Problem 98 - Chemistry and Chemical Reactivity

Question

The boron atom in boric acid, \(\mathrm{B}(\mathrm{OH})_{3},\) is bonded to three - OH groups. In the solid state, the \(-\mathrm{OH}\) groups are in turn hydrogen-bonded to - OH groups in neighboring molecules. (a) Draw the Lewis structure for boric acid. (b) What is the hybridization of the boron atom in the acid? (c) Sketch a picture showing how hydrogen bonding can occur between neighboring molecules.

Step-by-Step Solution

Answer

(a) Draw the Lewis structure with boron bonded to three \(-\mathrm{OH}\) groups. (b) The boron's hybridization is \(\mathrm{sp}^2\). (c) Neighboring molecules form hydrogen bonds between \(-\mathrm{OH}\) groups.

Step 1: Draw the Lewis Structure

Boric acid, \(\mathrm{B} (\mathrm{OH})_3\), consists of one boron atom and three hydroxyl (\(-\mathrm{OH}\)) groups. Boron has 3 valence electrons and each \(-\mathrm{OH}\) group contributes one valence electron from the oxygen atom and one hydrogen. The boron atom forms a single bond with each of the three oxygen atoms. Each oxygen atom, in turn, forms a single bond with a hydrogen atom. The octet of each oxygen atom is completed with lone pairs, making two lone pairs on each oxygen.

Step 2: Determine the Hybridization of Boron

The hybridization of an atom is determined by the number of regions of electron density around it. In boric acid, boron forms three single covalent bonds with the oxygen atoms. This set-up involves three areas of electron density, indicating \(\mathrm{sp}^2\) hybridization for the boron atom. This hybridization results in a trigonal planar structure around the boron atom.

Step 3: Illustrate Hydrogen Bonding

In the solid state, the \(-\mathrm{OH}\) groups of boric acid are hydrogen-bonded with \(-\mathrm{OH}\) groups of neighboring molecules. Each hydrogen atom in an \(-\mathrm{OH}\) group can act as a hydrogen bond donor, interacting with lone pairs on an oxygen atom of an adjacent molecule's \(-\mathrm{OH}\) groups. This creates an extended hydrogen-bonded network between the boric acid molecules.

Key Concepts

Lewis Structure of Boric Acid

The Lewis structure is a visual representation showing how atoms connect in a molecule. For boric acid, \(\mathrm{B}(\mathrm{OH})_3\), understanding this structure is key to understanding its bonding.
Boric acid consists of a central boron atom bonded to three hydroxyl \(\mathrm{OH}\) groups.
Boron itself has three valence electrons. Each \(\mathrm{OH}\) group contributes with one electron from the oxygen atom and one from the hydrogen to form single bonds with boron.
This means the boron atom forms a single covalent bond with each oxygen atom.
Additionally, each oxygen atom bonds with a hydrogen atom to complete the \(\mathrm{OH}\) group and brings its octet to completion with two lone pairs of electrons. In essence, each oxygen atom forms a single bond with its respective hydrogen.
This distinct arrangement highlights how all valence electrons are utilized in bonding except for those forming lone pairs on oxygen.

Boron Hybridization in Boric Acid

Hybridization is the concept that involves mixing atomic orbitals to create new hybrid orbitals suitable for pairing electrons.
In boric acid, boric atom's hybridization is determined by the electron domains surrounding it. With boric acid, the boron atom forms three single covalent bonds with the oxygen atoms of the \(\mathrm{OH}\) groups, resulting in three electron-density areas.
This arrangement indicates \(\mathrm{sp}^2\) hybridization for the boron atom.
The \(\mathrm{sp}^2\) hybridization produces three equivalent hybrid orbitals laid out in a trigonal planar shape around the boron.
This configuration allows the boron atom to efficiently overlap orbitals to form strong covalent bonds with each oxygen atom, while leaving no lone pairs on boron itself. It is this hybridization that is critical in giving boric acid its planar structure.

Hydrogen Bonding in Boric Acid

Hydrogen bonding is a type of intermolecular force that occurs when a hydrogen atom covalently bonded to a more electronegative atom, like oxygen, interacts with another electronegative atom.
In solid-state boric acid, this phenomenon is observed with the \(\mathrm{OH}\) groups. The hydrogen in each \(\mathrm{OH}\) acts as a bond donor, forming a hydrogen bond with the lone pair electrons on an oxygen atom of an adjacent molecule's hydroxyl group.
The hydrogen bonds are responsible for linking neighboring boric acid molecules together, creating a larger network.
This network enhances characteristic properties, such as increased stability and higher boiling and melting points, attributed to the strength of hydrogen bonds.

Understanding Electron Density in Boric Acid

Electron density refers to the probability of finding electrons in a particular area within a molecule. It plays a fundamental role in determining molecular shape and bond formation.
In boric acid, electron density is found around the bonds connecting the boron to the \(\mathrm{OH}\) groups. Each bond consists of shared electron pairs, composing areas of electron density.
These zones influence the molecular geometry as electron pairs repel each other, arranging themselves in a way to minimize repulsion. In boric acid, electron density around the bonded atoms results in the \(\mathrm{sp}^2\) hybridization that supports a trigonal planar shape.
Comprehending the distribution and interaction of electron density helps explain the structural and bonding aspects of boric acid.

Trigonal Planar Structure of Boric Acid

The trigonal planar structure arises from the spatial arrangement of three substituents surrounding a central atom.
In boric acid, the boron atom at the center has three \(\mathrm{OH}\) groups attached through single bonds.
The \(\mathrm{sp}^2\) hybridization of boron dictates that the \(\mathrm{OH}\) groups spread out evenly around the central atom.
This arrangement naturally forms a trigonal planar structure with bond angles of approximately 120°.
The planarity of the molecule is crucial for maintaining uniformity in bonding symmetry, and along with the absence of lone pairs on boron, it contributes to the chemical behavior and reactivity of boric acid.