Question

What are the possible resonance structures of `CH_3-CH=CH-CH=O` and `CH_3-COO^-`? What is the order of stability of the contributing resonance structures?

Answer 1

See below.

Explanation:

Just remember that the electrons will move to the more electronegative atom or the more positive atom.

(a) But-2en-al

There is no positive atom, but `"O"` is the most electronegative atom, so let's move electrons to the `"O"` atom.

`underbrace("CH"_3"CH=CHCH=O")_color(red)(bb "A") ⟷ underbrace("CH"_3"CH=CH"stackrelcolor (blue)(+)("C")"H-"stackrelcolor (blue)(-)("O"))_color(red)(bb "B")`

Now the carbon atom has a `+` charge, so let's move electrons to it.

`underbrace("CH"_3"CH=CH"stackrelcolor (blue)(+)("C")"H-"stackrelcolor (blue)(-)("O"))_color(red)(bb "B") → underbrace("CH"_3stackrelcolor (blue)(+)("C")"H-CH=CH-"stackrelcolor (blue)(-)"O")_color(red)(bb "C")`

Structure `bb "A"` has the lowest energy and is the major contributor.

Structures `bb "B"` and `bb "C"` are higher in energy and are minor contributors.

(b) Acetate ion

Here again, there is no positive atom, but the other `"O"` atom is another electronegative atom.

Let's move the electrons toward it.

You can also move the electrons in the carbonyl group to the carbonyl `"O"` atom to get the third structure shown below.

(from www.chem.ucla.edu)

Of the three structures above, the first two are equivalent and of equal energy.

The third structure is a minor contributor because it has three charges instead of one,

Answer 2

Although Ernest provided an attentive, helpful answer, I have provided the bond-line structures for the resonance structures, below.

For the first molecule, there aren't many resonance structures that adhere to the octet rule besides the one we start with,

`" "uparrowdownarrow`

`" "uparrowdownarrow`

`" "uparrowdownarrow`

`" "uparrowdownarrow`

The electrons move to the right with each resonance structure. The first is major, and the rest our minor due to its octet rule satisfaction.

For acetate, I think Ernest did a fine job that doesn't need any more detail.