Which of the following reactions is an example of a Lewis acid–base reaction, but not a Brønsted–Lowry acid–base reaction? (A) NH3 plus BH3 reacts reversibly to produce NH3BH3. (B) NH3 plus HCl reacts reversibly to produce NH4Cl. (C) NH3 plus CH3COOH reacts reversibly to produce NH4 1+ plus CH3COO 1−. (D) NH3 plus HNO3 reacts reversibly to produce NH4NO3. Or (E) NH3 plus H2O reacts reversibly to produce NH4 1+ plus OH 1−.
A Lewis acid–base reaction happens when a pair of electrons are donated from one species to an electron-deficient species. The species which donates the electron pair is called a Lewis base, while the substance that accepts the lone pair of electrons is called the Lewis acid. The type of bond that forms between Lewis acids and Lewis bases are called coordinate covalent bonds, which we can represent using an arrow to demonstrate which species are donating and receiving the electron pair.
A coordinate covalent bond can be defined as a covalent bond where the pair of electrons forming the bond is donated from one atom only. We can compare this type of reaction to a Brønsted–Lowry acid–base reaction. In a Brønsted–Lowry acid–base reaction, the species behaving as the Brønsted–Lowry acid loses protons, which in this case the term protons refers to hydrogen ions, while the Brønsted–Lowry base is the substance that gains protons. So a Brønsted–Lowry base gains hydrogen ions, while Lewis bases donate electron pairs. Brønsted–Lowry acids donate hydrogen ions, while Lewis acids gain electron pairs. So although almost all Brønsted–Lowry acids are also Lewis acids, it should be noted that not all Lewis acids are Brønsted–Lowry acids.
Using this information, let’s have a look at our answer choices. We can see that ammonia is present in all of the reactions. Because of its lone pair are nitrogen, ammonia can behave as a Brønsted–Lowry base by accepting a hydrogen ion, and it can behave as a Lewis base by donating an electron pair. In answer choice (A), the reaction between ammonia and BH3, we can see that boron is electron deficient with only six electrons in its valence shell. In this reaction, nitrogen donates an electron pair, forming a coordinate covalent bond with boron and producing NH3BH3. This reaction is a Lewis acid–base reaction as it involves the donation of an electron pair and is not a Brønsted–Lowry acid–base reaction as there is no hydrogen ion transfer occurring.
Therefore, the reaction that is an example of a Lewis acid–base reaction but not a Brønsted–Lowry acid–base reaction is answer choice (A). NH3 plus BH3 reacts reversibly to produce NH3BH3.