Factors affecting rxn

• Factors affecting the rate of Nucleophilic substitution of R - X
• Classification of alkyl halides
• Nucleophilic Substitution, SN & Alkyl Halides
• Factors affecting Rate of SN Reaction Halogenoalkanes (R - X):

  <|>The nature of nucleophile:

  • Better Nucleophile with negative charge, high electron density, lower electronegativity, less bulky
• The nature of halogen:

  • R - I > R - Br > C - Cl > C - F
  • Better Leaving Group to Worst Leaving Group
• The choice of solvent:

  • SN1: Polar Protic (water, Et - OH, RCOOH, NH3)
  • SN2: Aprotic (propanone, ethoxyethane, ethanenitrile, DMSO)
• Nucleophiles Definition:

  <|>A nucleophile is an electron-rich species capable of donating a pair of electrons to an electrophile to create a covalent bond
• It will love to attack a nucleus
• A nucleophile is a Lewis Base
• Factors affecting Rate of SN Reaction Halogenoalkanes (R - X):

  <|>The Role Of Charge:

  • Nucleophilicity Increases As An Atom’s Electron Density Increases
  • The more dense the negative charge on the ion/molecules is, the better the nucleophile
• Electronegativity:

  • The less electronegative the atom, the better nucleophile
  • Nucleophilicity increases with decreasing electronegativity
• Nucleophilicity Decreases With Increasing Steric Hindrance (“Bulkiness”):

  • The less bulky the nucleophile, the stronger the nucleophile
  • Steric hindrance decreases nucleophilicity but not basicity
• The nature of Halogen:

  <|>The rate of a nucleophilic substitution reaction depends on the strength of the carbon – halogen bond rather than the degree of polarization in the bond
• The C – I bond is the weakest and so most readily undergoes nucleophilic substitution
• The rate of reactions involving iodoalkanes is the highest
• The C – I bond is the weakest and most readily undergoes nucleophilic substitution
• Strength (kJ mol - 1) Bond Rate Increasing Reactivity Increasing Bond Strength decreasing
• Factors affecting Rate of SN Reaction Halogenoalkanes (R - X)
• Mechanisms of Nucleophilic Substitution: SN1 Unimolecular Nucleophilic Substitution, Rate = k[R - X] SN2 Bimolecular Nucleophilic Substitution, Rate = k[R - X][Nu]
• In SN2 reactions, polar aprotic solvents are more suitable as they leave the nucleophile "NAKED" and solvate Na+, leading to a faster rate of reaction
• In SN1 reactions, polar protic solvents are favored as they solvate the carbocation and LG via hydrogen bonding, forming a solvent shell
• Polar protic solvents slow down SN2 reactions, while polar aprotic solvents favor SN2 reactions
• There are periodic trends in leaving group ability: in polar protic solvents, nucleophilicity increases down a column of the periodic table as the size of the anion increases