Problem 75 - General Chemistry: Principles and Modern Applications

Question

The three following reactions are acid-base reactions according to the Lewis theory. Draw Lewis structures, and identify the Lewis acid and Lewis base in each reaction. (a) \(\mathrm{B}(\mathrm{OH})_{3}+\mathrm{OH}^{-} \longrightarrow\left[\mathrm{B}(\mathrm{OH})_{4}\right]^{-}\) (b) \(\mathrm{N}_{2} \mathrm{H}_{4}+\mathrm{H}_{3} \mathrm{O}^{+} \longrightarrow \mathrm{N}_{2} \mathrm{H}_{5}^{+}+\mathrm{H}_{2} \mathrm{O}\) (c) \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{O}+\mathrm{BF}_{3} \longrightarrow\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{OBF}_{3}\)

Step-by-Step Solution

Answer

For reaction (a), \(\mathrm{B}(\mathrm{OH})_{3}\) is the Lewis acid and \(\mathrm{OH}^{-}\) is the Lewis base. For reaction (b), \(\mathrm{H}_{3} \mathrm{O}^{+}\) is the Lewis acid and \(\mathrm{N}_{2} \mathrm{H}_{4}\) is the Lewis base. For reaction (c), \(\mathrm{BF}_{3}\) is the Lewis acid and \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{O}\) is the Lewis base.

Step 1: Chemical Reaction A - Lewis Structures and Identification

Given the reaction \(\mathrm{B}(\mathrm{OH})_{3}+\mathrm{OH}^{-} \longrightarrow\left[\mathrm{B}(\mathrm{OH})_{4}\right]^{-}\), \(\mathrm{B}(\mathrm{OH})_{3}\) becomes \(\left[\mathrm{B}(\mathrm{OH})_{4}\right]^{-}\), and thus accepts an electron pair from \(\mathrm{OH}^{-}\), so it is the Lewis acid. \(\mathrm{OH}^{-}\) donates an electron pair, so it's the Lewis base.

Step 2: Chemical Reaction B - Lewis Structures and Identification

Given the reaction \(\mathrm{N}_{2} \mathrm{H}_{4}+\mathrm{H}_{3} \mathrm{O}^{+} \longrightarrow \mathrm{N}_{2} \mathrm{H}_{5}^{+}+\mathrm{H}_{2} \mathrm{O}\), \(\mathrm{H}_{3} \mathrm{O}^{+}\) becomes \(\mathrm{H}_{2} \mathrm{O}\) by getting an electron from \(\mathrm{N}_{2} \mathrm{H}_{4}\). So, \(\mathrm{H}_{3} \mathrm{O}^{+}\) is the Lewis acid while \(\mathrm{N}_{2} \mathrm{H}_{4}\) is the Lewis base.

Step 3: Chemical Reaction C - Lewis Structures and Identification

Given the reaction \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{O}+\mathrm{BF}_{3} \longrightarrow\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{OBF}_{3}\), \(\mathrm{BF}_{3}\) becomes \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{OBF}_{3}\) by accepting a pair of electrons. So, \(\mathrm{BF}_{3}\) is the Lewis acid, while \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{O}\) by donating a pair of electrons is the Lewis base.

Key Concepts

Lewis structures

Lewis structures provide a visual representation of the arrangement of atoms and electrons in a molecule. They are fundamental when discussing Lewis acid-base reactions.
Understanding these structures helps in identifying how molecules interact in chemical reactions. In Lewis structures, each atom is represented by its chemical symbol. Bonding pairs and lone pairs of electrons are shown as dashes or dots.

When drawing Lewis structures, keep these points in mind:

• Identify the central atom, often the least electronegative one, except for hydrogen.
• Count all valence electrons of the atoms involved.
• Distribute electrons among atoms, first creating a single bond between atoms and then using remaining electrons to satisfy the octet rule for each atom.

This systematic approach helps in clearly visualizing how atoms are bonded and where potential chemical reactivity may occur.

Electron pair donation

A key aspect of Lewis acid-base theory is the concept of electron pair donation. A Lewis base is defined by its ability to donate an electron pair to a Lewis acid, which accepts the electron pair.
This interaction forms a coordinate covalent bond. In essence, the base donates a lone pair of electrons that the acid lacks.

For example, in the reaction a) \( \mathrm{B}(\mathrm{OH})_{3} + \mathrm{OH}^{-} \rightarrow \left[\mathrm{B}(\mathrm{OH})_{4}\right]^{-} \),\(\mathrm{OH}^{-}\) donates an electron pair to \(\mathrm{B}(\mathrm{OH})_{3}\), allowing \(\mathrm{B}(\mathrm{OH})_{3}\) to form a new bond.

• The base (electron pair donor) provides the electrons needed for bond formation.
• The acid (electron pair acceptor) is usually electron-deficient, making it receptive to electron pairs.

Understanding electron pair donation is crucial for predicting outcomes of chemical reactions and understanding molecular interactions.

Chemical reactivity

Chemical reactivity refers to the propensity of a substance to engage in chemical reactions. It is deeply influenced by a compound's structure and the presence of reactive sites within a molecule, such as lone electron pairs or electron-deficient atoms.
Lewis acids, characterized by their electron deficiency, and Lewis bases, known for their electron pair donation capacity, form specific interactions that dictate reactivity patterns in molecules.

In the reaction b) \( \mathrm{N}_{2} \mathrm{H}_{4} + \mathrm{H}_{3} \mathrm{O}^{+} \rightarrow \mathrm{N}_{2} \mathrm{H}_{5}^{+} + \mathrm{H}_{2} \mathrm{O} \),\(\mathrm{H}_{2} \mathrm{O}\) form results from the interaction of \(\mathrm{H}_{3} \mathrm{O}^{+}\) (acid) and \(\mathrm{N}_{2} \mathrm{H}_{4}\) (base), illustrating the reactivity potential due to electron pair involvement.

Molecular properties like electronegativity, geometry, and the presence of substituents also influence chemical reactivity and are essential to consider:

• Electronegativity differences impact how electrons are distributed across a molecule, affecting reactivity.
• Molecular geometry can dictate how easily molecules can interact with each other.

Understanding these factors aids in predicting and controlling chemical reactions in both organic and inorganic systems.