Nucleophilic Attack

Nucleophilic attack - Electrons flow from the nucleophile to the electrophile.  Nucleophilic attack can have one or more arrows, depending upon the nature of the electrophile.

Take NoteThe tail of the arrow starts on the Nucleophile (- charge or electron-rich center).  The head of the arrow ends on a Electrophile (+ charge or electron-poor center).

The electrons end up being sharing rather than transferred.  Note that the electrons in the above case become the bond between the Cl and the C atom of the electrophile.

Nucleophilic attack may require more than one curved arrow.

Take NoteThe nucleophilic center becomes neutral or positive, depending upon its nature.  If the nucleophilic center is negative, then it becomes neutral, and if the nucleophilic center is neutral it becomes positive.

This transformation could also be thought of as nucleophilic attack on a resonance structure, which needs only one arrow.

Recall that a π bond can also act as nucleophile.

Take Note
Note that only one of the carbon atoms from the pi bond uses the pair of electrons (from the pi bond) to attack the
electrophile.