Problem 17 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Among the following, the correct statement(s) is (are) [Adv. 2017] (a) \(\mathrm{Al}\left(\mathrm{CH}_{3}\right)_{3}\) has the three-centre two- electron bonds in its dimeric structure (b) \(\mathrm{BH}_{3}\) has the three-centre two-electron bonds in its dimeric structure (c) \(\mathrm{AlCl}_{3}\) has the three-centre two-electron bonds in its dimeric structure (d) The Lewis acidity of \(\mathrm{BCl}_{3}\) is greater than that of \(\mathrm{AlCl}_{3}\)

Step-by-Step Solution

Answer

Statements (b), (c), and (d) are correct.

Step 1: Understand the Concept of Three-Centre Two-Electron Bonds

A three-centre two-electron bond involves one electron pair shared between three atoms. It is often seen in compounds like boranes, where bridging hydrogen atoms are bonded to two boron atoms.

Step 2: Evaluate Statement (a)

Examine the structure of Al(CH3)3. In its dimeric form, Al2(CH3)6, aluminum methyl forms structures without three-centre two-electron bonds as the bridging occurs through the CH3 groups, not forming a classic three-centre bond.

Step 3: Evaluate Statement (b)

Consider BH3, which actually forms a dimer called diborane ( B2H6). In diborane, there are two bridging hydrogen atoms which contribute to the formation of three-centre two-electron bonds, making statement (b) true.

Step 4: Evaluate Statement (c)

Examine AlCl3, which forms a dimer ( Al2Cl6) where chlorine atoms connect the aluminum atoms in a bridging fashion, supporting the presence of three-centre two-electron bonds. Thus, statement (c) is true.

Step 5: Evaluate Statement (d)

Assess the acidity of BCl3 and AlCl3. BCl3 is more Lewis acidic than AlCl3 because boron can better accept electrons due to the stabilization it achieves by completing its electron deficiency. Thus, statement (d) is true.

Key Concepts

Lewis Acidity

Lewis acidity refers to the ability of a compound to accept an electron pair. This concept is central in understanding reactions where electron pair donation occurs between reagents. In the context of the given exercise, the Lewis acidity of both \( \text{BCl}_3 \) and \( \text{AlCl}_3 \) is examined. Boron in \( \text{BCl}_3 \) is more Lewis acidic than aluminum in \( \text{AlCl}_3 \). This is primarily because:

• Boron is smaller in size compared to aluminum, which makes its empty p-orbitals more accessible for bonding.
• Boron is more electron-deficient, creating a stronger need to accept electrons.

Thus, when \( \text{BCl}_3 \) encounters potential electron pair donors, it readily forms a bond by accepting these electrons, affirming its greater Lewis acidity compared to \( \text{AlCl}_3 \).

Dimeric Structures

Dimeric structures arise when two molecules bond together. This can involve complex bonding modes, including the sharing of electrons in multi-center bonds.For example, \( \text{AlCl}_3 \) forms a dimer, \( \text{Al}_2\text{Cl}_6 \), due to the saturation of the electron-deficient aluminum sites through chlorine bridging. This bridging results in a three-center two-electron bond that stabilizes the dimer structure. The interaction between two aluminum atoms and the bridging chlorine atoms allows for electron sharing, which is distinct from traditional covalent bonds.Similarly, \( \text{BH}_3 \) forms a dimeric structure in \( \text{B}_2\text{H}_6 \), known as diborane. Here, the hydrogen atoms form bridges between boron atoms, once again illustrating a three-center two-electron bond formation. This type of bond is a unique feature of dimeric structures found in certain compounds, particularly among boranes.

Boranes

Boranes are compounds of boron and hydrogen, typically resembling a series with varying boron-hydrogen ratios. They are known for their unique bonding and structural properties which make them chemically significant.One interesting structural feature of boranes is the presence of three-center two-electron bonds, particularly evident in diborane \( \text{B}_2\text{H}_6 \). In diborane, two hydrogen atoms form bridges with two boron atoms, allowing the three atoms to share two electrons simultaneously. This bonding is atypical compared to classic two-center two-electron bonds and highlights the novelty of boranes. The electron deficiency of boron facilitates these complex bonding modes, providing stability to borane structures through three-center two-electron interactions. Understanding boranes require a grasp of these multi-center bond formations, representing an advanced concept in molecular chemistry.