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Question Video: Identifying the Species That Would Not Act as a Lewis Base Chemistry

Which of the following species is unlikely to act as a Lewis base? [A] OH⁻ [B] NH₃ [C] Al³⁺ [D] F⁻ [E] H₂O

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Video Transcript

Which of the following species is unlikely to act as a Lewis base? (A) OH−, (B) NH3, (C) Al3+, (D) F−, or (E) H2O.

To answer this question, we must understand the definition of a Lewis base. A Lewis base is a substance that can donate a lone pair of electrons forming a bond. The bond that is formed is called a coordinate covalent bond and results from a Lewis acid–base reaction. A coordinate covalent bond is a covalent bond where the pair of electrons forming the bond is donated from one atom only. The species that is acting as the Lewis base donates the pair of electrons to form the coordinate covalent bond and can be considered the species that is electron rich, while the species receiving the electron pair, or the Lewis acid, can be considered electron deficient. The product which is formed in a Lewis acid–base reaction is called an adduct. The adduct is formed by the coordinate covalent bond between a Lewis acid and a Lewis base.

With this information in mind, let’s have a look at our answer choices. Answer choice (A) OH− or the hydroxide ion is an electron-rich species with three lone pairs of electrons and thus would act as a Lewis base. Since the question is asking us to identify the species that is unlikely to act as a Lewis base, we can eliminate answer choice (A). Answer choice (B) NH3 or ammonia is also considered electron rich with one lone pair of electrons and can behave as a Lewis base. We can eliminate answer choice (B).

Answer choice (C) Al3+ or the aluminum cation would be considered electron deficient with no lone pairs in its valence shell and would therefore be more likely to act as a Lewis acid. Answer choice (C) is the correct answer. Let’s rule out answer choices (D) and (E) just to be sure. Answer choice (D) F− or the fluoride ion is electron rich with four lone pairs of electrons and would behave as a Lewis base. Finally, answer choice (E) H2O or water is an electron-rich species with two lone pairs on the oxygen atom and can behave as a Lewis base.

Therefore the species that is unlikely to act as a Lewis base is answer choice (C), Al3+.

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