haloalkanes

• two factors affecting reactivity:

  • strength - going down halogen group, atoms get larger; increased shielding due to nucleus is further away from electrons

  → this means the mean enthalpy gets lower + bond are easier to break; if the mean enthalpy is higher, the halogen will less likely appear in a nucleophilic substitution reaction

  • polarity - F has highest electronegativity in group 17 so electronegativity decreases down the group

  → meaning the C-X bond decreases as only the molecules with highest priority are more likely to react
• Hydrolysis

  a nucleophilic reaction for halogens

  • H2O is the reactant that provides OH for the nucleophilic reaction

  →if water was used, one of the O-H bond would undergo heterolytic fission to produce OH-

  • this species attack the electron-deficient carbon atom in C-X bond

  →once the nucleophilic has bonded to the haloalkane, the OH undergoes heterolytic fission to produce a halide
• Measuring the rate of reaction

  rate of reaction is the change in concentration of reactants or products in a given time
• halogens with silver nitrate

  • results show the rate of reaction is fastest for primary haloalkanes + slowest for primary chloroalkanes

  
• Hydrolysis + bond enthalpy of C-halogen bonds

  • we can conclude bond enthalpy is the most important factor in hydrolysis of primary haloalkanes

  → the C-Cl bond is most polar so it’s most attractive to nucleophilic attack; due to highest polarity in bond the rate of reaction because bond is weaker + easier to break
• Hydrolysis using aqueous alkali

  • when a haloalkane is under reflux with an aqueous solution containing OH- ions, an alcohol is made

  →this is a nucleophilic reaction + example of hydrolysis; rate of reaction is faster than that using water