Question

Which of the following does not represent a Lewis acidbase reaction? (a) \(\mathrm{H}_{2} \mathrm{O}+\mathrm{H}^{+} \longrightarrow \mathrm{H}_{3} \mathrm{O}^{+}\) (b) \(\mathrm{NH}_{3}+\mathrm{BF}_{3} \longrightarrow \mathrm{H}_{3} \mathrm{NBF}_{3}\) (c) \(\mathrm{PF}_{3}+\mathrm{F}_{2} \longrightarrow \mathrm{PF}_{5}\) (d) \(\mathrm{Al}(\mathrm{OH})_{3}+\mathrm{OH}^{-} \longrightarrow \mathrm{Al}(\mathrm{OH})_{4}^{-}\)

Short answer

Reaction (c) does not represent a typical Lewis acid-base reaction.

Step-by-step solution

Understand Lewis Acid-Base Reactions

In a Lewis acid-base reaction, a Lewis acid is defined as a species that can accept an electron pair, while a Lewis base is a species that can donate an electron pair. The reaction involves the formation of a coordinate covalent bond where the base donates an electron pair to the acid.

Analyze Reaction (a)

Reaction (a) is \( \mathrm{H}_{2} \mathrm{O} + \mathrm{H}^{+} \longrightarrow \mathrm{H}_{3} \mathrm{O}^{+} \). Here, \( \mathrm{H}_{2} \mathrm{O} \) is donating a pair of electrons to \( \mathrm{H}^{+} \). This is a classic Lewis acid-base reaction, with \( \mathrm{H}^{+} \) acting as the Lewis acid and \( \mathrm{H}_{2} \mathrm{O} \) as the Lewis base.

Analyze Reaction (b)

Reaction (b) is \( \mathrm{NH}_{3} + \mathrm{BF}_{3} \longrightarrow \mathrm{H}_{3} \mathrm{NBF}_{3} \). Here, \( \mathrm{NH}_{3} \) donates an electron pair to \( \mathrm{BF}_{3} \). \( \mathrm{BF}_{3} \) is an electron-deficient species making it a Lewis acid, while \( \mathrm{NH}_{3} \) acts as a Lewis base. This is a Lewis acid-base reaction.

Analyze Reaction (c)

Reaction (c) is \( \mathrm{PF}_{3} + \mathrm{F}_{2} \longrightarrow \mathrm{PF}_{5} \). This reaction involves \( \mathrm{F}_{2} \) donating an electron pair to \( \mathrm{PF}_{3} \), forming \( \mathrm{PF}_{5} \). \( \mathrm{PF}_{3} \) acts as the Lewis acid by accepting the electron pair, and \( \mathrm{F}_{2} \) is the base.

Analyze Reaction (d)

Reaction (d) is \( \mathrm{Al}( ext{OH})_{3} + \mathrm{OH}^{-} \longrightarrow \mathrm{Al}( ext{OH})_{4}^{-} \). Here, hydroxide \( \mathrm{OH}^{-} \) donates an electron pair to \( \mathrm{Al(OH)}_{3} \), forming \( \mathrm{Al(OH)}_{4}^{-} \). This is a typical Lewis acid-base reaction, where \( \mathrm{Al(OH)}_{3} \) is acting as a Lewis acid.

Identify the Non-Lewis Acid-Base Reaction

After analyzing each option, all reactions (a), (b), and (d) can be classified as Lewis acid-base reactions. However, reaction (c) \( \mathrm{PF}_{3} + \mathrm{F}_{2} \longrightarrow \mathrm{PF}_{5} \) does involve a transfer of electron pairs and follows a different mechanism. Although similar, it primarily represents a halogen bond formation rather than a straightforward Lewis acid-base reaction due to the involvement of F2 as a molecule, not a free electron pair donor.

Key concepts

Electron Pair Donation

In a Lewis acid-base reaction, one main player is the Lewis base, which is essential for electron pair donation. This concept revolves around the Lewis base offering a pair of electrons to a Lewis acid, which is a key characteristic of this reaction type. - A Lewis base is rich in electrons and is capable of donating a pair of electrons.- The process of donating these electrons forms a connection known as a **coordinate covalent bond**.- An example of electron pair donation can be seen in water (\(\mathrm{H_2O}\)) donating electrons to a proton, \(\mathrm{H^+}\), which results in the formation of a hydronium ion (\(\mathrm{H_3O^+}\)).Understanding electron pair donation is crucial for deciphering many Lewis acid-base reactions, as it is the initial step that facilitates the formation of new compounds.

Coordinate Covalent Bond

A coordinate covalent bond, also known as a dative bond, is an essential outcome in Lewis acid-base reactions. This type of bond initiates from the electron pair donation by the Lewis base to the Lewis acid.- In a coordinate covalent bond, both electrons in the bond are supplied by just one of the atoms—the Lewis base.- This bond differs from a typical covalent bond, where each atom in the molecule contributes one electron.- An illustrative example is the interaction between ammonia (\(\mathrm{NH_3}\)) and boron trifluoride (\(\mathrm{BF_3}\)), where \(\mathrm{NH_3}\) donates an electron pair to \(\mathrm{BF_3}\) creating a stable molecule, \(\mathrm{H_3NBF_3}\).The creation of a coordinate covalent bond underscores the adaptability and reactivity of molecules in diverse chemical environments, amplifying their potential for forming new substances.

Electron-Deficient Species

In the context of Lewis acid-base reactions, electron-deficient species are vital players, often acting as the Lewis acids. These species are eager to accept electron pairs to fulfill their electron needs and stabilize their structure.- A classic example of an electron-deficient species is \(\mathrm{BF_3}\), which lacks a complete octet of electrons.- Such molecules are highly reactive and ready to accept electron pairs from an electron-rich Lewis base.- Identifying electron-deficient species in reactions helps in predicting the flow of electrons and understanding the potential reactivity of compounds.Recognizing electron-deficient species is crucial for predicting which molecules will act as Lewis acids, thus directing the course of chemical reactions.

Hydronium Ion Formation

The formation of a hydronium ion, \(\mathrm{H_3O^+}\), is a fundamental example of a Lewis acid-base reaction that many students encounter early in their chemistry studies. It exemplifies the straightforward electron pair donation process.- When water (\(\mathrm{H_2O}\)) encounters a proton (\(\mathrm{H^+}\)), it donates a pair of electrons to the proton.- This electron donation results in forming a hydronium ion, where the shared electrons from \(\mathrm{H_2O}\) create a new bond with \(\mathrm{H^+}\).- The water molecule acts as the Lewis base while the proton operates as the Lewis acid in this reaction.Understanding hydronium ion formation helps students grasp the broader concept of proton transfer in acid-base chemistry and its significance in many chemical processes.