The \(\mathrm{S}_{\mathrm{N}} 2\) reaction that occurs between an alkyl halide and a nucleophile is a very important process in organic chemistry. For this reason, it is necessary to recognize the nucleophilicity of various reagents. In the following pairs of compounds, indicate the more nucleophilic reagent. Explain your reasoning. (a) \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{3} \mathrm{~N} \quad\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{3} \mathrm{P}\) (b) \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{3} \mathrm{~N} \quad\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{2} \mathrm{~N}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SNa} \quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OK}\) There is much detail in this chapter, but that is not usually a problem for most students. Good nucleophiles can be distinguished from good bases, good leaving groups can be identified, and so on. What is hard is to learn to adjust reaction conditions (reagents, temperature, and solvent) so as to achieve the desired selectivity to favor the product of just one of these four reactions. That's hard in practice as well as in answering a question on an exam or problem set. To a certain extent, we all have to learn to live with this problem!

Step by step solution

01

Identify General Rule for Nucleophilicity

Nucleophilicity is influenced by several factors such as charge, electronegativity, solvent, and steric hindrance. Generally, a species with a negative charge is more nucleophilic than its neutral counterpart. Additionally, nucleophilicity decreases with increasing electronegativity.

02

Pair (a): Compare \((\mathrm{CH}_{3} \mathrm{CH}_{2})_{3} \mathrm{~N}\) and \( (\mathrm{CH}_{3} \mathrm{CH}_{2})_{3} \mathrm{P} \)

Phosphorus (P) is larger and less electronegative compared to nitrogen (N), making it more polarizable and capable of forming bonds more readily. Therefore, \( (\mathrm{CH}_{3} \mathrm{CH}_{2})_{3} \mathrm{P} \) is more nucleophilic than \( (\mathrm{CH}_{3} \mathrm{CH}_{2})_{3} \mathrm{N} \).

03

Pair (b): Compare \( (\mathrm{CH}_{3} \mathrm{CH}_{2})_{3} \mathrm{~N} \) and \( (\mathrm{CH}_{3} \mathrm{CH}_{2})_{2} \mathrm{~N}^{-} \)

A negatively charged nitrogen is a stronger nucleophile than a neutral nitrogen due to the extra electron density. Therefore, \( (\mathrm{CH}_{3} \mathrm{CH}_{2})_{2} \mathrm{~N}^{-} \) is more nucleophilic than \( (\mathrm{CH}_{3} \mathrm{CH}_{2})_{3} \mathrm{~N} \).

04

Pair (c): Compare \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SNa} \) and \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OK} \)

Sulfur (S) is less electronegative and more polarizable than oxygen (O), making it a better nucleophile, even though both species are negatively charged. Therefore, \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SNa} \) is more nucleophilic than \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OK} \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Nucleophilicity

Nucleophilicity refers to the ability of a chemical species to donate an electron pair and form a new chemical bond. This concept is crucial in understanding the \( \text{S}_\text{N}2 \) reaction. In organic chemistry, several factors influence nucleophilicity:

• Charge: A negatively charged species is typically more nucleophilic than a neutral one due to its higher electron density.
• Electronegativity: As electronegativity increases, nucleophilicity decreases because more electronegative atoms hold onto their electrons more tightly.
• Polarizability: Larger atoms or ions are more polarizable and thus more nucleophilic. They can easily distort their electron clouds to form bonds.
• Steric hindrance: Bulkier nucleophiles experience more steric hindrance and are less able to approach and attack electrophilic centers.

Understanding these factors helps in predicting the outcome of nucleophilic substitution reactions.

Organic Chemistry

Organic chemistry is the branch of chemistry that studies the structure, properties, composition, reactions, and synthesis of organic compounds. These are compounds containing carbon atoms bonded to hydrogen, oxygen, nitrogen, and other elements. The \( \text{S}_\text{N}2 \) reaction is a fundamental reaction in organic chemistry involving a single step where a nucleophile attacks the electrophile from the opposite side of the leaving group, resulting in the inversion of configuration:

• Reaction mechanism: A nucleophile approaches the electrophile (often an alkyl halide) and displaces the leaving group in a concerted, single-step process.
• Polar aprotic solvents: \( \text{S}_\text{N}2 \) reactions are favored in polar aprotic solvents that do not solvate nucleophiles, thereby increasing their reactivity.

These core concepts are crucial for understanding and manipulating organic reactions to synthesize desired compounds.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. It plays a vital role in nucleophilicity and reactivity in \( \text{S}_\text{N}2 \) reactions. Here’s how electronegativity impacts nucleophilicity:

• Lower Electronegativity: Atoms with lower electronegativity, such as phosphorus, hold their electrons less tightly and can better donate them, making them more nucleophilic compared to more electronegative atoms like nitrogen.
• Bond formation: Less electronegative atoms are more willing to form new chemical bonds by sharing their electron pairs.

For instance, in the comparison between triethylamine and triethylphosphine, phosphorus is more nucleophilic due to its lower electronegativity and higher polarizability.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to bulky groups obstructing the approach of reactants. This is particularly relevant in the context of nucleophilicity and the \( \text{S}_\text{N}2 \) mechanism:

• Effect on Nucleophilicity: Bulky groups on nucleophiles can hinder their approach to the electrophile, decreasing their nucleophilicity.
• \text{S}_\text{N}2 \ Mechanism: This mechanism requires a backside attack on the electrophile, and steric hindrance significantly impacts its efficiency. Smaller, less hindered nucleophiles are more effective in \( \text{S}_\text{N}2 \) reactions.

In nucleophilic substitutions, understanding steric hindrance helps in selecting the right nucleophile and designing efficient synthetic pathways.