Question

Which of the following can act as Lewis acid?

• A. \( H_2O \)
• B. \( HO^- \)
• C. \( F^- \)
• D. \( NH_3 \)
• E. \( AlCl_3 \)

Hint:

Electron-deficient species act as Lewis acids.

Answer 1

Answer: (E)

Step 1: Understanding Lewis Acids and Bases:
According to the Lewis theory of acids and bases:
A Lewis acid is a substance that can accept a pair of electrons. These are typically electron-deficient species.
A Lewis base is a substance that can donate a pair of electrons. These species must have at least one lone pair of electrons.
Step 2: Analyzing the Options:
(A) H\(_2\)O (Water): The oxygen atom in water has two lone pairs of electrons which it can donate. Therefore, H\(_2\)O acts as a Lewis base.
(B) HO\(^-\) (Hydroxide ion): The oxygen atom has three lone pairs and a negative charge. It is a strong electron-pair donor and acts as a Lewis base.
(C) F\(^-\) (Fluoride ion): The fluoride ion has four lone pairs and a negative charge. It readily donates an electron pair and acts as a Lewis base.
(D) NH\(_3\) (Ammonia): The nitrogen atom in ammonia has one lone pair of electrons, which it can donate. Therefore, NH\(_3\) acts as a Lewis base.
(E) AlCl\(_3\) (Aluminum chloride): In AlCl\(_3\), the central aluminum atom is bonded to three chlorine atoms. Aluminum is in Group 13 and has only 3 valence electrons. After forming three single bonds, it has only 6 electrons in its valence shell, not a full octet. This electron deficiency makes it eager to accept a pair of electrons to complete its octet. Therefore, AlCl\(_3\) acts as a Lewis acid.
Step 3: Final Answer:
Among the given options, AlCl\(_3\) is the only one that can act as a Lewis acid.