Resonance Between Three Atoms

The next common patterns you will see in organic chemistry

Atom with an incomplete octet next to a π bond.

For example in the following case we have a pi bond adjacent to a C atom with a + charge. The + charge is a clue that it does not have a full octet. It only has 6 electrons surrounding it.
Atom with a lone pair of electrons next to a π bond.

Here we have a pi bopnd adjacent to a C atom with a negative "-" charge. The "-" charge is a clue that it has a lone pair and a full octet.

Atom with Incomplete Octet next to π bond

It's important to be able to draw other important resoance structures, when needed. Here is an example using generic atoms.

A good example of this is the following propene cation (allylic cation).

Which carbon has the greatest positive charge? ;-)

The resonance hybrid would look like the following in which the charge is shared between each terminal carbon. Each C-C bond is also the exact same length and there length is somewhere between that of normal C-C single and a C=C double bond.

The propene cation's electrostatic potential map is shown below. Each carbon is identical and each C-C bond is identical as our simple resonance would predicts. Blue indicates more positive charge. Suppose you were a nucleophile (Nu^-), which carbon would you attack?

Atom with lone pair next to π bond

When the atom next to a π bond has a lone pair of electrons, obtaining the other resonance structure is slightly different. In this case there are two arrows and one of the arrows starts on a lone pair.

The allylic anion shown below is a good example.

Practice Problem

Which carbon has the largest negative charge?

Answer: They both have the exact same charge! Since we don't know what the charge is we say each carbon has δ^-.

Practice Problem

Is there a difference between each C-C bond length?

Each C-C bond is the exact same length.

Practice Problem

Can you draw the resonance hybrid?