Curved Arrows and Reactions

We introduced curved arrow notation in our discussion of resonance to illustrate how to convert between different resonance structures.  At that time we stressed that the electrons don't really move as the arrows do, the structures are not in equilibrium, and it's just a paradigm for obtaining the different resonance (Lewis) structures.

When we use curved arrows to describe reaction mechanisms, you can imagine that this is how the electrons flow during the reaction.  The flow of electrons in a chemical reaction is analogous to the flow of electrons in wires and circuits.  Electrons flow in wires and circuits because of a charge potential or charge difference.  Electrons flow from electron sources to an electron sink because of a charge or potential difference.  A source is like a battery, a source of electrons, while a sink is a device that accepts electrons (maybe a fan or light bulb).

Bond Dissociation

Our first example of the use of curved arrows is to illustrate heterolytic bond cleavage.  Hetero is Latin for "different", while lytic or lysis is Latin for breaking apart.  This heterolytic bond cleavage means to break apart into different pieces.  The different pieces refer to the electrons (i.e. two electrons go with one of the atoms).

Heterolytic bond cleavage

Notice how the two electrons on the bond are now on the Br giving it a negative charge, while the hydrogen atom becomes a proton.  The electrons flowed to a more electronegative or polarizable element, Br.  We illustrate this with the double-headed curved arrow.  The double head on the arrow indicates two electrons move.  The source is the bond, this is where the arrow starts.  The sink is the rather electronegative bromine atom at the end of the arrowhead.

Homolytic bond cleavage

With homolytic cleavage the bond breaks into two similar radical pieces.  Homolytic cleavage produces two radicals while heterolytic cleavage produces anion/cation pairs.

Bond Formation

The reverse of the above two reactions results in the formation (genesis) of a bond.  These are referred to as heterogenic and homogenic bond formation.

Heterogenic bond formation

Homogenic bond formation