Reactions of nucleophile at saturated carbons

• What are key factors influencing the reaction pathway?

  • Substrate structure
  • Leaving group
  • nucleophile and base
  • solvent
  • temperature
• How does substrate structure reaction pathway?

  • reaction site hindered or not
  • Stabilisation of intermediates or transition states by subtituents
• How does leaving group ability impact reaction pathway?

  Stabilisation of leaving group (pka)
• how does nucleophile affect?

  • Steric bulk
  • hard/soft
  • pKa
• How does solvents affect reaction pathway?

  Stabilises TS or intermediate by solvation

  • polar protic stabilises cationic intermediate
  • Polar aprotic solvates nucleophile counter, increase nucleophile strength
• how does temperature affect the reaction pathway?

  Entropy factors favour elimination
• What is an SN1 reaction?

  Unimolecular (first order),

  • two steps
  • formation cation intermediate
• what is an SN2 reaction?

  Bimolecular (second order)

  • one step
• what does the rate of reaction depend for an SN1 reaction?

  • ONLY on the concentration of the substrate
• How many transition states to an SN1?

  2
• What determines the enantiomers formed from a SN1 reaction?

  Racemization (S/R) chiral centres
• What is the rate determining step for an SN2 reaction?

  product formation
• what is the rate of reaction for SN2 reaction?

  depends on concentration of the substrate

  Depends on the concentration of the nucleophile
• What occurs at the SN2 transition state?

  The filled p orbital at the central carbon atom shares one pair of electrons between the old and new bond (new= nucleophile, old = leaving)
• What is responsible for the geometry of the transistion state?

  HOMO-LUMO interactions
• what is a good HOMO-LUMO interaction?

  a large coefficient
• What is the HOMO of SN2?

  Lone pair on Nucleophile
• what is the LUMO of SN2?

  C-LG sigma*
• what is a back-side attack?

  Nucleophilic attack from the back of a molecule.
• What is the transition state geometry of a SN2 reaction?

  trigonal bipyramidal
• Does racemisation occur in SN2 reactions?

  No, inversion of configuration instead
• Can SN1 occur at sp2 centres?

  no
• Why cant sp2 occur with Sn1?

  no stabilisation from pi orbitals, so no overlap possible as pi orbitals are orthogonal to empty Sp2
• Does SN2 occur at sp2 centres?

  no
• why does does no SN2 occur at aromatic rings?

  Approach od nucleophile into a C-LG sigma* is blocked by the rest of the aromatic ring.

  • Inversion of configuration impossible