2.4 Drawing Good Arrows

2.4 Drawing Good Arrows

We need to practice drawing arrows now that we know how to identify good and bad arrows.

The head of an arrow must go to form a bond or a lone pair, and the tail of an arrow must come from a bond or a lone pair.
It makes sense to look for bonds or lone pairs when going from one resonance structure to another if we are given two resonance structures and asked to show the arrow(s) that get us from one resonance structure to the other.

Look for double bonds or lone pairs that are disappearing.

It will tell us where to put the arrow's tail.

There are no lone pairs in this example, but there is a double bond that is disappearing.

We know we need to put the tail of our arrow on the double bond.

We need to know where to put the arrow's head.

We look for lone pairs and double bonds.
There is a new pair on the oxygen atom.

When we move a double bond up onto an atom to form a lone pair, we create two charges: a positive charge on the carbon atom that lost its bond and a negative charge on the oxygen atom that got a lone pair.

This is an important issue.
In the previous chapter, formal charges were introduced, and now they are instrumental in drawing resonance structures.
In the next section of this chapter, we will look at the formal charges and push arrows.

It's easy to see how to push one arrow that will get us from one resonance structure to another.

Let's do an example.

Let's look at the differences between the drawings.

We look for any double bonds or lone pairs that are disappearing.
Oxygen is losing a single pair, and the C on the bottom is also disappearing.
This will tell us that we need two arrows.

We need two tails to lose a single pair and a double pair.

Let's look for double bonds or lone pairs.

A C with a negative charge is appearing and a C with a lone pair is not.
This shows us that we need two heads and two arrows.

We know we need two arrows.

Let's start at the top.
We lose a pair from the oxygen atom.