Problem 40 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

 extnormal{SnCl}_{4} is not a Lewis acid.

Step 1: Define Lewis Acid

A Lewis acid is a chemical compound that can accept an electron pair. It is usually an atom or molecule with an incomplete octet or vacant orbitals that can accommodate additional electrons.

Step 2: Analyze  extnormal{SnCl}_{4}

 extnormal{SnCl}_{4} could potentially act as a Lewis acid because tin (Sn) is in group 14 and can expand its valence shells to accept electrons. It has available 3d orbitals which could participate in accepting electron pairs.

Step 3: Analyze  extnormal{BeCl}_{2}

 extnormal{BeCl}_{2} is a linear molecule with beryllium (Be) only having two covalent bonds, which means it has empty orbitals and can accept electron pairs, making it a potential Lewis acid.

Step 4: Analyze  extnormal{BF}_{3}

 extnormal{BF}_{3} is known to be a strong Lewis acid. Boron (B) only has three valence electrons and by being bonded to three fluorine atoms, it does not complete its octet, making it able to accept an electron pair to achieve a stable configuration.

Step 5: Analyze  extnormal{AlCl}_{3}

 extnormal{AlCl}_{3} is also a Lewis acid. Aluminum (Al) in  extnormal{AlCl}_{3} has only six electrons in its outer shell and thus can accept electrons to complete its octet.

Step 6: Determine the Exception

Among the compounds,  extnormal{SnCl}_{4} is the exception as a Lewis acid. While it can potentially accept electrons, it already has a complete octet and is generally stable, therefore, it doesn't exhibit typical Lewis acid behavior like the others.

Key Concepts

Electron Pair Acceptor

A key feature of Lewis acids is their ability to act as electron pair acceptors. This means that Lewis acids have the potential to form a bond by accepting a pair of electrons from a donor, usually referred to as a Lewis base. This process is essential in various chemical reactions where electron-rich species donate electrons to electron-poor species.
For example, the interaction between boron trifluoride (\(\text{BF}_3\)) and ammonia (\(\text{NH}_3\)) is a classic representation. Here, \(\text{BF}_3\) acts as the Lewis acid because it can accept a lone pair of electrons from the nitrogen atom in ammonia.
The capacity of a compound to act as an electron pair acceptor greatly influences its chemical reactivity and the types of reactions it can undergo. Many reactions involving the formation of new bonds rely on this transfer of electron pairs, highlighting the importance of Lewis acids in chemical synthesis and catalysis.

Incomplete Octet

In chemistry, the octet rule is a simple guideline that states atoms tend to form bonds until they are surrounded by eight electrons. However, certain elements such as boron and aluminum often present exceptions to this rule.
These exceptions, known as incomplete octets, occur in compounds like \(\text{BF}_3\) and \(\text{AlCl}_3\).

• Boron in \(\text{BF}_3\) is bonded to three fluorine atoms and contains only six electrons around it, leaving it eager to accept an additional electron pair to reach a stable octet configuration.
• Similarly, \(\text{AlCl}_3\) has aluminum surrounded by six electrons, making it capable of accepting more electrons to fill the octet.

This ability to accept additional electrons due to an incomplete octet defines their role as Lewis acids. By understanding the concept of incomplete octets, we can explain why some compounds are strong electron pair acceptors and predict their behavior in chemical reactions.

Vacant Orbitals

The presence of vacant orbitals is another significant characteristic contributing to the behavior of certain compounds as Lewis acids. Vacant orbitals are empty spaces in the valence shell of an atom that can accommodate additional electrons.
Elements like beryllium in \(\text{BeCl}_2\) are prime examples where available orbitals allow the molecule to participate in Lewis acid reactions.
For instance:

• \(\text{BeCl}_2\) has linear geometry with beryllium having empty 2p orbitals that can accept electron pairs.
• In \(\text{SnCl}_4\), tin can potentially use its d orbitals to accept electrons, although it tends to prefer maintaining its stable structure without acting prominently as a Lewis acid.

The availability of these vacant orbitals enables these compounds to engage in the formation of additional bonds, thereby showcasing their Lewis acidic nature. Understanding this concept aids in predicting and explaining various chemical interactions and the reactivity of compounds.