Reaction mechanism

• Addition reaction

  Going from a triple bond to a double bond, or from a double to a single bond
• Elimination reaction

  Going from an alkane to an alkene
• Addition reaction (cyclohexene to cyclohexane)

  1. Add hydrogen gas using a metal catalyst

  2. Add two hydrogens across the double bond
• Elimination reaction (2-butanol to alkene)

  1. React with concentrated sulfuric acid with heat

  2. Remove/eliminate a hydrogen atom and the hydroxyl group
• Substitution reaction

  Replace a chlorine atom with an OH group
• Carbocation rearrangement

  • Secondary carbocation adjacent to a more substituted tertiary carbon
  • Rearrange to form a more stable tertiary carbocation intermediate
• Electrophilic addition reaction (1-butene + HBr)

  1. Hydrogen atom goes on less substituted carbon

  2. Bromine atom goes on more substituted carbon

  3. Form secondary carbocation intermediate

  4. Bromide ion combines with carbocation
• Nucleophilic addition reaction (cyclopentanone + NaBH4 + H2O)

  1. Borohydride ion releases hydride to attack carbonyl

  2. Oxygen gains negative charge

  3. Hydrogen from water added
• Carbocation rearrangement (secondary carbocation next to tertiary carbon)

  1. Hydride shifts from tertiary to secondary carbon

  2. Positive charge moves to where hydride left
• Carbocation rearrangement (secondary carbocation next to quaternary carbon)

  Methyl shift occurs
• Ring expansion rearrangement (5-membered to 6-membered ring)

  1. Break bond between carbons 2 and 6

  2. Reform bond between carbons 1 and 6

  3. Positive charge on carbon 2
• E1 elimination reaction (secondary alkyl halide + H2O)

  1. Leaving group leaves first to form carbocation

  2. Water abstracts proton to form double bond
• E2 elimination reaction (secondary alkyl halide + OH-)

  1. Strong base removes hydrogen and kicks out leaving group in one step

  2. Forms major Zaitsev product and minor Hofmann product
• Enolate formation (beta-hydroxy ketone + OH-)

  1. Base removes acidic alpha hydrogen

  2. Negative charge stabilized by resonance
• Concentration of the base

  First order with respect to the substrate
• Beta hydroxy ketone

  Ketone with a hydroxyl group on the beta carbon
• E1CB reaction

  1. Acidic alpha hydrogen removed by base

  2. Enolate ion formed

  3. Elimination of H and OH

  4. Formation of alpha-beta unsaturated ketone
• Primary alkyl halide

  Favours SN2 over E2 reaction
• SN2 reaction

  1. Nucleophile attacks from back

  2. Leaving group expelled

  3. Substitution of halide with nucleophile
• SN1 reaction

  • Leaving group leaves first, forming carbocation
  • Nucleophile attacks carbocation
• SN1 reaction mechanism

  1. Leaving group leaves

  2. Carbocation formed

  3. Nucleophile attacks carbocation

  4. Hydrogen abstracted by another nucleophile
• Free radical substitution

  • Radical initiation
  • Radical propagation
• Free radical substitution mechanism

  1. Bromine radicals formed

  2. Bromine radical reacts with alkane

  3. Carbon radical reacts with bromine
• Electrophilic aromatic substitution

  • Electrophile adds to benzene ring
  • Base removes proton to regenerate aromaticity
• Electrophilic aromatic substitution mechanism

  1. Electrophile addition

  2. Proton removal by base
• Nucleophilic aromatic substitution


  • Nucleophile attacks carbon with leaving group
  • Leaving group expelled
• Nucleophilic aromatic substitution mechanism


  1. Nucleophile addition

  2. Leaving group elimination
• Nucleophilic aromatic substitution (elimination-addition)

  1. Hydrogen and leaving group removed

  2. Nucleophile added
• Benzyne intermediate formed in nucleophilic aromatic substitution
• Aniline produced from bromobenzene and sodium amide