Organic chemists use curly arrows to represent reaction mechanisms

Organic chemists use curly arrows to represent reaction mechanisms 

You have seen several examples of curly arrows so far and you may already have a general idea of what they mean. The representation of organic reaction mechanisms by this means is so important that we must now make quite sure that you do indeed understand exactly what is meant by a curly arrow, how to use it, and how to interpret mechanistic diagrams as well as structural diagrams. 

A curly arrow represents the actual movement of a pair of electrons from a filled orbital into an empty orbital. You can think of the curly arrow as representing a pair of electrons thrown, like a climber s grappling hook, across from where he is standing to where he wants to go. In the simplest cases, the result of this movement is to form a bond between a nucleophile and an electrophile. Here are two examples we have already seen in which lone pair electrons are transferred to empty atomic orbitals. 

Note the exact position of the curly arrow as the value of this representation lies in the precision and uniformity of its use. The arrow always starts with its tail on the source of the moving electrons, representing the filled orbital involved in the reaction. The head of the arrow indicates the final destination of the pair of electrons—the new bond between oxygen and hydrogen or oxygen and aluminium in these examples. As we are forming a new bond, the head of the arrow should be drawn to a point on the line between the two atoms. 

When the nucleophile attacks an antibonding orbital, such as the weak Br-Br bond we have just been discussing, wc shall need two arrows, one to make the new bond and one to break the old. 

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