I won't try to draw one here but take for example, CH3CH2COO-. This has two resonance structures but they look the same to me. The CH2-C single bond can rotate and it looks like you can just flip the COO- section and it looks like the same structure. Maybe that's the point. Any enlightenment?
Sheryl
The two resonating structures of the acid (which you haven't addressed) are non-equivalent and they add some stabilization to the molecule. However, in the anion, the two structures are equivalent, as you point out, because of the rotation of the bond. But equivalent resonating structures stabilize MORE than non-equivalent resonating structures and the anion has more stability (due to resonance) than does the acid.
And as you point out, "That is the point."
You're correct in observing that the two resonating structures of CH3CH2COO- appear to be the same. However, it's important to understand that resonance structures are not meant to represent different physical forms of the molecule, but rather different electron distributions within the molecule.
In the case of CH3CH2COO-, the two resonating structures depict the movement of electrons. The negative charge is delocalized, meaning that it is not localized to a single atom, but spread out over multiple atoms. This allows the negative charge to be distributed more evenly, increasing the stability of the anion.
To better understand the resonating structures, let's break down the molecule. CH3CH2COO- can be thought of as a combination of two resonance contributors: CH3CH2COO- and CH3CH2COO-. The former has a double bond between the carbon and oxygen atoms, while the latter has a single bond between them.
The resonance occurs because the double bond and the single bond can rotate freely around each other. This rotation allows the electrons to shift between the carbon and oxygen atoms, resulting in the distribution of negative charge throughout the molecule.
Although it may seem like the two resonating structures are the same, they are not. The movement of electrons from the oxygen atom to the adjacent carbon atom in one structure is different from the movement in the other structure. The exchanging of electron positions contributes to the overall stability of the anion.
In summary, the two resonating structures of CH3CH2COO- depict the movement of electrons, leading to the delocalization of the negative charge within the molecule. While they may appear similar, their distinct electron distributions contribute to the increased stability of the anion compared to the acid form.