Question

What type of infrared absorption bands due to hydroxyl groups would you expect for trans-cyclobutane-1,2-diol and butane-1,2-diol (a) in very dilute solution, (b) in moderately concentrated solution, and (c) as pure liquids? Give your reasoning.

Short answer

Increased hydrogen bonding from dilute to pure forms broadens and shifts OH IR bands to lower frequencies.

Step-by-step solution

Understanding Hydroxyl Groups

Before we analyze the infrared absorption bands, it's important to understand how hydroxyl (OH) groups behave. Hydroxyl groups can participate in hydrogen bonding, affecting their infrared (IR) absorption characteristics.

Infrared Spectroscopy Basics

In infrared spectroscopy, hydroxyl (OH) groups typically show a strong broad band due to O-H stretching. The position and shape of this band can indicate the type and extent of hydrogen bonding.

Analyzing Dilute Solutions

In very dilute solutions of trans-cyclobutane-1,2-diol and butane-1,2-diol, the hydroxyl groups are more isolated and typically do not interact strongly with each other. As a result, the IR absorption band for the OH stretching is usually sharp and appears around 3600-3650 cm\(^{-1}\) for isolated OH groups.

Analyzing Moderately Concentrated Solutions

As the concentration increases in solution, hydroxyl groups come in closer proximity leading to more intermolecular hydrogen bonding. This results in a broader and slightly red-shifted (lower wavenumber) IR band for the OH stretching, typically seen around 3400-3500 cm\(^{-1}\).

Analyzing Pure Liquids

For pure liquids, like trans-cyclobutane-1,2-diol and butane-1,2-diol, there is extensive hydrogen bonding among OH groups. This results in even broader and further red-shifted IR absorption bands, frequently appearing between 3200-3400 cm\(^{-1}\). Such extensive hydrogen bonding also causes a band to emerge for in-plane bending.

Key concepts

Hydroxyl Groups

Hydroxyl groups are functional groups characterized by an oxygen atom bonded to a hydrogen atom, represented as -OH. They are commonly found in alcohols and can significantly influence a molecule's properties in various chemical reactions. When present in a molecule, these groups are highly polar, making them capable of forming hydrogen bonds.
This ability to engage in hydrogen bonding results in unique physical characteristics and reactivity. These traits are crucial when examining a compound's infrared spectroscopy because the bonding environment directly affects the observed absorption bands.

Hydrogen Bonding

Hydrogen bonding is a vital intermolecular force where a hydrogen atom connects with an electronegative atom such as oxygen, nitrogen, or fluorine. This type of bonding is much weaker than covalent or ionic bonds but is stronger than van der Waals forces.
In hydrogen bonding, the positive hydrogen atom is attracted to the lone electron pairs on electronegative elements. This gives water its high boiling point and ice its unique crystal structure. In the case of alcohols, like trans-cyclobutane-1,2-diol and butane-1,2-diol, hydrogen bonds occur between the hydroxyl groups, altering their physical properties and IR spectroscopy data.

OH Stretching Band

The OH stretching band is a key feature in infrared spectroscopy. It represents the vibrational movement between the oxygen and hydrogen atoms in a hydroxyl group.
This band is significant because its position and shape can reveal crucial information about intermolecular forces like hydrogen bonding in the sample. For isolated OH groups, such as in very dilute solutions, the IR absorption band appears sharp and at a higher wavenumber, typically between 3600-3650 cm\(^{-1}\). As intermolecular interaction increases, this band tends to become broader and shift to lower wavenumbers.

Wavenumber

In infrared spectroscopy, the wavenumber is the unit used to express frequencies of light absorbed by molecules. It is defined as the number of wave cycles in a unit length and is usually measured in reciprocal centimeters (cm\(^{-1}\)).
The wavenumber is inversely proportional to wavelength and directly proportional to frequency. It is crucial in identifying functional groups within a sample since each functional group, like the hydroxyl group, absorbs at specific wavenumber ranges due to its characteristic vibrational frequencies.

Intermolecular Hydrogen Bonding

Intermolecular hydrogen bonding occurs between molecules and significantly affects their properties, including boiling and melting points, solubility, and the behavior in infrared spectroscopy.
In more concentrated solutions or pure liquids, hydrogen bonding between hydroxyl groups leads to extensive network formation. This bonding results in broader and shifted OH stretching bands in the IR spectrum, as seen at lower wavenumbers, usually between 3200-3400 cm\(^{-1}\). Such bonding causes noticeable changes in the overall spectroscopic profile, crucial for understanding how molecules interact in different states or concentrations.