Curved Arrow Notation

Curved arrows or curved arrows notation is a very powerful tool you will learn in organic chemistry. In general, we use curved arrows to show the movement of electrons. It is used in two important areas of organic chemistry you must master; 1) resonance theory and 2) reaction mechanisms. However, there is a distinct conceptual difference in using curved arrows for each of these areas. With resonance, the electrons aren't moving, we just use the curved arrows to help us find other possible resonance structures! On the other hand, when we use curved arrows for reaction mechanisms you can imagine the electrons moving with your arrows.  As we will begin to see, reactions are like the flow of electrons in wires.  The electrons flow from places where there is an abundance (Nucleophile or source) to places where they are a deficiency of electrons (Electrophile or sink).

Types of Curved Arrows

• Full-headed arrows are used for two-electron movements.       
  
  
  
  
• Half-headed arrows are used for single-electron movements.
  
  
  
  

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.