Problem 144 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following, the correct statement is (a) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{NH}_{3}\) is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional (b) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{PH}_{3}\) is a better electron donor because the lone pair of electrons occupies \(s p^{3}\) orbital and is more directional (c) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{NH}_{3}\) is a better electron donor because the lone pair of electrons occupies \(s p^{3}\) orbital and is less directional (d) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{PH}_{3}\) is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional

Step-by-Step Solution

Answer

The correct statement is (c).

Step 1: Understand the Concepts

To begin, recognize that 'better electron donor' refers to a molecule's ability to donate its lone pair of electrons to another atom or molecule. In this context, evaluate the orbital in which the lone pair resides and its effects on directionality and electron donation.

Step 2: Assess Electron Configuration

For both \( NH_3 \) and \( PH_3 \), the central atom (nitrogen or phosphorus) has a lone pair of electrons. In \( NH_3 \), the lone pair resides in an \( sp^3 \) hybrid orbital, which is more directional. In \( PH_3 \), the lone pairs are in a less directional orbital.

Step 3: Examine Directionality

The directionality of orbitals affects electron donation. \( sp^3 \) hybrid orbitals are tetrahedrally oriented and more directed, thus enhancing the effectiveness of electron donation in \( NH_3 \). The lone pair in \( PH_3 \) is in an almost pure 's' orbital, having less directionality.

Step 4: Compare Electron Donor Ability

Considering the directional nature of \( sp^3 \) hybridized orbitals in \( NH_3 \) and the nondirectional nature of the 's' orbital in \( PH_3 \), \( NH_3 \) is the better electron donor. This is because the directional nature of the \( sp^3 \) hybrid orbital allows the electron pair to be more readily donated.

Step 5: Identify the Correct Option

Based on the information, \( NH_3 \) is a better electron donor due to its lone pair residing in an \( sp^3 \) orbital, making it more directional. Therefore, the correct answer is option (c).

Key Concepts

Lone Pair Electrons

Lone pair electrons refer to pairs of valence electrons that are not involved in bonding. Unlike bonding electrons, lone pairs are not shared with other atoms and sit on a single atom. They play a significant role in the shape and reactivity of molecules.
In molecules such as ammonia (\(NH_3\)) and phosphine (\(PH_3\)), the central atoms have lone pairs. Nitrogen in (\(NH_3\)) and phosphorus in (\(PH_3\)) each contain one lone pair of electrons. These electrons are critical in determining their ability to function as electron donors. Electron donors can transfer their lone pair to electron-deficient areas in other molecules.
Understanding where these electrons reside — whether in hybridized orbitals or more spherically-influenced orbitals — can significantly dictate the effectiveness with which these molecules donate electrons. When lone pair electrons are housed in more directional orbitals, like those in (\(NH_3\)), they can be more potent donors due to their concentrated electron cloud shape.

Orbital Directionality

Orbital directionality is a key concept in understanding how atoms share or donate electrons. The geometry of an orbital determines its directionality, which subsequently affects how available the electrons within that orbital are for bonding.
In the case of (\(NH_3\)) and (\(PH_3\)), the directionality of the orbitals housing the lone pairs plays a major role in their properties as electron donors. The lone pair in (\(NH_3\)) resides in an (\(sp^3\)) hybrid orbital, which gives it a specific directionality. These orbitals form a tetrahedral shape, increasing the effectiveness of these electrons to be donated due to their pointed, concentrated form.
This directional feature of the orbital leads to more prominent electron interactions compared to (\(PH_3\)). In phosphine, the lone pair is found within an almost pure 's' orbital, whose spherical shape imparts less directionality. As a result, the electrons in (\(PH_3\)) are not as effective in bonding interactions, explaining why (\(NH_3\)) is a superior electron donor.

sp3 Hybridization

In chemical bonding, (\(sp^3\)) hybridization is crucial for understanding molecular shape and reactivity. This occurs when one s orbital and three p orbitals combine, forming four equivalent hybrid orbitals. These (\(sp^3\)) orbitals have a tetrahedral geometry with a bond angle of 109.5°, significant in influencing a molecule's three-dimensional shape.
The presence of (\(sp^3\)) hybridization in (\(NH_3\)) plays a pivotal role in its ability to donate electrons effectively. The nitrogen atom mixes its orbitals to create these strong, directional hybrid orbitals, where its lone pair of electrons is situated. The heightened directionality allows the lone pair to readily engage in forming bonds with electron-deficient centers.
In contrast, (\(PH_3\)) does not utilize (\(sp^3\)) hybridization to the same extent. Its lone pair resides in orbitals with less hybrid nature, impacting the molecule’s geometry and electron donation efficacy. Understanding such distinctions in hybridization can be key to predicting how molecules will interact chemically.