Course Topics

Free Radical Mechanism

Free radical substitution in alkanes involves three steps:

Step Process Example (Chlorination of Methane)
Initiation Formation of free radicals Cl₂ → 2Cl• (with UV light or heat)
Propagation Chain reaction steps Cl• + CH₄ → HCl + •CH₃
•CH₃ + Cl₂ → CH₃Cl + Cl•
Termination Radical combination Cl• + Cl• → Cl₂
•CH₃ + •CH₃ → C₂H₆
•CH₃ + Cl• → CH₃Cl
CRITICAL CONCEPT: The reactivity order of halogens in free radical substitution is F₂ > Cl₂ > Br₂ > I₂. However, only chlorination and bromination are practically useful in the laboratory because fluorine is too violent and iodine is too slow.
Memory Tip

Remember the three steps: Initiation (starts), Propagation (continues), Termination (ends). Like a chain reaction that needs to be started, keeps going, and eventually stops!

Preparation of Alkenes

Major methods for preparing alkenes:

Method Reagents/Conditions Example
Dehydration of Alcohols Conc. H₂SO₄, Al₂O₃, or H₃PO₄ at high temperature C₂H₅OH → C₂H₄ + H₂O
Dehydrohalogenation Alcoholic KOH, heat CH₃CH₂Cl → CH₂=CH₂ + HCl
Dehalogenation of Vic-Dihalides Zn dust in alcohol CH₂Br-CH₂Br + Zn → CH₂=CH₂ + ZnBr₂
Kolbe’s Electrolysis Electrolysis of dicarboxylic acid salts NaOOC-CH₂-CH₂-COONa → CH₂=CH₂ + 2CO₂ + 2NaOH + H₂
CRITICAL CONCEPT: The ease of dehydration follows the order: Tertiary alcohol > Secondary alcohol > Primary alcohol. This is because tertiary carbocations are more stable than secondary, which are more stable than primary.
Memory Tip

Dehydration = Remove H₂O, Dehydrohalogenation = Remove HX, Dehalogenation = Remove X₂. All are elimination reactions!

Structure & Reactivity of Alkenes

Structure of Ethene (C₂H₄):

  • Carbon atoms are sp² hybridized
  • Trigonal planar geometry with bond angles of 120°
  • C=C bond length: 1.34 Å (shorter than C-C single bond: 1.54 Å)
  • One σ-bond and one π-bond between carbon atoms
  • π-bond is weaker and electrons are more exposed

Reactivity: Alkenes are highly reactive due to:

  1. Weak π-bond that can be broken easily
  2. Exposed π-electrons that are available for electrophilic attack
  3. Ability to undergo addition reactions
sp² hybridization: 1s² + 2p³ → 3 sp² orbitals + 1 p orbital
Memory Tip

sp² = 3 orbitals in plane (trigonal planar) + 1 p orbital perpendicular. The p orbitals overlap sideways to form the π-bond!

Addition Reactions of Alkenes

Major addition reactions:

Hydrogenation

  • Catalyst: Pt, Pd, or Ni
  • Product: Alkane
  • Exothermic process
  • CH₂=CH₂ + H₂ → CH₃-CH₃

Halogenation

  • Reagent: Cl₂ or Br₂ in CCl₄
  • Product: Vicinal dihalide
  • Test for unsaturation
  • CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br

Hydrohalogenation

  • Reagent: HCl, HBr, HI
  • Follows Markovnikov’s rule
  • Product: Alkyl halide
  • CH₂=CH₂ + HBr → CH₃-CH₂Br

Hydration

  • Reagent: H₂SO₄ then H₂O
  • Product: Alcohol
  • Industrial method for ethanol
  • CH₂=CH₂ + H₂O → CH₃CH₂OH
CRITICAL CONCEPT – Markovnikov’s Rule: “In the addition of an unsymmetrical reagent to an unsymmetrical alkene, the negative part of the adding reagent goes to that carbon which has the least number of hydrogen atoms.” Example: CH₃-CH=CH₂ + HBr → CH₃-CHBr-CH₃ (not CH₃-CH₂-CH₂Br)
Memory Tip

Markovnikov’s Rule: “The rich get richer” – the carbon with more hydrogens gets the hydrogen from HX!

Oxidation Reactions of Alkenes

Reaction Reagent/Conditions Product Significance
Hydroxylation Cold dilute KMnO₄ (Baeyer’s test) 1,2-diol (glycol) Test for C=C bond
Oxidative Cleavage Hot concentrated KMnO₄ or K₂Cr₂O₇ Carboxylic acids/ketones Structure determination
Ozonolysis O₃ then Zn/H₂O Aldehydes/ketones Locate double bond position
Epoxidation Peroxy acids Epoxides Synthetic intermediate
CRITICAL CONCEPT: Ozonolysis is particularly important for determining the position of double bonds in alkenes. The alkene is cleaved at the double bond, and the products reveal the original structure.
Baeyer’s Test: R-CH=CH-R’ + [O] + H₂O → R-CH(OH)-CH(OH)-R’
Memory Tip

Cold KMnO₄ gives diols (adds OH groups), hot KMnO₄ cuts the molecule (cleavage), O₃ also cuts but gives different products!

Preparation & Structure of Alkynes

Preparation methods:

  1. Dehydrohalogenation of vicinal dihalides: Two steps with strong base (alc. KOH)
  2. Dehalogenation of tetrahalides: With active metals (Zn)
  3. From calcium carbide: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂

Structure of Acetylene (C₂H₂):

  • Carbon atoms are sp hybridized
  • Linear geometry with bond angle of 180°
  • C≡C bond length: 1.20 Å (shortest C-C bond)
  • One σ-bond and two π-bonds between carbon atoms
  • π-bonds are perpendicular to each other
Memory Tip

sp hybridization = 2 orbitals linear + 2 p orbitals perpendicular. Like two balloons tied together with strings at right angles!

Acidity of Alkynes & Reactions

Acidity of Terminal Alkynes: Terminal alkynes (RC≡CH) are weakly acidic because:

  • sp hybridized carbon has 50% s-character
  • Higher s-character means higher electronegativity
  • Can form acetylide ions with strong bases
  • Acidity order: Terminal alkyne > NH₃ > Alkene > Alkane
HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃

Reactions of Alkynes:

Reaction Conditions Product
Hydrogenation H₂ with Pt/Pd/Ni Alkane (complete reduction)
Partial Hydrogenation H₂ with Lindlar’s catalyst cis-Alkene
Dissolving Metal Reduction Na in liquid NH₃ trans-Alkene
Hydration H₂O, H₂SO₄, HgSO₄ Aldehyde (from acetylene) or ketone
Memory Tip

Acidity increases with s-character: sp³ (25% s) < sp² (33% s) < sp (50% s). Terminal alkynes can lose H⁺ because sp carbon is more electronegative!

Benzene: Structure & Stability

Resonance in Benzene: Benzene is a resonance hybrid of two Kekulé structures:

CRITICAL CONCEPT – Resonance Energy: Benzene is more stable than expected by 150.5 kJ/mol. This extra stability (resonance energy) explains why benzene undergoes substitution rather than addition reactions.
Compound Expected Heat of Hydrogenation Actual Heat of Hydrogenation Resonance Energy
Cyclohexene -119.5 kJ/mol -119.5 kJ/mol 0 kJ/mol
1,3-Cyclohexadiene -239 kJ/mol -231.5 kJ/mol 7.5 kJ/mol
Benzene (as 1,3,5-cyclohexatriene) -358.5 kJ/mol -208 kJ/mol 150.5 kJ/mol

Molecular Orbital Picture:

  • Each carbon is sp² hybridized
  • Six p-orbitals overlap to form delocalized π-electron cloud
  • All C-C bonds are equal (1.397 Å) – between single (1.54 Å) and double (1.34 Å) bond lengths
  • Planar hexagonal structure with bond angles of 120°
Memory Tip

Benzene: 6 carbons, 6 hydrogens, 6 π-electrons, hexagonal shape, 120° angles, 1.397 Å bonds. Everything is “6” or symmetrical!

Electrophilic Substitution in Benzene

General Mechanism: Two-step process involving carbocation intermediate (arenium ion)

Reaction Reagents Electrophile Generated Product
Nitration Conc. HNO₃ + H₂SO₄ NO₂⁺ (nitronium ion) Nitrobenzene
Halogenation X₂ + FeX₃ (Lewis acid) X⁺ (halonium ion) Halobenzene
Sulfonation Conc. H₂SO₄ or SO₃ SO₃ (sulfur trioxide) Benzenesulfonic acid
Friedel-Crafts Alkylation R-Cl + AlCl₃ R⁺ (carbocation) Alkylbenzene
Friedel-Crafts Acylation RCOCl + AlCl₃ RCO⁺ (acylium ion) Aryl ketone
CRITICAL CONCEPT: Benzene requires strong electrophiles because the delocalized π-electron cloud is stable. The reaction proceeds through a three-step mechanism: (1) Generation of electrophile, (2) Attack on benzene ring forming arenium ion, (3) Loss of H⁺ to regenerate aromaticity.
Memory Tip

Electrophilic substitution: Electrophile attacks benzene, forms arenium ion (resonance stabilized), then loses H⁺ to restore aromaticity!

Comparison of Hydrocarbon Reactivity

Most Reactive
Moderately Reactive
Least Reactive
Hydrocarbon Type of Bond Characteristic Reaction Reactivity Towards Electrophiles Reason
Alkenes C=C (π-bond) Electrophilic Addition Highest Weak π-bond, exposed electrons
Alkynes C≡C (2 π-bonds) Electrophilic Addition Moderate Tightly held π-electrons
Benzene Delocalized π-system Electrophilic Substitution Low (requires strong E⁺) Resonance stabilization
Alkanes C-C (σ-bond) Free Radical Substitution Very Low Strong σ-bonds, non-polar
CRITICAL CONCEPT: The reactivity order for electrophilic reactions is: Alkenes > Alkynes > Benzene > Alkanes. However, for nucleophilic reactions, alkynes are more reactive than alkenes due to the electronegative sp-hybridized carbon atoms.

Ortho/Para Directors (Activators)

  • -NH₂, -NHR, -NR₂
  • -OH, -OR
  • -alkyl groups
  • Increase electron density
  • Make ring more reactive

Meta Directors (Deactivators)

  • -NO₂, -CN
  • -COOH, -CHO, -COR
  • -SO₃H
  • Decrease electron density
  • Make ring less reactive
Memory Tip

Alkenes: Easy to attack (weak π), Alkynes: Harder to attack (tight π), Benzene: Very hard (resonance stabilized), Alkanes: Almost impossible (strong σ)!