key term - [M - 18]+

Definition

[M - 18]+ is a key term in the context of spectroscopy of alcohols and phenols. It refers to a characteristic fragment ion that is commonly observed in the mass spectra of these organic compounds, formed by the loss of water (H2O) from the molecular ion [M]+.

5 Must Know Facts For Your Next Test

1. The [M - 18]+ fragment ion is commonly observed in the mass spectra of alcohols and phenols due to the loss of a water molecule from the molecular ion.
2. This fragmentation pattern is characteristic of compounds containing a hydroxyl (-OH) functional group, as the water loss is facilitated by the presence of this group.
3. The intensity of the [M - 18]+ peak relative to the molecular ion [M]+ can provide information about the position and substitution patterns of the hydroxyl group in the molecule.
4. The presence and relative abundance of the [M - 18]+ peak can be used to confirm the identity of an alcohol or phenol compound in mass spectrometry analysis.
5. Understanding the [M - 18]+ fragmentation pattern is crucial for interpreting mass spectra and elucidating the structure of unknown alcohols and phenols.

Review Questions

• Explain the formation of the [M - 18]+ fragment ion in the mass spectra of alcohols and phenols.
  • The [M - 18]+ fragment ion is formed in the mass spectra of alcohols and phenols due to the loss of a water molecule (H2O) from the molecular ion [M]+. This fragmentation process is facilitated by the presence of the hydroxyl (-OH) functional group in these compounds. The water loss occurs through a rearrangement reaction, where the hydrogen atom from the hydroxyl group is transferred to the oxygen, leading to the elimination of water and the formation of the [M - 18]+ fragment ion. This characteristic fragmentation pattern is observed due to the stability of the resulting fragment ion and provides valuable information about the structure and identity of the analyzed alcohol or phenol compound.
• Discuss the significance of the [M - 18]+ peak in the interpretation of mass spectra of alcohols and phenols.
  • The [M - 18]+ peak in the mass spectra of alcohols and phenols holds significant importance in the interpretation and identification of these compounds. The relative abundance of this fragment ion compared to the molecular ion [M]+ can provide information about the position and substitution patterns of the hydroxyl group. A stronger [M - 18]+ peak indicates that the hydroxyl group is more readily eliminated, which is influenced by factors such as the steric environment and the stability of the resulting fragment ion. Additionally, the presence and intensity of the [M - 18]+ peak can be used as a characteristic fingerprint to confirm the identity of an unknown alcohol or phenol compound during mass spectrometry analysis. Understanding the [M - 18]+ fragmentation pattern is crucial for structural elucidation and the interpretation of mass spectra in the context of spectroscopy of alcohols and phenols.
• Evaluate the role of the [M - 18]+ fragment ion in differentiating between structurally similar alcohols and phenols using mass spectrometry.
  • The [M - 18]+ fragment ion plays a crucial role in differentiating between structurally similar alcohols and phenols using mass spectrometry. The position and substitution pattern of the hydroxyl group can significantly influence the intensity and relative abundance of the [M - 18]+ peak in the mass spectrum. By analyzing the [M - 18]+ fragmentation pattern, it is possible to gain insights into the molecular structure and distinguish between isomeric compounds that may have the same molecular formula but different arrangements of the hydroxyl group. For example, the [M - 18]+ peak may be more prominent in the mass spectrum of a primary alcohol compared to a secondary or tertiary alcohol, due to differences in the ease of water elimination. Similarly, the [M - 18]+ fragmentation can help differentiate between phenols with different substitution patterns on the aromatic ring. This detailed understanding of the [M - 18]+ fragmentation behavior is essential for the accurate identification and structural elucidation of alcohols and phenols using mass spectrometry.

Related terms

Molecular Ion [M]+: The peak in a mass spectrum that corresponds to the intact, unfragmented molecule, representing the parent compound.

Fragmentation: The process by which a molecule breaks apart into smaller fragment ions during mass spectrometry analysis.

Mass Spectrometry:

An analytical technique that measures the mass-to-charge ratio of ions to determine the chemical composition and structure of a compound.