Problem 37 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The compound that is not a Lewis acid is : (a) \(\mathrm{BF}_{3}\) (b) \(\mathrm{AlCl}_{3}\) (c) \(\mathrm{BeCl}_{2}\) (d) \(\mathrm{SnCl}_{4}\)

Step-by-Step Solution

Answer

The compound that is not a Lewis acid is \\( \mathrm{SnCl}_4 \\\).

Step 1: Understand Lewis Acids

A Lewis acid is defined as a chemical species that can accept an electron pair. Typically, they are electron-deficient and have an incomplete octet, allowing them to accept electron pairs from a Lewis base to form a stable compound.

Step 2: Analyze \\( \mathrm{BF}_3 \\\)

Boron trifluoride (\( \mathrm{BF}_3 \)) is an electron-deficient compound because boron has only 6 electrons in its valence shell and can accept an electron pair. Thus, \( \mathrm{BF}_3 \) is a Lewis acid.

Step 3: Analyze \\( \mathrm{AlCl}_3 \\\)

Aluminum chloride (\( \mathrm{AlCl}_3 \)) is also electron-deficient. Aluminum has 3 valence electrons and forms 3 bonds with chlorine, giving it an incomplete octet. Therefore, \( \mathrm{AlCl}_3 \) can accept electron pairs, making it a Lewis acid.

Step 4: Analyze \\( \mathrm{BeCl}_2 \\\)

Beryllium chloride (\( \mathrm{BeCl}_2 \)) is similar, as beryllium is able to form only two bonds with chlorine, leaving it with an incomplete octet. This enables \( \mathrm{BeCl}_2 \) to act as a Lewis acid.

Step 5: Analyze \\( \mathrm{SnCl}_4 \\\)

Tin(IV) chloride (\( \mathrm{SnCl}_4 \)) has a complete octet. Tin is capable of forming robust covalent bonds with four chlorine atoms, thereby completing its octet. As it has no deficiency of electrons, \( \mathrm{SnCl}_4 \) does not act as a Lewis acid.

Step 6: Conclusion

Based on the analysis, \( \mathrm{SnCl}_4 \) is the compound that does not act as a Lewis acid because it has a complete octet.

Key Concepts

Electron Pair Acceptor

A Lewis acid is defined as a substance that can accept a pair of electrons. This is a fundamental concept in chemistry where Lewis acids participate in reactions by accepting electron pairs from Lewis bases.
Lewis acids can include:

• Cations, like \\( \text{Fe}^{3+} \)
• Molecules with central atoms which have empty orbitals, like \( \mathrm{BF}_3 \)
• Neutral molecules or species that can rearrange to form bonds with available electron pairs

When a Lewis acid accepts an electron pair, it usually forms a coordinate covalent bond. This means both electrons in the bond come from the same atom, typically the Lewis base. Understanding the ability of a substance to serve as an electron pair acceptor helps chemists predict its reactivity in numerous chemical reactions.

Incomplete Octet

An incomplete octet is a scenario where atoms do not have 8 electrons in their valence shell, which is less than the stable octet configuration. This situation often occurs in certain elements from the second period of the periodic table, particularly those with a smaller number of valence electrons, such as boron and beryllium.
For example:

• Boron in \( \mathrm{BF}_3 \) has 3 valence electrons, forming three bonds with fluorine. Thus, it reaches only 6 electrons.
• Beryllium in \( \mathrm{BeCl}_2 \) also forms two bonds, resulting in only 4 electrons around beryllium.

These elements, due to their incomplete octet, are often highly reactive. Their tendency to complete the octet by forming additional bonds, commonly by accepting electron pairs, makes them potent Lewis acids.

Electron Deficiency

Electron deficiency refers to a lack of enough electrons to fulfill the octet rule, which makes atoms or molecules prone to accept electron pairs. This characteristic is commonly found in molecules and species which do not have enough valence electrons to form a full octet through covalent bonding alone.
Electron-deficient compounds often include:

• Molecules like \( \mathrm{BF}_3 \) and \( \mathrm{AlCl}_3 \), where the central atom has fewer than 8 electrons.
• Metal hydrides, such as \( \mathrm{BeH}_2 \), which also display electron deficiency.

This deficiency provides these compounds with the capability to act as Lewis acids. Since these atoms or molecules are on the lookout to stabilize their electron configuration, they readily accept electrons from other species, making them essential players in various chemical transformations and catalytic processes.