Question

Consider the following nucleophiles: \(\mathrm{H}_{2} \mathrm{O}, \quad \mathrm{CH}_{3} \mathrm{COO}^{-}, \quad \overline{\mathrm{O}} \mathrm{H}, \quad \mathrm{CH}_{3} \mathrm{O}^{-}\) 2 3 The correct order of decreasing nucleophilicity is, (a) \(1>2>3>4\) (b) \(4>3>2>1\) (c) \(4>1>2>3\) (d) \(4>2>3>1\)

Short answer

(b) \(4>3>2>1\) is the correct order of decreasing nucleophilicity.

Step-by-step solution

Understanding Nucleophilicity

Nucleophilicity refers to the ability of a chemical species to donate an electron pair to an electrophile, usually in a chemical reaction. It's primarily governed by factors such as charge, electronegativity, steric effects, and the solvent.

Analyzing the Nucleophiles

The nucleophiles given are: \(\mathrm{H}_{2}\mathrm{O}\), \(\mathrm{CH}_{3}\mathrm{COO}^{-}\), \(\mathrm{OH}^{-}\), and \(\mathrm{CH}_{3}\mathrm{O}^{-}\). Two are negatively charged anions (\(\mathrm{CH}_{3}\mathrm{COO}^{-}\) and \(\mathrm{CH}_{3}\mathrm{O}^{-}\)), suggesting they are stronger nucleophiles than the neutral \(\mathrm{H}_{2}\mathrm{O}\). \(\mathrm{OH}^{-}\) is also negatively charged.

Evaluating Negative Charge

Negatively charged species usually have higher nucleophilicity. Therefore, \(\mathrm{OH}^{-}\) and \(\mathrm{CH}_{3}\mathrm{O}^{-}\) will generally be stronger nucleophiles than \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{CH}_{3}\mathrm{COO}^{-}\). \(\mathrm{CH}_{3}\mathrm{O}^{-}\) is expected to be more nucleophilic than \(\mathrm{OH}^{-}\) due to the electron-releasing effects of the \(\mathrm{CH}_{3}\) group, which increases electron density on the oxygen atom.

Considering the Neutral Nucleophile

\(\mathrm{H}_{2}\mathrm{O}\), being a neutral molecule, is the weakest nucleophile among the choices. The lack of a negative charge reduces its ability to donate electrons.

Assessing the Carboxylate Anion

\(\mathrm{CH}_{3}\mathrm{COO}^{-}\), a carboxylate ion, is a resonance-stabilized anion, which tends to lower its nucleophilicity when compared to negatively charged, non-resonance-stabilized species like \(\mathrm{CH}_{3}\mathrm{O}^{-}\) and \(\mathrm{OH}^{-}\). However, it is more nucleophilic than \(\mathrm{H}_{2}\mathrm{O}\).

Ordering Nucleophilicity

Based on the analysis, the order of decreasing nucleophilicity is: \(\mathrm{CH}_{3}\mathrm{O}^{-} > \mathrm{OH}^{-} > \mathrm{CH}_{3}\mathrm{COO}^{-} > \mathrm{H}_{2}\mathrm{O}\). Therefore, the correct order is (b) \(4>3>2>1\).

Key concepts

Chemical Species

Chemical species refer to atoms, molecules, ions, or radicals that participate in a chemical process. Each chemical species is characterized by its own unique set of properties and behaves differently in chemical reactions. For example, the compounds water ewline ( ext{H}_2 ext{O} ), acetate ion ewline ( ext{CH}_3 ext{COO}^- ), hydroxide ion ewline ( ext{OH}^- ), and methoxide ion ewline ( ext{CH}_3 ext{O}^- ) all serve as potential nucleophiles in chemical reactions.
The ability of these chemical species to act as nucleophiles depends heavily on their structure and charge.

• Negatively charged ions tend to be more nucleophilic due to their electron-rich nature.
• Neutral species, although capable, are typically less nucleophilic because they do not have an extra electron density to offer readily.

Understanding the nature of chemical species gives insight into their reactivity and helps predict how they might interact in different chemical environments.

Electron Pair Donation

Electron pair donation is a fundamental concept in chemistry, describing the mechanism by which nucleophiles react with electrophiles.
A nucleophile, in essence, seeks to donate an electron pair to another chemical species, the electrophile, which is typically electron-deficient.

• For example, the hydroxide ion ewline ( ext{OH}^- ) is an excellent nucleophile because its negative charge renders it electron-rich, ready to donate an electron pair.
• Similarly, the methoxide ion also donates electron pairs efficiently due to its negative charge.

Understanding this concept is crucial as it explains much of the behavior of chemical species in reactions. The process of electron pair donation is essential for the formation of new bonds, which is the crux of many organic reaction mechanisms.

Negative Charge

The presence of a negative charge on a chemical species greatly influences its nucleophilicity.
Negatively charged species possess more electrons than protons, giving them excess electron density.
This makes them more potent donors of electron pairs:

• Hydroxide ion ewline ( ext{OH}^- ) and methoxide ion ewline ( ext{CH}_3 ext{O}^- ), for example, carry negative charges, which enhances their nucleophilicity compared to neutral molecules like water.
• The comparison between negatively charged species and neutral ones often highlights the increased reactivity and capability of electron donation by negatively charged nucleophiles.

In turn, this ability to donate electron pairs explains why negatively charged species often lead to faster and more efficient chemical reactions.

Resonance Stabilization

Resonance stabilization is a concept where a chemical species achieves stability due to the delocalization of its electrons.
This is common in conjugated systems and ions like the acetate ion ewline ( ext{CH}_3 ext{COO}^- ).

• In the acetate ion, the negative charge is delocalized over the two oxygen atoms through resonance.
• While this delocalization gives extra stability to the molecule, it reduces the molecule's nucleophilicity because the electron pair is less localized and thus less available for donation.

In contrast, ions like methoxide ewline ( ext{CH}_3 ext{O}^- ) and hydroxide are not resonance-stabilized. Their electrons are highly concentrated, making them more reactive nucleophiles. Comprehending resonance stabilization helps predict not only the stability of a chemical species but also its expected behavior as a nucleophile in reactions.