key term - Electron-Withdrawing Groups

Definition

Electron-withdrawing groups are functional groups or substituents in a molecule that have a strong affinity for electrons, making them attractive to electrons. This property can significantly influence the reactivity, stability, and spectroscopic properties of the molecule.

5 Must Know Facts For Your Next Test

1. Electron-withdrawing groups can stabilize negative charges and destabilize positive charges in a molecule, affecting its reactivity.
2. The presence of electron-withdrawing groups can shift the $^1$H NMR signals of nearby protons to lower field (higher ppm) due to the deshielding effect.
3. Electron-withdrawing groups can increase the dienophilicity of alkenes, making them more reactive in Diels-Alder reactions.
4. Substituents with electron-withdrawing properties can deactivate the aromatic ring towards electrophilic aromatic substitution reactions.
5. Electron-withdrawing groups can increase the acidity of alcohols, phenols, and carboxylic acids by stabilizing the conjugate base.

Review Questions

• Explain how the presence of electron-withdrawing groups can affect the reactivity of alkynes in the formation of acetylide anions.
  • Electron-withdrawing groups on alkynes can increase the acidity of the terminal hydrogen, making it more readily removed to form the acetylide anion. This is because the electron-withdrawing group stabilizes the resulting negatively charged carbon, making the deprotonation more favorable. The increased acidity of the alkyne allows for the formation of the reactive acetylide species, which can then undergo further reactions.
• Describe the influence of electron-withdrawing groups on the chemical shifts observed in $^1$H NMR spectroscopy.
  • Electron-withdrawing groups can deshield nearby protons in a molecule, causing their $^1$H NMR signals to shift to a higher chemical shift (lower field). This is because the electron-withdrawing group reduces the electron density around the protons, making them less shielded from the external magnetic field. The magnitude of the chemical shift change depends on the strength of the electron-withdrawing effect and the proximity of the protons to the substituent.
• Analyze how electron-withdrawing groups can impact the characteristics and reactivity of the Diels-Alder reaction.
  • Electron-withdrawing groups on the dienophile (the alkene component) in a Diels-Alder reaction can increase its dienophilicity, making it more reactive towards the diene. This is because the electron-withdrawing group stabilizes the partial positive charge that develops on the dienophile during the transition state, lowering the activation energy and making the reaction more favorable. Additionally, electron-withdrawing groups can influence the regio- and stereochemistry of the Diels-Alder adduct by altering the electron distribution and steric effects.

Related terms

Inductive Effect:

The ability of a substituent to either attract or repel electrons through the sigma bonds of a molecule, affecting the electron density at other positions.

Resonance Effect:

The ability of a substituent to stabilize or destabilize a molecule by participating in resonance, altering the electron distribution.

Electronegativity:

The ability of an atom to attract shared electrons in a chemical bond, which determines the strength of electron-withdrawing or electron-donating effects.