Show how resonance can occur in the following organic ions: (a) acetate ion, \(\mathrm{CH}_{3} \mathrm{CO}_{2}{ }^{-}\); (b) enolate ion, \(\mathrm{CH}_{2} \mathrm{COCH}_{3}\) which has one resonance structure with \(\mathrm{a} \mathrm{C}=\mathrm{C}\) double bond and an \(-\mathrm{O}^{-}\)group on the central carbon atom; (c) allyl cation, \(\mathrm{CH}_{2} \mathrm{CHCH}_{2}^{*}\); (d) amidate ion, \(\mathrm{CH}_{3} \mathrm{CONH}^{-}\)(the \(\mathrm{O}\) and \(\mathrm{N}\) atoms are both bondod to the second \(C\) atom).

The acetate ion (\( \text{CH}_3\text{CO}_2^- \) ) exemplifies the concept of resonance in organic chemistry, showcasing stability through delocalization of electrons. During resonance, the electrons originally in a lone pair on the negatively charged oxygen atom can move to form a double bond with the central carbon atom. Concomitantly, the \text{π} electrons of the existing carbon-oxygen double bond shift to the other oxygen atom. This electron redistribution effectively oscillates, creating two resonant structures where the negative charge is shared between the two oxygen atoms.

Resonance stabilization is vital because it decreases the potential energy of the molecule, leading to increased stability. For students, visualizing resonance in the acetate ion involves drawing resonance structures that illustrate electron delocalization. This can be a tricky concept; therefore, practicing by drawing the contributing structures and using arrows to indicate electron movement can enhance understanding.

Enolate ions (\( \text{CH}_2\text{COCH}_3 \) ) are significant in organic chemistry, especially in various organic reactions such as aldol condensation. An enolate ion has one resonance structure where \text{π} electrons from a C=C double bond move towards the oxygen atom on the central carbon, resulting in that oxygen gaining a negative charge.

The electron movement yields an alternate resonance form where the negative charge once on the carbon-carbon double bond is now delocalized onto the oxygen. Understanding enolate resonance involves recognizing that these ions have dual character; they can act as both nucleophiles due to the negative charge and as bases due to the lone pair of electrons. Grasping the implications of this dual nature is crucial for predicting the reactivity and outcome of reactions involving enolates.

The allyl cation (\( \text{CH}_2\text{CHCH}_2^+ \) ) demonstrates resonance in species with a positive charge. In this case, the resonance allows the allyl cation to delocalize the positive charge over multiple carbon atoms. Specifically, when \text{π} electrons from the double bond shift to the carbon atom at the end, the positive charge then moves from the original carbon to the adjacent carbon.

The understanding of how positive charge can be spread over a molecule via resonance is important because it too contributes to the stability of the ion. The allyl cation is involved in many reactions, including polymerizations and substitutions, and recognizing the different locations of the positive charge can aid in predicting the molecule's reactivity.

Amidate ions (\( \text{CH}_3\text{CONH}^- \) ) exhibit resonance where the lone pair of electrons on the nitrogen atom can jump in to form a double bond with the adjacent carbon atom. This causes the \text{π} electrons from the carbon-oxygen double bond to shift onto the oxygen, thus dispersing the negative charge between the oxygen and the nitrogen atoms.

In examining these resonance forms, it's important to highlight that the delocalization of charge across adjacent atoms contributes to enhanced stability, similar to acetate and enolate ions. The resonance in amidate ions is a central theme in understanding the reactivity and stability of amide bonds, especially in biochemical systems such as proteins.