RX

• R-Cl: PCl3 / PCl5 / SOCl2 OR dry HCl + heat

  R-BR: PBr3 OR dry HBr + heat

  R-I: red P & I2 OR dry HI + heat
• why backside attack

  X blocks approach of Nu from front side, it also repels Nu since both are electron rich
• factors affecting Sn2

  1. C-X strength
  2. favours less steric hindrance (less alkyl grps)
• Sn1

  • unimolecular
  • rate = k[halogenoalkane] order: 1
• Sn2

  • bimolecular
  • rate = k[halogenoalkane][Nu] order: 2
• Sn1 factors:

  1. C-X strength
  2. carbocation stability: favours e- donating grps that stabilise CC by dispersing positive charge at C atom
• special cases that undergo Sn1 as they form stable carbocation

  • allylic halide & benzylic halide
  • in carbocation formed, empty p orbital of positively charged C overlaps with e- cloud of adjacent C=C bond, allowing pi electrons of C=C bond to delocalise over postive charged C & disperse positive charge, stabilizing carbocation
• why halogenoarenes dont undergo NS??

  partial dbl bond in C-X bond:

  • p orb containing lp e- on X overlaps with pi e- cloud of benzene ring, allowing lp of e- in p orb of X to delo into pi e- clould of benzene ring, resulting in pdb character in C-X bond, more E req to overcome the stronger bond

  steric hindrance & e- repulsion due to benzene ring

  Sn2: benzene ring blocks Nu attack at e- deficient C atom from side directly opposite X atom as its pi e- cloud repels lp e- of incoming Nu

  Sn1: unable to form stable carbocation
• tests

  1. add aq NaOH, heat
  2. cool mixture
  3. acidify with HNO3
  4. add aq AgNO3
• tests

  1. add ethanolic AgNO3
  2. warm