Electron-withdrawing and donating effects

In this lesson, we will learn:

• To recall the definitions of electron-withdrawing group (EWG) and electron-donating groups (EDG).
• To understand the effects of EWGs and EDGs on nucleophile and electrophile strength.
• To understand the underlying electronic effects that produce these properties.
• To apply mesomeric and inductive effects to predict nucleophile and electrophile strength.

• When running an organic reaction to make a desired product, the reactivity of both the electrophile and nucleophile need to be thought about.
  The more electron rich a nucleophile, or electron poor the electrophile, the better. This is because of the smaller HOMO-LUMO gap discussed in Nucleophiles and electrophiles . These conditions will lead to greater reactivity and a higher product yield; the smaller the HOMO-LUMO gap, the greater tendency for reactant bonds to break and product bonds to form.
  This can be predicted in reactions because substituents in organic molecules have electron withdrawing$\,$ or electron donating$\,$ effects.
  
  
• An electron donating group$\,$ (EDG) has the net effect of increasing electron density in a molecule through the carbon atom it is bonded to. By increasing electron density on adjacent carbon atoms, EDGs change the reactivity of a molecule:
  • EDGs make nucleophiles stronger. With EDGs attached, a nucleophilic center is even more electron rich and ready to attack electrophilic sites.
  • EDGs make carbon centers weaker electrophiles and less reactive to nucleophiles, because any (partial) positive charge it has will be minimized or nullified if the EDG is strong enough.
  Examples of good electron donating groups are groups with lone pairs to donate, such as:
  • The oxygen anion, -O^-
  • Alcohol groups, -OH
  • Amine groups, -NH₂ or -NR₂
  • Ethers, -OR
  • Alkyl groups are also weakly electron-donating.
  
  
• An electron withdrawing group$\,$ (EWG) is a group that reduces electron density in a molecule through the carbon atom it is bonded to. By reducing electron density on adjacent carbon atoms, EWGs change the reactivity of a molecule:
  • EWGs make electrophiles stronger, because the electron-withdrawing effect makes any carbon center even more electron deficient than before.
  • EWGs make any nucleophilic species less reactive, for the same reason as they strengthen electrophiles. Nucleophiles need electron density to react with electrophiles; if an EWG is ‘withdrawing’ electrons, this is taking away the source of the nucleophile’s strength!
  The strongest EWGs are groups with pi bonds to electronegative atoms:
  • Nitro groups (-NO₂)
  • Aldehydes (-CHO)
  • Ketones (-C=OR)
  • Cyano groups (-CN)
  • Carboxylic acid (-COOH)
  • Esters (-COOR)
  Halogens are also electron-withdrawing; the effect gets weaker going down the group.
  
  
• Electron-withdrawing and donating properties come from two different electronic effects that we need to understand:
  • The mesomeric effect$\,$ (M) is a group’s ability to delocalize electrons through resonance structures.
    Resonance$\,$ is a state where a chemical compound has multiple ‘forms’ that are readily interconverting, due to the movement of delocalized electrons through the structure. This is what benzene does with its double bonds.
    Molecules with resonance forms$\,$ that readily interconvert can stabilize points of localized charge.
    By taking up another structure, the charge is ‘shared’ across other atoms. This is the basis for the stability of aromatic rings.
    • A group with a positive mesomeric effect$\,$ (+M) is an electron-donating group that ‘pushes’ electrons onto the carbon atom it is bonded to, usually via a lone pair that can make a resonance structure. This increases electron density on carbon and beyond. See the image below:
      
      To find +M groups, look for single bonds to atoms with lone pairs. Examples are:
      • The alkoxide anion (-O^-) and alcohol group (-OH)
      • Amino group (-NH₂) and alkyl analogues (-NR₂)
      • Ether (-OR)
      
      
    • A group with a negative mesomeric effect$\,$ (-M) is an electron-withdrawing group that ‘pulls’ electrons out from the carbon atom and the rest of the structure it is attached to.
      To do this a group needs pi orbital overlap to delocalize electrons; double bonds to electronegative atoms that ‘want’ electrons make this more likely. See the image below:
    
    To find -M groups, look for double bonds to oxygen and nitrogen!
    Examples are:
    • Nitro groups (-NO₂)
    • Cyano groups (-CN)
    • Carbonyl groups such as aldehydes (-CHO) and ketones (-COR)
    • Esters (-COOR)
    
    
  • The inductive effect$\,$ (I) is a group’s ability to polarize a sigma bond through electronegativity. This is more straightforward than the mesomeric effect:
    • A group with a positive inductive effect (+I) will increase electron density by polarizing the sigma bond. This is normally seen as a weak effect due to:
      • Alkyl groups (-R)
      • Aromatic ring substituents (-C₆H₅)
      
      
    • A group with a negative inductive effect (-I) decreases electron density on the carbon atom by polarizing the sigma bond. This is the effect seen when carbon bonds to electronegative atoms like:
      • Halogens (-F, -Cl. -Br, -I)
      • Oxygen (-OR)
      • Nitrogen (-NR₂)
      See the image below for examples of both:
  
  
• The mesomeric and inductive effects don’t directly impact one another, but both must be considered when looking at group attachments.
  The mesomeric effect dominates over the inductive effect in most cases. For example, ethers or alcohols, which are -I but +M, are both widely recognized as electron-donating groups (EDGs) because of their mesomeric effect.