Question

The compounds \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}\) and \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) both show parent ions in their mass spectra at \(m / z=62 .\) How do the fragmentation patterns allow you to distinguish between the two compounds?

Short answer

Ethanolthiol shows sulfur-based fragment ions, while ethylene glycol loses water, distinguishing them.

Step-by-step solution

Analyze the Molecular Structure

First, we need to examine the molecular structure of each compound. The compound \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH} \) is an thiol, specifically ethanolthiol, whereas \( \mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH} \) is a diol, specifically ethylene glycol.

Consider the Mass Spectra Characteristics

In mass spectrometry, different functional groups often fragment in characteristic ways. The thiol group \( \mathrm{-SH} \) in ethanolthiol can lead to specific fragmentation patterns, often resulting in the loss of \( \mathrm{HS}^+ \) or \( \mathrm{H}^+ \). For ethylene glycol, the presence of two hydroxyl groups \( \mathrm{-OH} \) can lead to fragmentation patterns showing losses such as water \( \mathrm{H}_2\mathrm{O} \), resulting in distinct peaks.

Identify Key Fragment Ions

For \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH} \), we should expect to see fragment ions resulting from the cleavage of the \( \mathrm{C-S} \) bond. Common fragment ions might appear at \( m/z = 31 \), corresponding to \( \mathrm{CH}_3 \mathrm{CH}_2^+ \), and around \( m/z = 45 \), indicative of loss of \( \mathrm{H}_2\mathrm{S} \). For \( \mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH} \), fragmentation due to the loss of water can create a peak at \( m/z = 44 \) or fragments due to breaking of \( \mathrm{C-O} \) bonds can lead to other characteristic peaks.

Compare Fragmentation Patterns

Compare the fragmentation patterns: ethanolthiol will likely show significant peaks associated with sulfur-based fragment ions, possibly due to the \( \mathrm{C-S} \) bond breakage. Ethylene glycol will instead display peaks that reflect its ability to lose water (\( m/z = 44 \)) and may not have any sulfur-specific peaks. These differences help us distinguish one compound from the other.

Key concepts

Fragmentation Patterns

In mass spectrometry, when molecules are ionized and fragmented, they break down into smaller pieces that form distinct patterns called fragmentation patterns. These patterns are like fingerprints for different compounds and are crucial for identifying them. Each type of bond in a molecule can break in characteristic ways, leading to specific fragment ions.

For example, a molecule containing a sulfur atom, like ethanolthiol (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH} \)), will often display fragmentation patterns including sulfur-based fragments. You might notice fragments that reveal a loss of parts like \( \mathrm{HS}^+ \) or other sulfur fragments. Such features cause peaks at specific mass-to-charge (\( m/z \)) ratios which help identify the molecule uniquely.

Conversely, a molecule like ethylene glycol (\( \mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH} \)) typically fragments in a way that showcases its two hydroxyl groups \( \mathrm{-OH} \). The common fragmentation involves the loss of water, which is noticeable as a peak at \( m/z = 44 \). By analyzing these patterns, chemists distinguish between similar compounds by identifying unique peaks in their spectra.

Functional Groups

Functional groups are sets of atoms within molecules that impart specific chemical properties to those molecules. They are key to understanding reactivity and fragmentation patterns in mass spectrometry. Functional groups affect how a molecule behaves under the conditions of spectrometry and which fragments are likely to be observed.

The thiol group \( \mathrm{-SH} \) in ethanolthiol is one such functional group. It has unique properties that lead to characteristic fragmentation patterns, such as the cleavage of the sulfur-hydrogen bond. This can produce fragment ions, making it easier to identify the presence of sulfur or specific thiol compounds in a sample.

Diols like ethylene glycol have two hydroxyl groups \( \mathrm{-OH} \), resulting in different fragmentation behaviors. Diols can lose water molecules during fragmentation, creating subtle shifts in the spectrum and unique peaks like \( m/z = 44 \). By focusing on these patterns, scientists leverage the presence of specific functional groups to accurately interpret mass spectrometry results.

Thiol and Diol Structures

Understanding the structures of thiols and diols is essential for interpreting their mass spectra. Thiols, characterized by the presence of the \( \mathrm{-SH} \) group, have unique properties due to the sulfur atom. Ethanolthiol, for instance, can form ions by losing parts of its thiol group or other sulfur-related fragments, which help distinguish it in spectral analysis.

Diols, on the other hand, are defined by having two hydroxyl \( \mathrm{-OH} \) groups. In ethylene glycol, this dual presence leads to the possibility of losing water during fragmentation. This behavior results in identifiable peaks such as \( m/z = 44 \), which can help differentiate diols from thiols in mass spectrometry.

• Thiol Structure: Includes an \( \mathrm{-SH} \) functional group, notable for sulfur-related fragmentation.
• Diol Structure: Contains two \( \mathrm{-OH} \) groups, often losing water and giving specific spectral peaks.

The structural differences between thiols and diols significantly influence their fragmentation in mass spectrometry, allowing for their distinct identification.