Alkyl Halides

• Alkyl halide can be prepared from:

  • alkene by halogenation in UV light
  • alkane by radical halogenation
  • alcohol using SOCl2 or PBr3
• Radical halogenation takes place in diffuse sunlight or heat, as if in strong sunlight it can be explosive - radical initiator are more convenient to use and other sources of halogen in the lab
• For the regioselectivity of radical halogenation, 2' product is favoured due to stability
• Reactivity-selectivity principle states that the more reactive a reagent is, the less selective it is
• Example of the reactivity-selectivity principle is Br radial is less reactive than Cl radical so has to select the easiest H to remove which results in 2' product to be more common, while Cl radial can selective other positions more easily
• Radical next to C=C can be stabilised by delocalising across the system - resonance view explains this
• Sn2 is substitution nucleophilic bimolecular, with 2 referring to kinetics. There is a transition state.

  • first step - Y- and RX collide and overcome activation energy to form TS
  • second step - second activation energy is overcome to form product
• Sn1 is substitution nucleophilic unimolecular, with 1 referring to the kinetics. There is no transition state.

  • first step - X leaves and forms carbenium ion as polar solvents favour dissociation
  • second step - Nu attacks carbenium ion forming the product
• Sn2 - reaction rate depends on [Y-] and [RX]

  Sn1 - reaction rate depends on [RX]
• Carbenium ion is flat and has orbital lobes on either side, so Nu can attack from either side which results in two possible products that differ in their stereochemistry
• For Sn2 mechanism, Nu attacks RX from behind so the configuration becomes turned inside out
• Elements of H-X can be eliminated by a good base which may act as Nu also
• When TS has a partial double bond it is a pseudo-alkene. Stability of pseudo-alkene determines ease of reaching TS, E2 reactivity parallels alkene stability
• E2 mechanism involves a simple collision between base and RX
• Where there is a choice within the same molecule E2TS, the more stable with lower energy the E2TS structure is preferred so the more stable alkene product is formed
• E1 mechanism involves a carbenium ion intermediate
• The more substituted C=C is preferred in the product
• E1 and Sn1 shared a common RLS so often occur in the same reaction
• Simple alkyl halides show no prominent characteristic features in UV-vis or IR. MS show characteristic isotope patterns for halogenated organic compounds