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Substituent Effects

Here is a table that shows the effect of substituents on a benzene ring have on the rate and orientation of electrophilic aromatic substitution reactions.
 
Table of Substituent Effects These effects are a combination of RESONANCE and INDUCTIVE effects (see review in Ch12)
The effects are also important in other reactions and properties (e.g. acidity of the substituted benzoic acids).

Here are some general pointers for recognising the substituent effects:

  • The H atom is the standard and is regarded as having no effect.
  • Activating groups increase the rate
  • Deactivating groups decrease the rate
  • EDG = electron donating group
  • EDG can be recognised by lone pairs on the atom adjacent to the p system, eg: -OMe
  • except -R, -Ar or -vinyl (hyperconjugation, p electrons)
  • EWG = electron withdrawing group
  • EWG can be recognised either by the atom adjacent to the p system having several bonds to more electronegative atoms, or

  • having a formal +ve or
    d +ve charge, eg: -CO2R, -NO2
  • EDG / activating groups direct ortho / para
  • EWG / deactivating groups direct meta
  • except halogens (-X) which are deactivating BUT direct ortho / para
  • EDG add electron density to the p system making it more nucleophilic
  • EWG remove electron density from the p system making it less nucleophilic

Thought provoking questions.....

  • Why are esters (-OCOR) and amides (-NHCOR) less activating than ethers (-OR) and amines (-NH) ? the lone pairs on O in esters and amides are also involved in resonance with the C=O group and therefore can not be donated to the arene as readily
  • Why do esters and amides appear in the table twice, onceas an EDG and once as an EWG ? if the arene is connected via the heteroatom, the lone pairs can activate but if it is connected via the C=O group then it will deactivate
  • Why are amines (-NH) better activators than alcohols (-OH) ? N is less electronegative than O so it is a better electron donor


  © Dr. Ian Hunt, Department of Chemistry, University of Calgary

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