Question

Pick the strongest nucleophile. (a) : \(\mathrm{CH}_{3}\) (b) \(\mathrm{NH}_{2}\) (c) \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}:\) (d) \(\mathrm{H}^{\ominus{\ominus}}\)

Short answer

The strongest nucleophile is \(\mathrm{NH}_2\).

Step-by-step solution

Define a Nucleophile

A nucleophile is a species that donates a pair of electrons to an electrophile to form a chemical bond in a reaction. Nucleophiles are electron-rich and seek positively charged or electron-deficient centers.

Analyze Available Options

Examine each given species based on their capability to donate electrons and form bonds.(a) \(\mathrm{CH}_3\) - Methyl radical, not negatively charged.(b) \( \mathrm{NH}_2 \) - Has a lone pair and a negative charge, making it a strong nucleophile.(c) \( \mathrm{H}-\mathrm{C} \equiv \mathrm{C}: \) - The acetylide ion is strong but less nucleophilic than \(\mathrm{NH}_2\).(d) \( \mathrm{H}^{\ominus} \) - Hydride ion, not typically acting as a nucleophile, more of a base.

Compare Electronegativity and Charge

Nucleophilicity typically increases with the decrease in electronegativity and increasing negative charge. Here, \(\mathrm{NH}_2\) has a high nucleophilicity due to its negative charge and less electronegative nitrogen atom compared to \(\mathrm{H}^{\ominus}\) and \(\mathrm{C}\) in \( \mathrm{H}-\mathrm{C} \equiv \mathrm{C}: \).

Identify the Strongest Nucleophile

Based on the evaluation, \( \mathrm{NH}_2 \) is the strongest nucleophile among the given options due to its negative charge and availability of a lone pair for donation.

Key concepts

Electron Donation

In chemistry, electron donation is a crucial process where an electron-rich species, like a nucleophile, donates electrons to an electron-poor species, known as an electrophile. This donation forms a chemical bond during a reaction. Electron-rich species have a surplus of electrons, typically found in their lone pairs or multiple bonds, making them ready donors.

• Electron-Rich Species: Typically, nucleophiles are rich in electrons. They could be ions with negative charges or neutral molecules with free electron pairs.
• Effect of Charge: Negative charges can enhance the electron-donating ability, increasing nucleophilicity.
• Mechanism: The electrons from the nucleophile are transferred to form a stable chemical bond with the electrophile, facilitating reactions such as nucleophilic substitution.

Understanding electron donation helps in grasping how some chemical reactions are driven and why certain molecules are more reactive than others in specific contexts.

Chemical Bond

A chemical bond is a force that holds atoms or ions together in a molecule or compound. It is the result of electron sharing (in covalent bonds) or electron transfer (in ionic bonds) between atoms, providing stability and releasing energy during formation.

• Covalent Bonds: Formed by sharing electron pairs between atoms, often occurring in organic compounds. These shared pairs come from electron donation by nucleophiles to electrophiles.
• Ionic Bonds: Result from electron transfer, typically between metals and non-metals, creating ions held together by electrostatic forces.
• Bond Formation: Stabilizes involved atoms, releasing energy to achieve lower energy states.

Grasping the nature of chemical bonds, especially those involving nucleophiles and electrophiles, aids in understanding molecular structure and reactivity patterns.

Nucleophilicity

Nucleophilicity refers to the ability of a nucleophile to donate electrons and form a chemical bond with an electrophile. It often depends on several factors, including charge, electronegativity, and the solvent used.

• Charge: Negatively charged species are generally more nucleophilic due to the increased electron density that can be donated.
• Electronegativity: Lower electronegativity usually correlates with higher nucleophilicity, as such atoms are less likely to hold onto their electrons.
• Solvent Effects: Polar protic solvents can hinder nucleophilicity by forming solvation shells around nucleophiles, whereas aprotic solvents often enhance nucleophilicity.

By understanding nucleophilicity, chemists can predict which species will more readily participate in chemical reactions, specifically those involving electron transfer.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a bond. It's a pivotal concept affecting both the nucleophilicity and reactivity of molecules.

• Trend: Electronegativity increases across a period and decreases down a group in the periodic table.
• Impact on Nucleophiles: Atoms with lower electronegativity often make stronger nucleophiles, as they do not tightly hold their electrons.
• Role in Chemical Reactions: Determines the polarity of bonds and the direction of electron flow, influencing reaction mechanisms.

Comprehending electronegativity helps explain why certain atoms form specific types of bonds and how they interact in chemical reactions.