A curly arrow represents the movement of a pair of electrons from a fi lled 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.

A curly arrow always starts with its tail resting on the symbol representing a pair of electrons in a fi lled orbital—in this case the lone pair or the negative charge (which actually represents a lone pair). The head of the arrow indicates the final destination of the pair of electrons—the new bond between oxygen and hydrogen or oxygen and boron in these examples. As we are forming a new bond, the head of the arrow should be drawn to a point somewhere on the line between the two atoms. Why does a curly arrow represent two electrons? Well, as you saw in Chapter 4, it takes two electrons to make a bond, and in these two cases those electrons come from a lone pair. We use a different sort of arrow for movements of one electron, as you will see in Chapters 24 and 37. When the nucleophile attacks an antibonding orbital, such as the weak Br–Br bond we have just been discussing, we need two arrows, one to make the new bond and one to break the old.

The bond-making arrow is the same as before—it starts on the nucleophile’s lone pair and ends near the electrophile—but the bond-breaking arrow is new. This arrow shows that the two electrons in the bond move to one end (a bromine atom) and turn it into an anion. As always, the arrow starts on something representing a pair of electrons in a fi lled orbital—the Br–Br σ bond. It should start in the centre of the bond and its head should rest on the atom (Br in this case) the electrons are heading for. Another example is the attack of a base on the strong acid HBr.

It is not important how much curvature you put into the arrows—as long as they curl enough to distinguish them from straight reaction arrows, they can be as curly as you like. Neither does it matter whether they go to the left or the right, or whether they curve up or down as long as they begin and end in the right places. The mechanism below is just as correct:

● Curly arrows always start on something representing a pair of electrons: • a negative charge

• a lone pair

• or a bond

and end at the point those electrons are moving to.