Substitution & Elimination Reactions: SN2 , SN1 , E2 , & E1

SN2 Reaction
SN1 Reaction
E2 Reaction
E1 Reaction
Download pdf

• Nucleophilic Substitution Reactions (SN2 and SN1) replace a leaving group with a nucleophile (Nu: or Nu: - )
• Elimination Reactions (E2 and E1) generate a double bond by loss of " A+ " and " B: - "
• They may compete with each other

Nucleophilic Substitution Reactions - SN2 Reaction:

Reaction is:

• Stereospecific (Walden Inversion of configuration)
• Concerted - all bonds form and break at same time
• Bimolecular - rate depends on concentration of both nucleophile and substrate
  
  
• Substrate: Best if primary (one substituent on carbon bearing leaving group), works if secondary, fails if tertiary
  
  
• Nucleophile: Best if more reactive (i.e. more anionic or more basic)
  
  
• Leaving Group: Best if more stable (i.e. can support negative charge well):
  • TsO- (very good) > I- > Br- > Cl- > F- (poor)
  • RF , ROH , ROR , RNH2 are NEVER Substrates for SN2 reactions
  • Leaving Groups on double-bonded carbons are never replaced by SN2 reactions
    
    
• Solvent: Polar Aprotic (i.e. no OH) is best: for example dimethylsulfoxide ( CH3SOCH3 ), dimethylformamide ( HCON(CH3)2 ), acetonitrile ( CH3CN ). Protic solvents (e.g. H2O or ROH) deactivate nucleophile by hydrogen bonding but can be used in some cases

Nucleophilic Substitution Reactions - SN1 Reaction:

Reaction is:

• Non-stereospecific (attack by nucleophile occurs from both sides)
• Non-concerted - has carbocation intermediate
• Unimolecular - rate depends on concentration of only the substrate

Substrate: Best if tertiary or conjugated (benzylic or allylic) carbocation can be formed as leaving group departs, never primary

Nucleophile: Best if more reactive (i.e. more anionic or more basic)

Leaving Group: Same as SN2: best if more stable (i.e. can support negative charge well):

TsO- (very good) > I- > Br- > Cl- > F- (poor)

However, tertiary or allylic ROH or ROR' can be reactive under strongly acidic conditions to replace OH or OR

Solvent: Same as SN2: Polar Aprotic (i.e. no OH) is best: for example dimethylsulfoxide ( CH3SOCH3 ), dimethylformamide ( HCON(CH3)2 ), acetonitrile ( CH3CN ). Protic solvents (e.g. H2O or ROH) deactivate but can be used in some cases

Elimination Reactions - E2 Reaction:

Reaction is:

• Stereospecific (Anti-periplanar geometry preferred, Syn-periplanar geometry possible)
• Concerted - all bonds form and break at same time
• Bimolecular - rate depends on concentration of both base and substrate
• Favoured by strong bases

Elimination Reactions - E1 Reaction:

Reaction is:

• Non-stereospecific- follows Zaitsev (Saytseff) Rule
• Non-concerted - has carbocation intermediate - favoured for tertiary leaving groups
• Unimolecular - rate depends on concentration of only the substrate
• Does NOT occur with primary alkyl halides (leaving groups)
• Strong acid can promote loss of OH as H2O or OR as HOR if tertiary or conjugated carbocation can be formed