Question

Which reactant in each of the following pairs is more nucleophilic? Explain. (a) \(-\mathrm{NH}_{2}\) or \(\mathrm{NH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}\) (c) \(\mathrm{BF}_{3}\) or \(\mathrm{F}^{-}\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{P}\) or \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) (e) \(\mathrm{I}^{-}\) or \(\mathrm{Cl}^{-}\) (f) \(-\mathrm{C} \equiv \mathrm{N}\) or \(-\mathrm{OCH}_{3}\)

Short answer

(a) -NH2, (b) CH3CO2^-, (c) F^-, (d) (CH3)3P, (e) I^-, (f) -OCH3

Step-by-step solution

Understand Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile, forming a chemical bond. It is influenced by charge, electronegativity, solvent effects, and steric hindrance.

Compare (a) -NH2 vs NH3

The -NH2 ion has an extra pair of electrons and a negative charge, making it more nucleophilic than neutral NH3, which lacks the negative charge that enhances electron donation.

Compare (b) H2O vs CH3CO2^-

CH3CO2^- carries a negative charge, increasing its nucleophilicity over neutral H2O. The negative charge in CH3CO2^- makes it better at donating an electron pair in reactions.

Compare (c) BF3 vs F^-

BF3 is an electron-deficient molecule acting as a Lewis acid, whereas F^- is a fluoride ion with a complete octet and negative charge, making it a stronger nucleophile.

Compare (d) (CH3)3P vs (CH3)3N

Phosphorus is less electronegative than nitrogen, so (CH3)3P, with a lone pair on phosphorus, is more nucleophilic than (CH3)3N, whose lone pair is less willing to participate in bonding.

Compare (e) I^- vs Cl^-

In polar protic solvents, nucleophilicity is affected by the size of anions. I^- is larger and more polarizable than Cl^-, thus it is more nucleophilic in such solvents due to less solvation and greater ease of electron pair donation.

Compare (f) -C≡N vs -OCH3

The -C≡N ion has less electron density ready for donation as a nucleophile compared to -OCH3. The electronegative oxygen in -OCH3 has a higher electron-donating capacity due to the lone pairs, making it more nucleophilic.

Key concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. Imagine it as a competitive trait that shows how badly an atom wants electrons from nearby atoms. The more electronegative an atom is, the more it pulls electrons towards itself. For example, elements like fluorine, oxygen, and nitrogen have high electronegativities. They hold onto their electrons tightly, which makes them less likely to donate electron pairs.In nucleophilicity, the less electronegative an atom within a molecule tends to be a better electron pair donor. This is because its electrons are held more loosely and are ready to form new bonds. For example, in the comparison between \(\text{CH}_3\text{CO}_2^-\) and \(\text{H}_2\text{O}\), we find that the acetate ion (with a less electronegative carbon) is more nucleophilic than water.

Lewis acid

A Lewis acid is any species that can accept an electron pair, thus forming a new bond. Think of it like a molecular vacancy sign saying, "Room for electrons here!" Lewis acids are often electron-deficient and ready to accept electrons from nucleophiles. Common examples include molecules like \(\text{BF}_3\).In the comparison between \(\text{BF}_3\) and \(\text{F}^-\), the boron trifluoride ( ext{BF}_{3}) acts as a Lewis acid. It is looking for electrons because it has an incomplete octet. On the other hand, \(\text{F}^-\) is not a Lewis acid but a nucleophile since it can donate its electrons to complete the octet of other atoms.

Electron donation

Electron donation is the act of giving away an electron pair to form a bond. Species that donate electrons are called nucleophiles. They are key players in chemical reactions, ready to share their electrons to help build new molecules. They find their targets in electrophiles, which are eager to accept these electrons and form stable bonds.In comparing \(\text{-NH}_{2}\) and \(\text{NH}_{3}\), the stronger electron donor is \(\text{-NH}_{2}\). The negative charge greatly enhances its electron donation capabilities compared to the neutral \(\text{NH}_{3}\). More charge usually implies a greater willingness and ability to donate electrons to form bonds.

Polarizability

Polarizability refers to how easily an electron cloud in a molecule can be distorted. Larger anions tend to have more flexible electron clouds, which means they are more polarizable. This property increases their nucleophilicity, especially in polar protic solvents where such anions are less tightly held by solvent molecules.A good example is the comparison between \(\text{I}^-\) and \(\text{Cl}^-\). Although iodine is larger than chlorine, the larger size of \(\text{I}^-\) makes it more polarizable. This results in \(\text{I}^-\) being more nucleophilic compared to \(\text{Cl}^-\), because its electron cloud can more easily engage in the formation of new bonds.