Question

Which of the following has the highest nucleophi-licity? (a) \(\mathrm{F}\) - (b) \(\mathrm{OH}\) (c) \(\mathrm{CH}_{3}-\) (d) \(\mathrm{NH}_{2}^{-}\)

Short answer

(c) \(\mathrm{CH}_{3}^{-}\) is the strongest nucleophile due to low electronegativity and strong electron pair donation.

Step-by-step solution

Understand Nucleophilicity

Nucleophilicity refers to a chemical species' ability to donate a pair of electrons to an electrophile. It is influenced by charge, electronegativity, steric hindrance, and the solvent in which the reaction is taking place.

Evaluate the Negative Charge

The more negative the charge on a molecule, the greater its nucleophilicity tends to be because it has more electrons to donate. All given options have a negative charge, so this factor alone does not differentiate them.

Consider Electronegativity

A less electronegative atom shares its electrons more readily, enhancing nucleophilicity. Therefore, the nucleophilicity decreases with increasing electronegativity. The order of electronegativity for involved atoms is: \(F > O > N > C\).

Analyze Steric Hindrance

Steric hindrance occurs when the size of groups attached to the nucleophile affects its ability to react. Larger groups provide more hindrance. Among the given options, \(\mathrm{CH}_3^-\) has a bulkier methyl group but still allows for good nucleophilicity.

Compare Each Option

Compare the nucleophiles by combining the factors: \(\mathrm{F}^-\) (high electronegativity), \(\mathrm{OH}^-\) (less electronegative than \(\mathrm{F}^-\)), \(\mathrm{CH}_3^-\) (less electronegative than \(\mathrm{OH}^-\)), and \(\mathrm{NH}_2^-\) (even less electronegative than previous). \(\mathrm{CH}_3^-\) and \(\mathrm{NH}_2^-\) appear as strong nucleophiles.

Conclusion and Answer Selection

Considering all factors, \(\mathrm{CH}_3^-\) is the least electronegative with a strong electron-donating capability due to its low level of electronegativity compared to other atoms involved. This makes it the strongest nucleophile among the given choices.

Key concepts

Electronegativity

Electronegativity measures an atom's ability to attract and hold onto electrons. In the context of nucleophilicity, electronegativity plays a crucial role. Less electronegative atoms are generally better nucleophiles because they don't hold their electrons as tightly, making them more willing to share these electrons with electrophiles.
In our example, the electronegativity order is:

• Fluorine (\( \mathrm{F} \)
• Oxygen (\( \mathrm{O} \)
• Nitrogen (\( \mathrm{N} \)
• Carbon (\( \mathrm{C} \)

From this order, carbon with the lowest electronegativity will be more likely to act as a strong nucleophile compared to nitrogen, oxygen, and fluorine. As a consequence, \( \mathrm{CH}_3^- \) exhibits stronger nucleophilicity among the options provided.

Negative Charge

The negative charge of a molecular species indicates that it has extra electrons available to donate. This increases its nucleophilicity.
All species in this example (\( \mathrm{F}^- \), \( \mathrm{OH}^- \), \( \mathrm{CH}_3^- \), \( \mathrm{NH}_2^- \)) carry a negative charge, which potentially makes them strong nucleophiles. However, charge alone doesn't determine nucleophilicity. Other factors, such as electronegativity and steric hindrance, must also be taken into account.
Nevertheless, in settings where electronegativity is constant, an increase in negatively charged species generally corresponds to increased nucleophilicity.

Steric Hindrance

Steric hindrance refers to the physical blockage that prevents a reaction from taking place. The size and bulkiness of atoms or groups attached to the nucleophile affect how freely it can approach an electrophile.
Even though \( \mathrm{CH}_3^- \) has a bulkier methyl group, it remains an effective nucleophile due to the low electronegativity of carbon, which ensures that the electrons are still easily donated despite the slight steric hindrance.
In contrast, nucleophiles with higher electronegativity and steric hindrance might become less effective, as seen in \( \mathrm{F}^- \), where its high electronegativity dominates the scenario.

Nucleophiles

Nucleophiles are chemical species that donate an electron pair to form a chemical bond in a reaction. The key indicators of a strong nucleophile are low electronegativity, higher negative charge, and minimal steric hindrance.

• The nucleophilicity series explained in this context highlights how the combination of low electronegativity and minimal steric hindrance enhances nucleophile strength.
• Strong nucleophiles make vigorous participants in reactions.
• An understanding of nucleophilicity allows chemists to predict reaction pathways and outcomes.

In this situation, \( \mathrm{CH}_3^- \) emerges as the most potent nucleophile because it balances a larger size with low electronegativity, thereby effectively donating electrons in a reaction.