Question

Why is a carboxylate anion more resistant to attack by nucleophilic agents, such as \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{O}^{\ominus}\), than is the corresponding ester?

Short answer

Carboxylate anions are more stable due to resonance stabilization, reducing electrophilicity and making them less susceptible to nucleophilic attack compared to esters.

Step-by-step solution

Understanding Nucleophilic Attack

Nucleophiles are electron-rich species that seek positively charged or electron-deficient sites. Nucleophilic attack typically occurs at carbon atoms that are bonded to electronegative atoms, making them more electrophilic.

Analyzing Carboxylate Anion

A carboxylate anion ( ext{RCOO}^-) has resonance stabilization. The negative charge is delocalized between the two oxygen atoms, making the overall structure more stable and less electrophilic. This stabilization makes them less reactive to nucleophilic attacks compared to less stable species.

Examining Ester Reactivity

Esters have a carbonyl group ( ext{RCOOR'}) where the carbonyl carbon is an electrophilic center. The carbonyl carbon doesn't have resonance stabilization similar to carboxylate ions, so it's more susceptible to attack by nucleophiles.

Comparative Stability and Reactivity

The carboxylate anion's resonance stability means the electron density is spread out, making the carbonyl carbon less reactive towards nucleophiles. In contrast, esters have a more localized positive charge on the carbonyl carbon, making it more likely to undergo nucleophilic attack.

Key concepts

Nucleophilic Attack

In a chemical reaction, nucleophiles are important players that have an affinity for positively charged centers. They are species rich in electrons, which naturally seek out and form bonds with electron-deficient atoms, often behaving like a magnet to positive charges. When assessing nucleophilic attack, it typically targets carbon atoms, especially those bonded to electronegative elements such as oxygen or nitrogen. These atoms, due to their electronegativity, draw electrons towards themselves, leaving the carbon atom with a partial positive charge. This charge deficiency at the carbon atom makes it an ideal site for nucleophilic attack, as it readily accepts electrons from nucleophiles.

Resonance Stabilization

The concept of resonance stabilization offers an insight into the stability of molecules like carboxylate anions. In a carboxylate ion, the negative charge resulting from deprotonation is delocalized over two oxygen atoms. This distribution is depicted through resonance structures, which illustrate the electron delocalization within the molecule. Resonance structures aid in stabilizing molecules by allowing the charge to spread out rather than concentrating on a single atom, thus reducing its reactivity. Because of this stabilization, carboxylate anions are a lot less vulnerable to nucleophilic attacks compared to molecules without such resonance. They enjoy a stable electron cloud, diminishing their need to acquire additional electrons, which is key to their lower reactivity.

Ester Reactivity

Esters are chemical compounds featuring a carbonyl group. The carbon in this group acts as an electrophilic center; however, unlike carboxylate anions, esters lack resonance stabilization across the oxygen atoms in the carboxylate ion. This absence of resonance in esters means that the carbonyl carbon retains a concentrated positive character, making it susceptible to nucleophilic attacks. Due to this electronegative draw in esters, the carbonyl carbon is subtly deprived of electrons, and is thus more reactive compared to the stabilized carbonyl in carboxylates. Furthermore, in an ester, the adjacent oxygen atom draws electron density away from the carbonyl carbon, enhancing its positive charge and electrophilic nature.

Electrophilic Centers

In chemical reactions, electrophilic centers are atoms that accept electron pairs from nucleophiles. These are typically atoms within molecules that possess either incomplete octets or are bonded to more electronegative elements. Within carboxylate anions and esters, the difference in these centers' reactivity can be attributed to resonance and localization of charges. Carboxylate anions, through resonance, distribute their negative charge, reducing the electrophilic nature of their carbonyl carbon. On the other hand, esters focus their positive charge on their carbonyl carbon due to the absence of resonance structures, increasing its electrophilicity. This means electrophilic centers significantly dictate how molecules like esters and carboxylates interact with nucleophiles—with esters being more reactive thanks to their distinct electrophilic sites.