Polar reaction mechanisms are the backbone of organic chemistry, showing how electrons move during chemical transformations. Curved arrow notation visually represents this electron flow, helping us understand how bonds break and form in reactions.

Nucleophiles and electrophiles are key players in polar reactions. By following the curved arrows, we can predict reaction outcomes and understand intermediate steps. This knowledge is crucial for grasping more complex organic reactions and synthesis strategies.

Polar Reaction Mechanisms and Curved Arrow Notation

Electron movement in polar reactions

• Curved arrows visually represent the flow of electrons from an electron-rich species (nucleophile) to an electron-poor species (electrophile) in a chemical reaction
• Single-headed curved arrows show the movement of a single electron in radical reactions or single electron transfers
• Double-headed curved arrows illustrate the movement of an electron pair in most polar reactions (nucleophilic attacks, proton transfers)
• The tail of the curved arrow begins at the electron source (lone pair, negative charge, or the middle of a bond line)
• The head of the curved arrow points towards the electron sink (atom, positive charge, or the end of a bond line)
• Curved arrows can also be used to show electron movement in resonance structures

Rules for nucleophiles and electrophiles

• Nucleophiles are electron-rich species that donate electrons to form a new bond
  • Possess a lone pair, negative charge, or a pi bond
  • Function as Lewis bases ($\ce{OH-}$, $\ce{NH3}$, $\ce{CH3O-}$, alkenes)
• Electrophiles are electron-poor species that accept electrons to form a new bond
  • Possess a positive charge, partial positive charge, or an empty orbital
  • Function as Lewis acids ($\ce{H+}$, $\ce{BF3}$, $\ce{AlCl3}$, carbocations)
• In a polar reaction, the nucleophile attacks the electrophile, forming a new bond with the curved arrow starting from the nucleophile and pointing towards the electrophile
• The leaving group (if present) departs with an electron pair, breaking a bond, with the curved arrow starting from the bond and pointing towards the leaving group
• Bond polarity influences the direction of electron flow in reactions

Interpreting curved arrow notation

• Follow the flow of electrons indicated by the curved arrows to determine the reaction products
• Nucleophilic attack
  1. The nucleophile donates an electron pair to the electrophile
  2. A new bond forms
  3. The electrophile gains an electron pair, becoming more negative or less positive
• Proton transfer
  1. A base (proton acceptor) removes a proton ($\ce{H+}$) from an acid (proton donor)
  2. The curved arrow starts from the base and points towards the proton
• Leaving group departure
  1. A leaving group departs with an electron pair
  2. A bond breaks
  3. The curved arrow starts from the bond and points towards the leaving group
• Rearrangements involve intramolecular electron flow to form a more stable structure, with curved arrows showing the electron movement
• Combine the individual steps indicated by the curved arrows to predict the overall reaction product

Reaction Progress and Intermediates

• Curved arrows can represent the formation and transformation of reaction intermediates
• Transition states are high-energy arrangements of atoms between reactants and products or intermediates
• Electron density changes throughout the reaction, as indicated by the curved arrows