Intro Chem Handouts Substitution & Elimination React ions Page 1 of 3
SN2 , SN1 , E2 , & E1:
Substitution and Elimination Reactions
Nucleophilic Substitution Reactions - SN2 Reaction:
• Reaction is:
o Stereospecific (Walden Inversion of configuration)
o Concerted - all bonds form and break at same time
o Bimolecular - rate depends on concentration of both nucleophile and substrate
• Substrate:
o Best if primary (one substituent on carbon bearing leaving group)
o works if secondary, fails if tertiary
• Nucleophile:
o Best if more reactive (i.e. more anionic or more basic)
• Leaving Group: Best if more stable (i.e. can support negative charge well):
o TsO- (very good) > I- > Br- > Cl- > F- (poor)
o RF , ROH , ROR , RNH2 are NEVER Substrates for SN2 reactions
o Leaving Groups on double-bonded carbons are never replaced by SN2 reactions
• Solvent: Polar Aprotic (i.e. no OH) is best.
o For example dimethylsulfoxide ( CH3SOCH3 ), dimethylformamide
( HCON(CH3)2 ), acetonitrile ( CH3CN ).
o Protic solvents (e.g. H2O or ROH) deactivate nucleophile by hydrogen bonding
but can be used in some case
• 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 eac h other
