Pushing Curly Arrows

Chapter 5: Pushing Curly Arrows

Pushing Electrons and Curly Arrows

Lesson 1

Key Issues :

Recognise that each of the covalent bonds we draw as lines in structries correspond to a pair of electrons e.g. H-H means H : H, C=C means C::C
Drawing bond breaking and bond making processes.
Learning to see where the electrons come from and where they go.
Learn to check by balancing the charges

In the early stages of learning to use curly arrows it may be helpful to use Lewis structures to help you see those all important electrons. Note that the mechanisms throughout this website are drawn showing lone pairs, especially those on oxygen and nitrogen because they are important.

STUDY TIP : It's a good idea to get into the habit of showing the lone pairs.

Bond Breaking :
If we remove the electrons from a bond, then that bond is broken, here are two examples of reactions where a σ-bond is broken, note that in each case the curly arrow starts at the bond and ends on an atom:

Do the charges balance ? Yes, in both cases we have a neutral molecule going to +ve and -ve ions whose charges cancel.

Now an example of resonance where a π-bond is broken, again the curly arrow starts at the bond and ends on an atom:

breaking a pi bond between C and O

Do the charges balance ? Yes, we have a neutral molecule going to give a +ve and -ve which cancel.

Bond Making Processes:
If we move two electrons between a pair of atoms, then we make a bond between those atoms

Do the charges balance ? Yes, in each case +ve and -ve combine to give a neutral molecule.

In the 2nd and 3rd cases, because a bond already existed, we increased the bond order and created multiple bonds.