Carbon Acids and Ranking of Electron-Withdrawing Groups

5 CARBON ACIDS AND RANKING OF ELECTRON-WITHDRAWING GROUPS 

Carbon acids can serve as good examples for the principles of acidity. Often organic reactions in basic media start with deprotonation of an acidic C-H, so we will need to 

Which of the following, CN, Ph, NO2, is the best electron-withdrawing group Which is the poorest  

Groups that are not pi acceptors stabilize an adjacent anion less effectively, and their CH3 ewg pAfaS are not yet available. Very electronegative groups, such as trimethylammonium cation, N(CH3)3, and trifluoromethyl, CF3, can withdraw electron 

Other factors contribute, but often to a much lesser degree. Inductive and field effects from electronegative atoms can also stabilize the carbanion somewhat. Hydrogen bonding is very rare for carbanions. Aromaticity greatly stabilizes carbanions, but the only common example is cyclopentadienyl anion C5H5 . 

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