Question

There are two crystalline forms of oxalic acid, \(\mathrm{HOOC}-\mathrm{COOH},\) both involving hydrogen bonding. The alpha form consists of three-dimensional zig-zag chains of oxalic acid molecules; the beta form has long, linear chains of oxalic acid molecules. Draw Lewis structures that illustrate hydrogen bonding in each form.

Short answer

Alpha form has zig-zag chains, beta form has linear chains due to hydrogen bonding patterns.

Step-by-step solution

Understand Oxalic Acid Structure

Oxalic acid has the chemical formula \( ext{HOOC-COOH} \). Each molecule consists of two carboxylic acid groups (COOH). A carboxylic acid group contains a carbon double-bonded to an oxygen (carbonyl) and an OH group, where hydrogen bonding can occur.

Draw the Basic Lewis Structure

Start by drawing the basic Lewis structure for one molecule of oxalic acid (HOOC-COOH). You need two carbon atoms connected by a single bond. Each carbon connects to a carboxyl group (C=O with an -OH), ensuring all atoms have complete octets.

Illustrate Hydrogen Bonding in Alpha Form

In the alpha form, oxalic acid molecules are connected in a zig-zag pattern due to hydrogen bonds. Connect the hydrogen of the -OH group on one molecule to the oxygen of the C=O group on a neighboring molecule to make zig-zag chains.

Illustrate Hydrogen Bonding in Beta Form

In the beta form, oxalic acid molecules form linear chains. Align the molecules so that the hydrogen bonds form straight lines. Again, connect the hydrogen of the -OH group to the oxygen of the C=O group in adjacent molecules.

Key concepts

Oxalic Acid

Oxalic acid, which is represented by the chemical formula \( \text{HOOC-COOH} \), is a simple dicarboxylic acid. It consists of two carboxylic acid groups joined by two carbon atoms. Each carboxylic acid group contains a carbon atom double-bonded to an oxygen atom (the carbonyl group) and a hydroxyl group (\(-OH\)). This specific arrangement allows the molecule to participate in hydrogen bonding, a vital characteristic for forming different crystalline structures.

Oxalic acid is naturally found in various plants and is responsible for the sharp taste of rhubarb and spinach. In its pure form, oxalic acid appears as a colorless crystalline solid. The presence of two carboxyl groups allows it to form strong hydrogen bonds, which leads to the creation of distinct patterns depending on how the molecules align themselves.

• It is a strong organic acid with many industrial and household applications.
• Its high propensity for hydrogen bonding makes oxalic acid unique in forming different crystalline structures like alpha and beta forms.

Crystalline Structures

Crystalline structures refer to the organized and repeating arrangement of atoms or molecules in a solid. Oxalic acid can form different crystalline structures, specifically the alpha and beta forms, based on how its molecules are arranged.

In the alpha form, molecules of oxalic acid connect in a zig-zag pattern through hydrogen bonding. This complex arrangement creates a three-dimensional network that stabilizes the structure. The zig-zag assembly results from the way the hydrogen of one molecule's \(-OH\) group forms a hydrogen bond with the \(C=O\) group of a neighboring molecule.

In contrast, the beta form of oxalic acid features long, linear chains. Each molecule is positioned such that the hydrogen bonds form straight lines. This form is simpler than the alpha form but is still stabilized by hydrogen bonding between adjacent molecules.

• Alpha form is complex and zig-zagged, creating a 3D network.
• Beta form is simpler with straight linear chains.
• Hydrogen bonding is crucial in determining both forms.

Lewis Structures

Lewis structures are diagrams that represent the bonds between the atoms of a molecule and the lone pairs of electrons that may exist. When drawing Lewis structures for oxalic acid, they must show both the covalent bonds between atoms and the potential hydrogen bonds that can form.

To draw the Lewis structure for oxalic acid, you start with the backbone of the two carbon atoms bonded together. Each carbon atom in the structure is connected to a \(C=O\) group and an \(-OH\) group, representing the carboxylic acid moiety. Ensure each atom achieves a complete octet by correctly positioning the electrons.
When illustrating hydrogen bonding in the crystalline structures of oxalic acid, pay attention to how the \(-OH\) group's hydrogen links to the \(C=O\) group's oxygen, either zig-zagging for the alpha form or in straight lines for the beta form. Accurately drawing these connections helps visualize how the molecules hold together in each crystalline type.

• Focus on representing hydrogen bonds clearly.
• Ensure that all oxygen and hydrogen bonds are correctly depicted.