Learning Objective: To compare different molecular structures with their pKa values to understand which structures can easily form and effectively stabilize a carbanion.
Skill Level - Advanced
Prerequisites:
Acidity
Aromaticity
Electron withdrawing groups
Chapter: Reactive Intermediates
Sub-topic: Carbanion
Author's Note: Carbanions are similar to acids in their proton donation, with faster donation corresponding to stronger acid. However, a carbanion requires the conjugate base to be stable to effectively function as an electron donor that eventually will help form a new C-C bond. Therefore, this section covers examples of various molecular structures and their pKa values to further understand which molecules will function better as carbanion in organic reactions and, when they do, which three basic features allow their conjugate base stabilization.
Ease of Formation of Carbanion - Acidic Proton
We have learned that an acid is a proton (H+) donor. In organic chemistry, the formation of a carbanion is analogous to that of an acid.
A hydrocarbon molecule may act as a proton donor by cleaving the C-H bond and forming a carbanion, provided several conditions are met.
However, hydrocarbons' ability to form a carbanion depends on their ability to give up protons (H+), known as acidity. A hydrocarbon's H+ donating capacity is measured with a pKa value.
The lower the pKa value, the stronger the acid and the easier the carbanion formation.
For example, Methane has a pKa of 56, and trinitromethane HC(NO2)3 has a pKa of 0.17, making it a strong acid over methane.
The ability to lose H+ increases when the negative charge on a carbanion is delocalized or stabilized. The stabilization occurs when....
Three structural features that enhance ...
Subscribe to learn which molecular structures ease the formation of carbanion.
