An Electron Withdrawing Group (EWG) is a functional group or atom that draws electron density away from the rest of the molecule. This effect results from the group’s higher electronegativity compared to the atoms it is bonded to. Whether EWGs increase a molecule’s reactivity depends entirely on the specific chemical reaction taking place. In some reactions, electron withdrawal significantly slows the process, while in others, it accelerates the reaction rate. EWGs change the electron distribution, making certain sites more or less susceptible to attack by other chemical species.
How Electron Withdrawing Groups Function
Electron Withdrawing Groups use two primary physical mechanisms to pull electron density away from the molecule’s core structure.
The inductive effect involves the withdrawal of electrons through sigma (\(\sigma\)) bonds. This results from the difference in electronegativity between the EWG and the attached carbon atom. Highly electronegative atoms, such as fluorine or the nitrogen in a nitro group (\(\text{NO}_2\)), create a permanent dipole moment, causing a slight positive charge on the adjacent carbon. This electronic pull weakens quickly with distance, typically becoming negligible after three or four single bonds.
The resonance effect, also known as the mesomeric effect, involves the delocalization of electrons through pi (\(\pi\)) systems. This occurs when the EWG stabilizes a charge through the overlap of p-orbitals, often resulting in multiple resonance structures. Groups like carbonyls (C=O) or nitro groups (\(\text{NO}_2\)) attached to a conjugated system spread electron density over a larger area. The resonance effect is generally much stronger than the inductive effect and influences reactivity over greater distances.
Impact on Electrophilic Aromatic Substitution
In reactions like Electrophilic Aromatic Substitution (EAS), Electron Withdrawing Groups dramatically decrease molecular reactivity, a process known as deactivation. EAS involves a positively charged electrophile attacking an electron-rich aromatic ring. The EWG pulls electron density away from the ring, making it less nucleophilic and less attractive to the electrophile.
The reaction proceeds through a high-energy, positively charged intermediate called the \(\sigma\)-complex or arenium ion. EWGs destabilize this intermediate by intensifying the positive charge within the ring structure. This destabilization raises the activation energy for the rate-determining step, significantly slowing the reaction rate compared to a simple benzene ring. For example, a strong EWG like the nitro group (\(\text{NO}_2\)) can make the ring millions of times less reactive than benzene in nitration.
Most strong EWGs, such as the nitro or carboxylic acid groups, direct the incoming electrophile to the meta position. This preference occurs because attack at the ortho and para positions places the positive charge directly onto the carbon bearing the EWG. Since the EWG is electron-poor, this placement is highly unstable. The meta attack avoids placing the positive charge adjacent to the EWG, making it the least unfavorable path.
Impact on Nucleophilic Reactions and Acidity
In contrast to EAS, Electron Withdrawing Groups are highly effective at increasing reactivity in reactions that involve stabilizing a negative charge or creating an electron-deficient center. The principle at work is the stabilization of a negative charge, which lowers the energy of the transition state or intermediate.
Increased Acidity
EWGs significantly increase the acidity of compounds by stabilizing the resulting conjugate base. When an acid donates a proton (\(\text{H}^+\)), the remaining structure is a negatively charged anion. The EWG pulls electron density away from this anion, dispersing the negative charge and stabilizing the structure. A more stable conjugate base corresponds to a stronger parent acid, meaning the acid is more likely to give up its proton.
In carboxylic acids, replacing a hydrogen atom with a halogen like chlorine drastically lowers the \(\text{p}K_a\) value, making the acid stronger. The inductive effect stabilizes the carboxylate anion, and this effect is strongest when the EWG is closest to the acidic proton. Similarly, the resonance effect of a nitro group stabilizes the phenolate anion formed from phenol, making the compound significantly more acidic.
Nucleophilic Aromatic Substitution (NAS)
EWGs also dramatically accelerate the rate of Nucleophilic Aromatic Substitution (\(\text{S}_{\text{N}}\text{Ar}\)) reactions. Unlike EAS, NAS involves an electron-rich nucleophile attacking an electron-poor aromatic ring, requiring EWGs to activate the ring. The reaction proceeds through a negatively charged intermediate, known as a Meisenheimer complex.
The presence of a strong EWG, typically positioned ortho or para to the leaving group, stabilizes this transient negative charge through resonance. By delocalizing electron density into the EWG itself, the intermediate’s energy is lowered, accelerating the rate-determining step. The nitro group is a common activator for NAS, turning an unreactive aryl halide into a highly reactive substrate.
Reactions at Carbonyls
Increased reactivity is also seen in reactions involving the carbonyl group (\(\text{C=O}\)), such as nucleophilic acyl substitution. The carbonyl carbon already carries a partial positive charge due to the electronegativity of the oxygen atom. If an EWG is attached near this carbonyl, its inductive effect increases the magnitude of this positive charge. This makes the carbonyl carbon a stronger electrophile, enhancing its susceptibility to nucleophilic attack. This electronic enhancement makes the carbonyl carbon of acyl chlorides, which have a chlorine EWG, much more reactive toward nucleophiles than that of an amide.