Curved Arrow Notation

Curved arrows, or curved arrow notation, are powerful tools you will encounter in organic chemistry. Generally, we use curved arrows to illustrate the movement of electrons. Mastering this notation is crucial for two key areas: resonance theory and reaction mechanisms. However, it’s important to understand that the way we use curved arrows differs conceptually between these two contexts.

In resonance theory, the electrons aren’t actually moving; the arrows simply guide us in identifying alternative resonance structures. In contrast, when we apply curved arrows to reaction mechanisms, we can visualize the electrons moving along the path indicated by the arrows. As we will explore, reactions resemble the flow of electrons through wires, where electrons move from areas of high electron density (the nucleophile or source) to areas of low electron density (the electrophile or sink).

Types of Curved Arrows

• Full-headed arrows are used for two-electron movements, such as in polar (ionic) reactions.       
  
  
  
  
• Half-headed arrows are used for single-electron movements, such as with radical reactions.
  
  
  
  

When to Use Curved Arrows

• Resonance
  
  
  • Curved arrows show how you could obtain other resonance structures (i.e. alternative electron delocalization or distributions)
    
    
  • Curved arrows are within a single molecule (intramolecular-like)
    
    
    
• Reaction mechanisms
  
  
• Curved arrows show the flow of electrons, from nucleophile (source of electrons - lone pairs, pi bond, negative charge etc) to sink (positive charge and/or empty orbital)
  
  
• Curved arrows can extend from molecule to molecule (intermolecular), but also intramolecular.

Examples - Curved arrows in reaction mechanisms.

Consider the electrophilic addition of a proton (H+) to ethene.  The pi bonds electrons attack the proton.  Ethene by nature of its pi bond is a nucleophile and attacks the electrophile (proton - H⁺).  It's called electrophilic addition because we are adding an electrophile (H⁺) to the ethene.  Notice how the proton (H+) is now bonded to one carbon.  The other carbon atom is now positively charged.  We will talk about the formal charge shortly.

As another example consider an acid base reaction between.

Example - Curved arrows in Resonance

There are two possible Lewis structures (i.e. resonance structures) for formaldehyde shown below.  The structure on the left has a π bond between the C and O.  We can use a curved arrow to push those electros onto the O atom.  Notice the O atom in the structure on the right now has 3 lone pairs of electrons and a -1 charge.  We can also use an arrow on the structure on the right showing how to convert it back to the structure on the left.  We will learn more about resonance and reaction mechanisms soon.

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