Electrophiles

Common Electron Sinks (Electrophiles)

Electrons must flow to somewhere that is deficient in electrons, we call this an electron sink or electrophile.  An electrophilic center can be an atom with an empty orbital (Lewis and Bronstead acids) or a very polarized bond (R-X, C=0, C=NR, etc).  The Lowest Unoccupied Molecular Orbital (LUMO) in a molecule is the electron sink.

Lewis Acids

Lewis acids are electron-pair acceptors (electron sinks).  Carbocations, borane (BH₃), and AlCl₃ are all good electron sinks or Lewis acids or Electrophiles by virtue of their unfilled p orbitals.

In the following scheme, the lone pairs on the oxygen of THF (tetrahydrofuran) is an electron source while the empty p orbital of the Boron atom in borane (BH₃) is an electron sink.

Bronstead Acids

Protic acids are electron sinks.  The proton on the HCl is the electron sink (electrophilic center), while the acetone oxygen lone pairs are the source.

Polarized Bonds - Inductive Effect

There's a large array structures behaving as electron sinks that have C bound to electronegative atoms.  These can be roughly divided into alkyl halides and those compounds containing carbonyl groups or carbonyl like groups.

Alkyl halides 

By virtue of there highly polarized C-X bond, alkyl halides are electron sinks.  The higher electronegativity of Cl and Br than C leaves the C atom deficient in electrons.  While iodine is not much more electronegative than C, its polarizability suffice to make the C very electrophilic.  The characteristic reaction of alkyl halides is substitution.  

Carbonyl (C=O) and carbonyl like (C=NR, C≡N) containing compounds

Aldehydes, ketones, carboxylic acid derivatives and nitriles are all good electron sinks.  We can use resonance to show the electrophilic nature of the carbonyl carbon.  The true structure of formaldehyde is a hybrid of the two structures below.  The structure on the right indicates there is some positive character in the carbonyl carbon.  This carbonyl carbon is deficient in electrons making it a good electron sink (electrophile) and is readily attacked by the electron sources (nucleophiles).

The carbonyl carbon is attacked by the electron sources (nucleophile).