Question

Which of the following does not act as Lewis acid? (1) \(\mathrm{A} 1 \mathrm{Cl}_{3}\) (2) \(\mathrm{A} \mathrm{CCl}_{3} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (3) \(\mathrm{FeCl}_{3}\) (4) \(\mathrm{SiC} \mathrm{Cl}_{4}\)

Short answer

SiCl_{4} does not act as a Lewis acid.

Step-by-step solution

Define Lewis Acid

A Lewis acid is a chemical species that can accept a pair of electrons to form a coordinate covalent bond.

Analyze \(\mathrm{AlCl}_{3}\)

Aluminum chloride (\(\mathrm{AlCl}_{3}\)) can accept an electron pair, thus acting as a Lewis acid.

Analyze \(\mathrm{AlCl}_{3} \. 6 \mathrm{H}_{2} \mathrm{O}\)

The structure \(\mathrm{AlCl}_{3} \. 6 \mathrm{H}_{2} \mathrm{O}\) implies that it’s hydrated form of \(\mathrm{AlCl}_{3}\) and thus behaves as a Lewis acid.

Analyze \(\mathrm{FeCl}_{3}\)

Iron(III) chloride (\(\mathrm{FeCl}_{3}\)) can accept an electron pair, hence it acts as a Lewis acid.

Analyze \(\mathrm{SiCl}_{4}\)

Silicon tetrachloride (\(\mathrm{SiCl}_{4}\)) does not accept an electron pair; thus, it does not act as a Lewis acid.

Conclusion

Among the given compounds, \(\mathrm{SiCl}_{4}\) does not act as a Lewis acid.

Key concepts

Lewis acid definition

A Lewis acid is a chemical species that can accept a pair of electrons to form a new bond. This new bond is known as a coordinate covalent bond.
Lewis acids do not need to be positively charged; they just need to have an empty orbital to accommodate the incoming electron pair. Examples include \(\text{AlCl}_3\) and \(\text{FeCl}_3\).
Recognizing Lewis acids is important in chemistry as they play a key role in various reactions such as complex formation, catalysis, and organic synthesis.

Coordinate covalent bond

A coordinate covalent bond, sometimes known as a dative bond, is a type of covalent bond where both electrons come from the same atom.
In standard covalent bonds, each atom contributes one electron to the bond. However, in coordinate covalent bonds, one atom provides both electrons. This often happens when a Lewis base donates a pair of electrons to a Lewis acid.
For example, when \( \text{AlCl}_3 \) accepts an electron pair from a donor molecule (like \( \text{NH}_3 \)), they form a coordinate covalent bond. This results in a stable complex with the \( \text{AlCl}_3 \) now having a full outer shell.

Chemical species analysis

When analyzing chemical species to determine if they are Lewis acids, you should look for the following features:

• An incomplete octet, meaning fewer than eight electrons in the outer shell, e.g., \(\text{AlCl}_3\).
• The ability to expand their valence shell and accept additional electrons, such as seen in \(\text{FeCl}_3\).

Conversely, species like \( \text{SiCl}_4 \) with no propensity to accept electron pairs, do not act as Lewis acids.
By carefully examining these features, you can predict and understand the behavior of various chemical species in reactions.