Problem 84 - Chemistry: Atoms First

Question

Boric acid, \(\mathrm{H}_{3} \mathrm{BO}_{3}\), is not a Bronsted-Lowry acid but a Lewis acid. (a) Write an equation for its reaction with water. (b) Predict the shape of the anion thus formed. (c) What is the hybridization on the boron consistent with the shape you have predicted?

Step-by-Step Solution

Answer

The equation for the reaction with water is \( \mathrm{H}_{3}\mathrm{BO}_{3} + 2\mathrm{H}_{2}O \rightarrow \mathrm{B}(\mathrm{OH})_{4}^{-} + \mathrm{H}_{3}O^{+} \). The anion formed, \( \mathrm{ B}(\mathrm{OH})_{4}^{-} \), has a tetrahedral shape. The hybridization of boron in this anion is sp3.

Step 1: Writing the Reaction with Water

To write the equation for the reaction of boric acid with water, we recognize that it is a Lewis acid, which means it can accept a pair of electrons. The reaction involves the lone pair of electrons on the water's oxygen atom being donated to the empty orbital of boron, forming a coordinate covalent bond: \[ \mathrm{H}_{3}\mathrm{BO}_{3} + 2\mathrm{H}_{2}O \rightarrow \mathrm{B}(\mathrm{OH})_{4}^{-} + \mathrm{H}_{3}O^{+} \]

Step 2: Predicting the Shape of the Anion

The anion formed, \( \mathrm{B}(\mathrm{OH})_{4}^{-} \), will have a tetrahedral shape. This is because the boron atom has four groups of electrons around it now that it has accepted an additional pair of electrons from water. These groups repel each other equally, assuming a tetrahedral configuration to minimize repulsion.

Step 3: Determining the Hybridization of Boron

The hybridization of the boron atom in the \( \mathrm{B}(\mathrm{OH})_{4}^{-} \) anion can be concluded from its tetrahedral shape. In a tetrahedral molecule, the central atom is hybridized to have four equivalent sp3 hybrid orbitals, which accommodates the four groups attached to it. Therefore, the hybridization of boron in this anion is sp3.

Key Concepts

Boric Acid Reaction

Boric acid, commonly represented as \( \mathrm{H}_3\mathrm{BO}_3 \), is a unique compound in the world of acids because it behaves as a Lewis acid, rather than the more commonly discussed Brønsted-Lowry acids. Understanding Lewis acid reactions is crucial since they expand upon the traditional acid-base reactions by including non-proton exchange reactions. In the case of boric acid, the reaction with water does not involve the release of a proton, but rather the acceptance of an electron pair.

Boric acid's reaction with water is illustrated by the chemical equation \[ \mathrm{H}_3\mathrm{BO}_3 + 2\mathrm{H}_2O \rightarrow \mathrm{B}(\mathrm{OH})_4^{-} + \mathrm{H}_3O^{+} \]. Here, boric acid accepts a pair of electrons from the hydroxide ion (\(\mathrm{OH}^{-}\)), which comes from water. This interaction results in the formation of a negative ion, \(\mathrm{B}(\mathrm{OH})_4^{-}\), and a hydronium ion, \(\mathrm{H}_3O^{+}\), signifying a complete Lewis acid reaction.

Chemical Hybridization

Chemical hybridization is a key concept in understanding how atoms bond in molecules to form specific shapes. At the core of chemical hybridization is the idea that atomic orbitals within an atom can mix, or 'hybridize,' to create new orbitals that can optimally overlap with orbitals from other atoms during bonding.

In boric acid's reaction with water, the central boron atom undergoes hybridization. Before reacting, the boron atom in boric acid possesses an empty p orbital that is capable of accepting a pair of electrons. Upon reaction, this boron atom undergoes sp3 hybridization. This means one s orbital and three p orbitals on the boron atom combine to form four equivalent sp3 hybrid orbitals. Each of these can form a sigma bond with a hydroxide group, leading to the overall stability of the resultant \(\mathrm{B}(\mathrm{OH})_4^{-}\) ion.

Molecular Geometry

Molecular geometry, which refers to the three-dimensional arrangement of atoms in a molecule, plays a fundamental role in determining the properties and reactivity of molecules. When predicting the shape of molecules such as the anion formed from boric acid, VSEPR (Valence Shell Electron Pair Repulsion) theory comes into play. According to this theory, the shape of a molecule is dictated by the repulsions between electron pairs located around a central atom.

For the boric acid reaction product, \(\mathrm{B}(\mathrm{OH})_4^{-}\), the molecular geometry is tetrahedral. This is because the boron atom in the center of the anion is surrounded by four groups of electrons—the four hydroxide ions. These electron groups repel each other and naturally adopt an arrangement in space that minimizes their mutual repulsions, leading to a tetrahedral shape. This geometric arrangement is critical to understanding many chemical reactions and interactions, including how complex molecules like proteins fold and how drugs fit into their target sites within the body.