Question

In a protic solvent, which of the following halogens would be the best nucleophile? A \(\mathrm{Br}^{-}\) B \(\mathrm{Cl}^{-}\) C \(\mathrm{F}^{-}\) D \(I^{-}\)

Short answer

\(I^{-}\)

Step-by-step solution

- Understand the Nature of a Protic Solvent

A protic solvent is capable of hydrogen bonding because it contains O-H or N-H groups. Examples include water, alcohols, and carboxylic acids.

- Determine the Effect of a Protic Solvent on Nucleophilicity

In a protic solvent, the nucleophilicity of halide ions decreases as we move up the periodic table. This is due to solvation effects, where smaller ions are more strongly solvated.

- Compare the Halide Ions

Compare the halide ions considering the solvation effect: - \[ \text{F}^{-} \] (smallest ion, most strongly solvated, weakest nucleophile)- \[ \text{Cl}^{-} \] (larger than \[ \text{F}^{-} \] , less solvated, stronger nucleophile than \[ \text{F}^{-} \])- \[ \text{Br}^{-} \] (larger than \[ \text{Cl}^{-} \], even less solvated, stronger nucleophile than \[ \text{Cl}^{-} \] )- \[ \text{I}^{-} \] (largest ion, least solvated, strongest nucleophile)

- Identify the Best Nucleophile

Considering the solvation effects in a protic solvent, \[ \text{I}^{-} \] will be the best nucleophile among the halogens listed.

Key concepts

nucleophilicity

Nucleophilicity refers to the ability of a molecule or an ion to donate a pair of electrons to form a new chemical bond. It's a key concept in understanding chemical reactivity and mechanisms. In general, a good nucleophile is characterized by:

• High negative charge density
• Low electronegativity
• Small size (in cases of similar atoms, though this changes in protic solvents)

A strong nucleophile will readily attack an electrophile, which is a species that accepts electron pairs. It's essential to note that nucleophilicity can vary dramatically depending on the solvent and the specific conditions of the reaction.

solvation effects

Solvation effects describe how solvent molecules interact with solutes, which are the particles dissolved in the solvent. When dealing with ionized species such as halide ions, the type of solvent can significantly influence nucleophilicity. In a protic solvent, which has hydrogen-bonding capability, solvation can heavily affect smaller ions.

Protic solvents like water, alcohols, and carboxylic acids can form hydrogen bonds with anions. This greatly stabilizes them by surrounding them with a shell of solvent molecules. Bigger ions are less strongly solvated due to their larger radius and lower charge density, making them better nucleophiles in such solvents. Smaller ions like \(\text{F}^{-}\) are more heavily solvated, reducing their nucleophilicity because the solvent shell makes it harder for them to attack electrophiles.

halide ions

Halide ions, which include \(\text{F}^{-}\), \(\text{Cl}^{-}\), \(\text{Br}^{-}\), and \(\text{I}^{-}\), vary widely in their nucleophilicity depending on the solvent. These anions are formed when halogens gain an electron, and they play a significant role in many nucleophilic substitution reactions:

• \(\text{F}^{-}\): Very small and highly electronegative. Its nucleophilicity is low in protic solvents because it's heavily solvated.
• \(\text{Cl}^{-}\): Larger than fluoride but still quite small. Less strongly solvated compared to \(\text{F}^{-}\), making it a better nucleophile in protic solvents.
• \(\text{Br}^{-}\): Larger and even less solvated, thus an even better nucleophile in protic solvents.
• \(\text{I}^{-}\): The largest halide ion. It's the least solvated and, therefore, the best nucleophile in protic solvents.

Understanding these differences helps predict which halide ion will be the most reactive under different conditions.

protic solvents

Protic solvents are those that can form hydrogen bonds because they contain O-H or N-H groups. Examples include water \(\text{H}_2\text{O}\), alcohols like ethanol (\(\text{C}_2\text{H}_5\text{OH}\)), and carboxylic acids such as acetic acid (\(\text{CH}_3\text{COOH}\)). The ability to hydrogen bond has a substantial impact on reaction mechanisms, especially in nucleophilic substitution reactions involving halide ions.

In these contexts, the solvent can stabilize the nucleophile to differing extents:

• Smaller ions, such as \(\text{F}^{-}\), are heavily solvated by hydrogen bonding, which reduces their nucleophilicity.
• Larger ions, like \(\text{I}^{-}\), are less strongly solvated, thus maintaining a higher nucleophilicity.

This explains why in a protic solvent, \(\text{I}^{-}\) is the best nucleophile despite being larger than the other halide ions.