SN1 vs SN2: substrate

• what does the substrate structure determine in SN1 reactions?

  Stability of carbocation intermediate
• what does the substrate structure determine in SN2?

  Stabilisation of transition state p orbital
• How do you stabilise a positively charged carbon?

  make it less positive
• how to stabilise a carbocation?

  • sigma donation
  • pi donation
  • lone pair donation
• what is the strongest way to stabilise a carbocation?

  lone pair donation
• what is hyperconjugation?


  C+ is adjacent to alkyl groups
• How to increase the effects of hyperconjugation?

  more bonds participating
• what is the stability of carbocations?

  primary < secondary < tertiary
• what is pi donation?

  C+ is adjacent double bonds
• how to increase affects of pi donation?

  Stronger effect if more than one double bond conjugated with C+
• what is lone pair donation?

  C+ adjacent to heteroatom
• what is a heteroatom?

  lone pair
• what is the relative strength of lone pair donation?

  N > O > S > Cl
• when do primary substrates undergo SN1?

  when carbocation is stabilised
• Is there acetal formation in SN2?

  no
• Is there Sn2 for a tertiary alkyl bromide?

  No
• what type of alkyl bromide is favourable for SN2?

  Primary alkyl bromide
• what type of reaction do unhindered alkyl bromides undergo?

  SN2
• what does lowering the energy of the P orbitals do to the transition state in SN2?

  stabilises it
• what R groups would stabilise the SN2 transition state?

  Alkenyl, aryl (C=O, C=N and alkynyl)
• How does the side on overlap with p orbitals stabilise the transition state?

  Lowers the LUMO of C-LG therefore closer to HOMO
• What can stabilise transition state?


  electron donating or withdrawing groups
• what are the most reactive SN2 substrates?

  2-halocarbonyls
• Simple alkyl group

  A) Bad

  B) Excellent

  C) bad

  D) good

  E) ok

  F) ok

  G) Excellent

  H) bad

  I) bad

  J) bad

  
• Simple alkyl chains

  A) Good

  B) Good

  C) Good

  D) Good

  E) Excellent

  F) Ok but better Sn1

  G) Excellent

  H) Ok but SN1 better

  I) Bad

  J) Excellent