Problem 1 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The correct statement among the following is : [Main April 12, 2019 (I)] (a) \(\left(\mathrm{SiH}_{3}\right)_{3} \mathrm{~N}\) is planar and less basic than \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\). (b) \(\left(\mathrm{SiH}_{3}\right)_{3} \mathrm{~N}\) is pyramidal and more basic than \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\). (c) \(\left(\mathrm{SiH}_{3}\right)_{3} \mathrm{~N}\) is pyramidal and less basic than \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\). (d) \(\left(\mathrm{SiH}_{3}\right)_{3} \mathrm{~N}\) is planar and more basic than \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\).

Step-by-Step Solution

Answer

The correct statement is (c): \((\mathrm{SiH}_3)_3 \mathrm{N}\) is pyramidal and less basic than \((\mathrm{CH}_3)_3 \mathrm{N}\).

Step 1: Analyze the Geometry and Hybridization

First, we need to analyze whether \( (\mathrm{SiH}_3)_3 \mathrm{N} \) and \( (\mathrm{CH}_3)_3 \mathrm{N} \) are planar or pyramidal. \( (\mathrm{CH}_3)_3 \mathrm{N} \) is known to be pyramidal because the nitrogen is sp3 hybridized and has a lone pair of electrons, which does not participate in delocalization. \( (\mathrm{SiH}_3)_3 \mathrm{N} \) is also pyramidal due to similar hybridization and the presence of a lone pair on nitrogen.

Step 2: Evaluation of Basicity

Next, we evaluate the basicity of \( (\mathrm{SiH}_3)_3 \mathrm{N} \) and \( (\mathrm{CH}_3)_3 \mathrm{N} \). Basicity depends on the availability of the lone pair. In \( (\mathrm{CH}_3)_3 \mathrm{N} \), the lone pair on nitrogen is localized and more available than in \( (\mathrm{SiH}_3)_3 \mathrm{N} \), where it is spread over larger silicon atoms. Therefore, \( (\mathrm{CH}_3)_3 \mathrm{N} \) is more basic than \( (\mathrm{SiH}_3)_3 \mathrm{N} \).

Step 3: Compare Options with Explanations

Using the information from Steps 1 and 2, we compare the options:- (a) Incorrect: \( (\mathrm{SiH}_3)_3 \mathrm{N} \) is pyramidal and less basic.- (b) Incorrect: \( (\mathrm{SiH}_3)_3 \mathrm{N} \) is not more basic.- (c) Correct: \( (\mathrm{SiH}_3)_3 \mathrm{N} \) is pyramidal and less basic.- (d) Incorrect: \( (\mathrm{SiH}_3)_3 \mathrm{N} \) is not planar and not more basic.

Key Concepts

Molecular Geometry

Molecular geometry refers to the specific three-dimensional arrangement of atoms within a molecule. This geometry is vital because it influences many physical properties and chemical reactivity of the molecule.
The shape of a molecule can determine how well it fits specific receptors in biological systems, how it interacts with light, and its overall polarity, which affects solubility and boiling points.

For example, the geometry around nitrogen in trimethylamine \((\mathrm{CH}_3)_3 \mathrm{N}\) and trisilylamine \((\mathrm{SiH}_3)_3 \mathrm{N}\) is pyramidal. This results from the sp3 hybridization of the nitrogen atom, which forms three bonds and has one lone pair of electrons. This lone pair slightly distorts the tetrahedral arrangement, creating a pyramidal shape. This shape plays a critical role in determining the molecule's interaction with other substances, especially in terms of basicity.

Hybridization

Hybridization is a model that describes the mixing of atomic orbitals to produce new hybrid orbitals. These hybrid orbitals influence molecular bonding and geometry.

In \((\mathrm{CH}_3)_3 \mathrm{N}\) and \((\mathrm{SiH}_3)_3 \mathrm{N}\), the nitrogen atom undergoes sp3 hybridization. This means that one s orbital and three p orbitals from nitrogen mix to form four equivalent sp3 hybrid orbitals.

• Three of these hybrid orbitals form sigma bonds with three hydrogen atoms or silicon-hydrogen groups.
• The remaining hybrid orbital holds a lone pair of electrons.
  This hybridization accounts for the pyramidal shape of the molecules, as the lone pair on nitrogen pushes the bonded atoms slightly away, creating a non-planar configuration.

Understanding hybridization is essential for predicting the shape and bonding behavior of complex molecules.

Pyramidal Shape

The pyramidal shape is a molecular geometry that occurs when a central atom is surrounded by three bonded atoms and one lone pair of electrons. This shape resembles a pyramid with a triangular base.

The arrangement creates an asymmetrical shape around the central atom, as seen in molecules like trimethylamine \((\mathrm{CH}_3)_3 \mathrm{N}\) and trisilylamine \((\mathrm{SiH}_3)_3 \mathrm{N}\).

This pyramidal configuration is due to the electron repulsion between the lone pair and bond pairs reducing the bond angles from the ideal tetrahedral angle. The lone pair is more repulsive than the bonded pairs, causing a slight compression of the bond angles closer to the central atom.

• The lone pair's presence also affects basicity, as it is these electrons that are available to form new bonds with protons.
• In general, a more localized lone pair, as in \(\mathrm{CH}_3)_3 \mathrm{N}\), leads to a higher basicity than a more spread out lone pair, like in \(\mathrm{SiH}_3)_3 \mathrm{N}\).

This understanding of pyramidal shapes can help explain varying basicities in similar structured molecules.