Pushing Curly Arrows

Useful Concepts

Pushing Electrons and Curly Arrows

Lesson 1

Key Issues :

Recognise that the covalent bonds we draw as lines represent 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 and think about where the electrons come from and where they go.
Learn to check your work by balancing the charges (simple math)

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 (especially on C, N and O atoms)
Why ?
Because they will often give an insight into the reactions of these important atoms.

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 starting material molecule going to +ve and -ve products ions whose charges cancel (i.e. sum of charges = 0, (+1) + (-1) = 0).

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 starting material molecule going to give a product which has +ve and -ve centers so the charges 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, +ve and -ve combine to give a neutral molecule.

Now examples of resonance where a π-bond is being made, the curly arrow starts at lone pairs and ends between atoms making a new bond:

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

In these cases, because a bond already existed, we increased the bond order and created multiple bonds.