Problem 33 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Identify correct nucleophilicity order (i) \(\mathrm{PH}_{3}\) (ii) \(\mathrm{NH}_{3}\) (iii) \(\mathrm{AsH}_{3}\) (a) \(\mathrm{i}>\mathrm{ii}>\mathrm{iii}\) (b) ii \(>\mathrm{iii}>\mathrm{i}\) (c) \(\mathrm{i}>\mathrm{iii}>\mathrm{ii}\) (d) \(\mathrm{iii}>\mathrm{i}>\mathrm{ii}\)

Step-by-Step Solution

Answer

(d) \\({3} > {3} > {3}\) is the correct order.

Step 1: Understand Nucleophilicity

Nucleophilicity refers to the ability of a molecule to donate a pair of electrons to form a new chemical bond during a chemical reaction. It is influenced by factors like charge, electronegativity, and steric factors.

Step 2: Evaluate Electronegativity

Among \({3}\), \({3}\), and \({3}\), we know that electronegativity decreases as we move down the periodic table in Group 15. Thus, \({3}\) is more electronegative than \({3}\), which is in turn more electronegative than \({3}\). The higher the electronegativity, the less nucleophilic the molecule.

Step 3: Consider Atom Size and Nucleophilicity

As the size of the atom increases down a group, nucleophilicity usually increases because larger atoms have more diffused electron clouds, making electron donation easier. As such, the order is \({3} > {3} > {3}\).

Step 4: Choose the Correct Answer

Based on the understanding from Steps 2 and 3, recall that nucleophilicity increases down the group and the least electronegative atom is typically the most nucleophilic. Therefore, option (d) \({3} > {3} > {3}\) is the correct nucleophilicity order.

Key Concepts

Electronegativity

Electronegativity is a key concept in understanding the behavior of atoms in chemical reactions. It refers to the ability of an atom to attract electrons towards itself in a chemical bond. This property is crucial when discussing nucleophilicity, as more electronegative atoms hold onto their electrons tightly and are less willing to share them in bond formation.
In the periodic table, electronegativity generally increases across a period from left to right and decreases as you move down a group. This trend is essential for determining the nucleophilic strength of different elements. For instance, in Group 15 elements like \(\mathrm{NH}_3\) (ammonia), \(\mathrm{PH}_3\) (phosphine), and \(\mathrm{AsH}_3\) (arsine), electronegativity decreases from nitrogen to arsenic. Therefore:

• Nitrogen is the most electronegative of the group, making \(\mathrm{NH}_3\) less nucleophilic compared to \(\mathrm{PH}_3\) and \(\mathrm{AsH}_3\).
• Higher electronegativity means a stronger pull on inner electrons, making it harder for the atom to donate them in reactions.

This means that when choosing between these molecules for their ability to donate electron pairs, \(\mathrm{AsH}_3\) would likely be the most nucleophilic due to its lower electronegativity.

Atom Size

The size of an atom plays a significant role in nucleophilicity. Larger atoms tend to have more diffused electron clouds, which can actually aid in their ability to donate electrons. This is because the electron cloud's outermost electrons are shielded more by the inner electrons, making these outer electrons easier to donate.
In Group 15, as you move down the group from nitrogen to arsenic, atomic size increases:

• \(\mathrm{NH}_3\) contains nitrogen atoms, which are smaller compared to phosphorus and arsenic atoms in \(\mathrm{PH}_3\) and \(\mathrm{AsH}_3\).
• The larger size of \(\mathrm{PH}_3\) and \(\mathrm{AsH}_3\) gives their electron clouds a wider reach, aiding nucleophilic reactions by making it easier to approach and donate to electrophiles.

Atom size impacts not just nucleophilicity, but the general reactivity of a compound in chemical reactions.

Group 15 Elements

Group 15 elements, also known as the nitrogen group, consist of \(\mathrm{N}\), \(\mathrm{P}\), \(\mathrm{As}\), \(\mathrm{Sb}\), and \(\mathrm{Bi}\). These elements are p-block elements and show diverse chemical properties.
Within Group 15, several trends impact their nucleophilicity and reactivity:

• As you descend the group, elements exhibit an increase in metallic character. Nitrogen is a non-metal, phosphorus and arsenic are metalloids, while antimony and bismuth are metals.
• This has implications for their chemical reactions, as non-metals generally form covalent bonds by sharing electrons, while metals prefer to lose electrons to form cations.
• The group shows a decrease in electronegativity from nitrogen to bismuth, influencing their proclivity to engage in nucleophilic reactions.

Understanding the trends in Group 15 can give insight into the varying behavior of molecules like \(\mathrm{NH}_3\), \(\mathrm{PH}_3\), and \(\mathrm{AsH}_3\), with respect to their ability to participate in chemical reactions as nucleophiles.