Nucleophilic Attack

The second fundamental reaction mechanism pattern is Nucleophilic Attack, which describes the flow of electrons from an electron-rich species (a nucleophile) to an electron-poor species (an electrophile). The number of curved arrows used to illustrate a nucleophilic attack depends on the nature of the electrophile.

In this example, the chloride ion acts as a nucleophile, using its lone pair to attack the electron-deficient carbon of the tert-butyl cation.

Key Principles of Nucleophilic Attack:

• Arrow Origin and Destination: A curved arrow always originates from the nucleophile (a negative charge or electron-rich center, such as a lone pair or pi bond) and points to the electrophile (a positive charge or electron-poor center).
  
  
• Bond Formation: The electrons depicted by the arrow ultimately form a new covalent bond between the nucleophile and the electrophile. As seen in Figure 1, the electrons from the chloride ion become the bond between the chlorine and the carbon atom of the electrophile.

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.

Consequences of Nucleophilic Attack on the Nucleophile:

• The nucleophilic center's charge changes based on its initial state. If the nucleophilic center is negatively charged, it becomes neutral after forming the new bond. If the nucleophilic center is neutral (e.g., an alcohol oxygen with a lone pair), it typically becomes positively charged after forming the new bond.

You could in come cases show the resonance first and then the nucleophilic attack.

Recall that a pi (π) bond can also function as a nucleophile.

When a pi bond reacts with an electrophile, only one carbon atom of the pi bond forms a new bond with the electrophile, utilizing the shared electron pair from the pi bond to establish this connection.