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🥼organic chemistry review

Carbanions

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

A carbanion is a negatively charged carbon atom with three bonds and a lone pair of electrons. These reactive intermediates play a crucial role in various organic chemistry reactions and concepts, including formal charges, reaction intermediates, alkyne acidity, and Grignard reagents.

5 Must Know Facts For Your Next Test

  1. Carbanions are stabilized by the presence of electronegative atoms or groups, such as halogens, oxygen, or aromatic rings, which can delocalize the negative charge.
  2. Carbanions are commonly formed as reaction intermediates in nucleophilic substitution and elimination reactions involving alkyl halides.
  3. The acidity of alkynes allows for the formation of acetylide anions, which are carbanions that can then participate in various organic transformations.
  4. Grignard reagents, which are organomagnesium compounds, can be used to generate carbanions that can then react with electrophiles to form new carbon-carbon bonds.
  5. The stability and reactivity of carbanions are influenced by factors such as the hybridization of the carbon atom, the presence of adjacent heteroatoms, and the degree of conjugation or delocalization.

Review Questions

  • Explain how the concept of formal charges is related to the formation and stability of carbanions.
    • Formal charges are used to determine the distribution of electrons in a molecule, which is crucial for understanding the stability and reactivity of carbanions. When a carbon atom bears a negative formal charge, it indicates the presence of a carbanion, which is a reactive intermediate with three bonds and a lone pair of electrons. The stability of the carbanion is influenced by factors that can delocalize or stabilize the negative charge, such as the presence of electronegative atoms or groups nearby.
  • Describe the role of carbanions as reaction intermediates and how they are involved in the formation of acetylide anions and Grignard reagents.
    • Carbanions are commonly formed as reaction intermediates in various organic chemistry reactions, such as nucleophilic substitutions and eliminations involving alkyl halides. The acidity of alkynes allows for the formation of acetylide anions, which are carbanions that can then participate in further organic transformations. Additionally, Grignard reagents, which are organomagnesium compounds, can be used to generate carbanions that can then react with electrophiles to form new carbon-carbon bonds. The reactivity and stability of these carbanion intermediates are crucial in determining the outcome of these reactions.
  • Analyze the factors that influence the stability and reactivity of carbanions, and explain how these factors impact their role in organic chemistry reactions.
    • The stability and reactivity of carbanions are influenced by several factors, including the hybridization of the carbon atom, the presence of adjacent heteroatoms, and the degree of conjugation or delocalization. Carbanions with sp$^2$ or sp hybridization are generally more stable than those with sp$^3$ hybridization, as the p-orbital or triple bond can help delocalize the negative charge. The presence of electronegative atoms or groups, such as halogens, oxygen, or aromatic rings, can also stabilize the carbanion by allowing the negative charge to be shared or delocalized. The degree of conjugation or delocalization within the molecule can further enhance the stability of the carbanion, making it less reactive and more likely to participate in specific organic chemistry reactions. Understanding these factors is crucial for predicting the behavior and reactivity of carbanions in various organic transformations.