Question

Consider the following nucleophiles: \(\mathrm{H}_{2} \mathrm{O}, \quad \mathrm{CH}_{3} \mathrm{COO}^{-}, \quad \overline{\mathrm{O}} \mathrm{H}, \quad \mathrm{CH}_{3} \mathrm{O}^{-}\) \(\begin{array}{llll}\text { I II II I } & \text { II } & \text { III } & \text { IV }\end{array}\) The correct order of decreasing nucleophilicity is, (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) IV \(>\mathrm{III}>\mathrm{II}>\mathrm{I}\) (c) IV \(>\mathrm{I}>\mathrm{II}>\mathrm{III}\) (d) IV \(>\mathrm{II}>\mathrm{III}>\mathrm{I}\)

Short answer

The order is IV > III > II > I, so option (b) is correct.

Step-by-step solution

Identifying Nucleophilicity

Nucleophiles are species that donate an electron pair in a chemical reaction. In our list, we have: 1. \( \text{H}_2\text{O} \) (water) - a neutral molecule.2. \( \text{CH}_3\text{COO}^{-} \) (acetate ion) - a weakly basic anion.3. \( \overline{\text{O}}\text{H} \) (hydroxide ion) - a strong base and a strong nucleophile.4. \( \text{CH}_3\text{O}^{-} \) (methoxide ion) - also a strong base and nucleophile.

Comparing Negative Charge Effect

Negatively charged species are generally more nucleophilic than neutral ones because they have an extra electron to donate. Thus, \( \text{CH}_3\text{O}^{-} \), \( \overline{\text{O}}\text{H} \), and \( \text{CH}_3\text{COO}^{-} \) will be more nucleophilic than \( \text{H}_2\text{O} \). The order will begin with negatively charged species before the neutral water.

Considering Basicity and Resonance

Basicity often correlates with nucleophilicity. Both \( \text{CH}_3\text{O}^{-} \) and \( \overline{\text{O}}\text{H} \) are strong bases, but \( \text{CH}_3\text{O}^{-} \) is more basic because methyl groups donate electrons more readily than hydroxyl groups. Thus, \( \text{CH}_3\text{O}^{-} \) is more nucleophilic than \( \overline{\text{O}}\text{H} \).\( \text{CH}_3\text{COO}^{-} \) is less nucleophilic due to resonance stabilization, which delocalizes the negative charge.

Arranging the Order

Putting the above information together, we get the decreasing order of nucleophilicity as follows:\( \text{CH}_3\text{O}^{-} > \overline{\text{O}}\text{H} > \text{CH}_3\text{COO}^{-} > \text{H}_2\text{O} \).Therefore, the correct option is (b): IV > III > II > I.

Key concepts

Basicity

Basicity refers to the ability of a compound to accept protons, and it's closely related to nucleophilicity. The stronger the base, the more willing it is to share its lone pair of electrons. This increased willingness to share electrons makes it a stronger nucleophile. This concept is key when comparing the methoxide ion \( \text{CH}_3\text{O}^{-} \) and the hydroxide ion \( \overline{\text{O}}\text{H} \).

Both ions are strong bases, but methoxide is even stronger. This strength is partly due to the electron-donating effect of the methyl group \( \text{CH}_3 \), which enhances the electron density on the oxygen atom. Consider it a friend who generously splits their candy—methoxide's additional electrons make it more eager to pair with another atom. This is why in our list of nucleophiles, \( \text{CH}_3\text{O}^{-} \) ranks higher in nucleophilicity than \( \overline{\text{O}}\text{H} \).

• Electron-donating groups like methyl increase basicity and nucleophilicity.
• Basicity is not always parallel to nucleophilicity due to factors like solvent effects.

Resonance Stabilization

Resonance stabilization is an important concept in understanding why some molecules are less nucleophilic. It involves the delocalization of electrons across adjacent atoms in a molecule, which serves to stabilize the molecule by spreading out the negative charge. Think of it as a group of friends sharing the load of holding up a tent—less burden on each individual.

In the case of acetate ion \( \text{CH}_3\text{COO}^{-} \), resonance stabilization plays a significant role in reducing nucleophilicity. The negative charge on the oxygen atom can resonate, or move, throughout the acetate ion structure. This makes the ion's electrons less available to donate in reactions compared to anions that don't have this delocalization.

• Resonance allows electrons to move through a system, reducing nucleophilicity.
• Acetate, despite its negative charge, is less nucleophilic due to resonance.
• Resonance stabilization outweighs the charge factor in determining nucleophilic strength.

Negative Charge Effect

The negative charge effect is a straightforward principle: negatively charged species tend to be more nucleophilic than their neutral counterparts due to possessing excess electrons. These readily available electrons make negatively charged ions eager to interact with positively charged or electron-deficient species.

When we look at the list of nucleophiles \( \text{CH}_3\text{O}^{-}, \overline{\text{O}}\text{H}, \text{CH}_3\text{COO}^{-}, \text{and} \ \text{H}_2\text{O} \), we see a clear pattern. The two negatively charged ions, methoxide and hydroxide, are more nucleophilic than neutral water because they have more electron density to offer. Acetate ion, although less nucleophilic than the others due to resonance, still outranks neutral water.

• Negatively charged ions are generally more nucleophilic.
• Extra electrons in negatively charged species make them highly reactive.
• Water, being neutral, ranks lowest in nucleophilicity among the options.