Electron deficiency

Electron deficiency occurs when a compound has too few valence electrons for the connections between atoms to be described as covalent bonds. Electron deficient bonds are often better described as 3-center-2-electron bonds. Examples of compounds that are electron deficient are the boranes.

The term electron-deficient is also used in a more general way in organic chemistry, to indicate a pi-system such as an alkene or arene that has electron-withdrawing groups attached, as found in nitrobenzene or acrylonitrile. Instead of showing the nucleophilic character common with simple C=C bonds, electron-deficient pi-systems may be electrophilic and susceptible to nucleophilic attack, as is seen in the Michael addition or in nucleophilic aromatic substitution.

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