formation of more stable carbocation (3° > 2° > 1°)

Saytzeff's Rule and Evidence

Rule: In elimination reactions, the more substituted (more stable) alkene is the major product

Evidence:

• Product distribution analysis shows predominance of more substituted alkenes
• Thermodynamic stability correlates with product ratios
• Exception: Hofmann elimination (less substituted alkene favored due to steric hindrance)

E1 vs E2 Reactions

Factors Affecting E1 and E2 Reactions

E1 Factors:

• Substrate structure: Tertiary > Secondary
• Leaving group: Better leaving groups increase rate
• Solvent: Polar protic solvents stabilize carbocation
• Temperature: Higher temperature favors elimination

E2 Factors:

• Base strength: Stronger bases increase rate
• Base concentration: Higher concentration increases rate
• Substrate structure: Anti-periplanar arrangement required
• Leaving group: Better leaving groups increase rate

ALKENE REACTIONS

Ozonolysis

Mechanism:

1. Ozonide formation: Alkene + O₃ → Molozonide → Ozonide
2. Reductive workup: Zn/AcOH or Me₂S
3. Oxidative workup: H₂O₂

Products:

• Aldehydes and/or ketones depending on substitution pattern
• Used for structure determination of alkenes

Electrophilic Addition Reactions

General Mechanism:

1. Step 1: Electrophile attacks π bond forming carbocation
2. Step 2: Nucleophile attacks carbocation

Common Reactions:

• Hydrohalogenation: HX addition
• Hydration: H₂O addition (acid-catalyzed)
• Halogenation: X₂ addition
• Halohydrin formation: X₂ + H₂O

Markovnikov's Rule

Rule: In addition of HX to unsymmetrical alkenes, hydrogen adds to the carbon with more hydrogens, and X adds to the carbon with fewer hydrogens

Mechanism basis: Formation of more stable carbocation intermediate

Example:

• Propene + HBr → 2-bromopropane (major) + 1-bromopropane (minor)

Free Radical Addition Reactions

Anti-Markovnikov Addition:

• Occurs in presence of peroxides (ROOR)
• Mechanism: Free radical chain reaction
• Result: Opposite regioselectivity to ionic addition

Mechanism (HBr + peroxides):

1. Initiation: ROOR → 2RO• → RO• + HBr → ROH + Br•
2. Propagation:
   • Br• + Alkene → C-Br + C• (adds to less substituted carbon)
   • C• + HBr → C-H + Br•

CONJUGATED DIENES

Stability of Conjugated Dienes

Enhanced Stability due to:

1. Resonance: Delocalization of π electrons
2. Extended conjugation: Lower overall energy
3. Reduced heat of hydrogenation: Compared to isolated dienes

Stability Order: Conjugated dienes > Cumulated dienes > Isolated dienes

Diels-Alder Reaction

Reaction: [4+2] Cycloaddition reaction

• Diene: Must be in s-cis conformation
• Dienophile: Electron-deficient alkene (usually with electron-withdrawing groups)
• Product: Six-membered ring (cyclohexene derivative)
• Stereochemistry: Concerted, syn addition
• Endo rule: Endo product is kinetically favored

Requirements:

• Diene must be able to adopt s-cis conformation
• Dienophile should have electron-withdrawing groups

Electrophilic Addition to Conjugated Dienes

Products:

1. 1,2-Addition: Addition across one double bond
2. 1,4-Addition: Addition across the conjugated system

Mechanism:

1. Step 1: Electrophile attacks forming resonance-stabilized allylic carbocation
2. Step 2: Nucleophile can attack at C-2 or C-4

Temperature effects:

• Low temperature: Kinetic control → 1,2-addition favored
• High temperature: Thermodynamic control → 1,4-addition favored

Free Radical Addition to Conjugated Dienes

• Mechanism: Similar to simple alkenes but with resonance stabilization
• Products: Both 1,2- and 1,4-addition products possible
• Selectivity: Determined by stability of allylic radical intermediate

Allylic Rearrangement

Definition: Migration of double bond and functional group in allylic systems

Mechanism:

• Formation of resonance-stabilized allylic carbocation
• Nucleophile can attack either end of allylic system
• Results in rearranged products

Examples:

• SN1 reactions of allylic halides
• Acid-catalyzed rearrangements
• Enzymatic rearrangements

Characteristics:

• Driving force: Resonance stabilization of allylic intermediate
• Products: Mixture of constitutional isomers
• Selectivity: Depends on steric and electronic factors

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Key Concepts Summary

1. Hybridization: sp³ (alkanes), sp² (alkenes) determines geometry and bonding
2. Stability trends: More substituted alkenes are more stable
3. Elimination mechanisms: E1 (carbocation) vs E2 (concerted)
4. Addition reactions: Markovnikov (ionic) vs Anti-Markovnikov (radical)
5. Conjugation effects: Enhanced stability and unique reactivity patterns
6. Stereochemistry: Important in cycloadditions and addition reactions