Problem 94 - 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 - 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) Connect boron to three OH groups. (b) Boron is \(sp^2\) hybridized. (c) Illustrate hydrogen bonds between OH groups and neighboring molecules.

Step 1: Drawing the Lewis Structure

To draw the Lewis structure of boric acid, \(\mathrm{B(OH)}_3\), first identify the atoms involved. Boron (\(\mathrm{B}\)) can form three covalent bonds. Each \(\mathrm{OH}\) group has one oxygen (\(\mathrm{O}\)) and one hydrogen (\(\mathrm{H}\)), with the oxygen acting as the central atom in the group. The three \(\mathrm{OH}\) groups bond to the boron atom. Begin by placing the boron atom in the center and draw single bonds connecting it to each of the three \(\mathrm{OH}\) groups. Each oxygen will then have two lone pairs of electrons while sharing a single bond with hydrogen and boron.

Step 2: Determining Boron's Hybridization

Hybridization of an atom is determined by the number of groups (bonds and lone pairs) attached to it. For boron in \(\mathrm{B(OH)}_3\), there are three \(\mathrm{OH}\) groups bonded directly to it. Therefore, boron is surrounded by three regions of electron density, which implies \(sp^2\) hybridization. This is because three atomic orbitals (one \(s\) and two \(p\)) mix to form three \(sp^2\) hybrid orbitals.

Step 3: Sketching Hydrogen Bonding in Solid State

In the solid state, hydrogen bonding occurs between the \(\mathrm{-OH}\) groups of adjacent molecules. Sketch a neighboring boric acid molecule close to the first one, ensuring that the \(\mathrm{-OH}\) groups of one molecule can form hydrogen bonds with the \(\mathrm{OH}\) groups of the neighboring molecule. These hydrogen bonds are represented by dashed lines linking the hydrogen of one \(\mathrm{OH}\) group with the oxygen of another. Each oxygen forms hydrogen bonds with nearby hydrogens from adjacent molecules, stabilizing the solid structure.

Key Concepts

Lewis Structure

The Lewis structure is a simple way to represent molecules and their bonding in terms of the electrons shared between atoms. For boric acid (\( \mathrm{B(OH)}_3 \)), we begin with identifying the components: a boron atom bonded to three hydroxyl (\( \mathrm{OH} \)) groups. In the Lewis structure:

• Place the boron (\( \mathrm{B} \)) atom in the center.
• Draw single lines from boron to each of the three \( \mathrm{OH} \) groups. These lines represent single covalent bonds.
• Oxygen is the more electronegative atom in \( \mathrm{OH} \), and therefore, it carries two lone pairs of electrons. Thus, each oxygen in \( \mathrm{OH} \) has two lone pairs while sharing a bond with both hydrogen and boron.

This simple approach allows us to visually capture the balance of covalent bonds and lone pairs around boron, ensuring the representation respects the \( \mathrm{octet} \) rule wherever applicable, even though boron is an exception as it is stable with six valence electrons.
By understanding Lewis structures, students can better grasp how atoms share electrons, which is fundamental to predicting molecule geometry and reactivity.

Hybridization

Hybridization in chemistry helps us understand the shape and bonding of molecules by explaining how atomic orbitals mix to form hybrids. For boron in boric acid (\( \mathrm{B(OH)}_3 \)), this concept is vital:

• Boron is bonded to three \( \mathrm{OH} \) groups.
• We see three areas of electron density around boron.

When we speak of hybridization here, we mean how boron's atomic orbitals combine:

• Boron mixes one \( s \) orbital with two \( p \) orbitals, resulting in three \( sp^2 \) hybrid orbitals.

This \( sp^2 \) hybridization allows for the planar trigonal arrangement around boron, meaning the \( \mathrm{OH} \) groups are 120 degrees apart. Understanding this helps clarify not just the shape but the potential reactivity and interaction of molecules akin to boric acid.
The information about hybridization enriches our grasp of molecular geometry, influencing properties and how the molecule forms further connections through hydrogen bonds.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction, which occurs in compounds where a hydrogen atom is directly bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine. In boric acid (\( \mathrm{B(OH)}_3 \)), here's how hydrogen bonding plays out:

• The hydrogen atoms in one \( \mathrm{OH} \) group can form hydrogen bonds with the oxygen atoms of another molecule's \( \mathrm{OH} \) group.
• These bonds are not as strong as covalent bonds but significantly influence the structure and stability in the solid state.

How do they arrange? Picture one boric acid molecule next to another:

• Dotted lines represent hydrogen bonds, indicating the attraction between the \( \mathrm{O} \) on one molecule and the \( \mathrm{H} \) from another \( \mathrm{OH} \) group.

These hydrogen bonds result in a network that stabilizes the solid form of the compound. Recognizing and sketching these interactions are useful, as they demonstrate how molecules assemble collectively beyond simple covalent bonding.This vital concept not only explains the molecular arrangement and properties but also sets the stage for understanding more complex biochemical interactions. Understanding hydrogen bonding expands knowledge in various fields, including development of novel materials and comprehension of biological molecules.