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

Step-by-Step Solution

Answer

Option (c) is correct; 7NH_37 is a better electron donor due to more directional 7sp^37 orbitals.

Step 1: Understand Electron Donation

Evaluate what makes one molecule a better electron donor than another. A stronger donor usually has more available electrons and a directional lone pair. This is often affected by the hybridization of the orbital containing the lone pair.

Step 2: Analyze NH3

Ammonia (7NH_37) has its lone pair of electrons in an 7sp^37 hybridized orbital. This hybridization implies that the lone pair is more directional, making 7NH_37 a strong electron donor.

Step 3: Analyze PH3

Phosphine (7PH_37) also features its lone pair in a somewhat hybridized orbital, but the influence of phosphorus's larger atomic size means that the electron pair is less directional compared to 7NH_37.

Step 4: Compare Directionality and Hybridization

Compare 7NH_37 and 7PH_37 regarding the lone pair's orbital characteristics. 7NH_37 is expected to donate better due to higher directionality of its 7sp^37 lone pair, which is more concentrated in space than 7PH_37's.

Step 5: Evaluate Each Option

- Option (a) describes 7NH_37 as having a spherical 's' orbital, which incorrectly suggests low directionality. - Option (b) incorrectly claims 7PH_37 as a better donor due to its hybridization. - Option (c) correctly attributes the 7sp^37 hybridization and better donation to 7NH_37.

Key Concepts

NH3 vs PH3

The molecules ammonia (NH3) and phosphine (PH3) both have a central atom bonded to hydrogen atoms with a lone pair of electrons. However, their ability to donate electrons and act as electron donors varies significantly.

- **Ammonia (NH3):** - Ammonia is a better electron donor compared to PH3. - Its lone pair of electrons is present in an sp3 hybridized orbital, which makes it more directional. - This directionality implies that the lone pair is concentrated in space; thus, it can overlap more efficiently with empty orbitals of other atoms or molecules.
- **Phosphine (PH3):** - In contrast, phosphine has a lone pair also in a similar sp3 like environment, but it is less directional due to phosphorus's larger atomic orbitals. - The larger size of phosphorus means that the lone pair is less concentrated and thus less directional, making PH3 a weaker electron donor.
Overall, the better electron donation ability of NH3 is linked to its lone pair's enhanced concentration and directionality due to hybridization.

Hybridization

Hybridization is a concept that explains how atomic orbitals mix to form new hybrid orbitals. These hybrid orbitals are responsible for the formation of chemical bonds in molecules. It significantly influences the properties of compounds.

In the context of NH3 and PH3, hybridization affects the availability and directionality of lone pairs in these molecules.

- **NH3's Hybridization:** - The lone pair of electrons in NH3 is found in an sp3 hybridized orbital. - An sp3 orbital arises from the mixing of one s orbital and three p orbitals. - This type of hybridization increases the directionality of the lone pair, making it more effective in forming strong interactions with potential electron acceptors.
- **PH3's Hybridization:** - For PH3, while sometimes classified into hybridization frameworks, the unique properties of phosphorus result in less pronounced hybridization. - The larger atomic orbitals of phosphorus lead to a reduced effectiveness in directional bonding that NH3's sp3 hybridization facilitates.
In summary, the type and extent of hybridization play crucial roles in determining electron donation abilities, especially as seen in NH3 versus PH3.

Orbital Directionality

Orbital directionality refers to how the shape and orientation of an electron's orbital allow it to interact with other atoms. The more directional an orbital, the more focused its electron density, which enhances the overlapping capabilities with empty orbitals of other atoms.

In atoms involved in bonding:

• **Directional Orbitals:** Offer focused electron density, leading to stronger overlap and bonds.
• **Non-directional Orbitals:** Have a more diffuse electron presence, resulting in weaker interactions.

For NH3: - The sp3 hybridization ensures its lone pair is highly directional. - This directionality allows ammonia to act as an effective electron donor, as its electron pair can readily engage in interaction with positive or empty orbitals on other molecules, enhancing bond strength.
For PH3: - Its lone pair, even if housed in an sp3-like environment, is less directional due to phosphorus’s atomic structure. - Consequently, the electron donating ability of PH3 is weaker, making its interactions less potent.
Orbital directionality is a crucial concept when discussing electron donation and chemical reactivity, especially highlighting the differences between NH3 and PH3.