Question

Account for the presence of peaks at \(m / z 135\) and 107 in the mass spectrum of 4-methoxybenzoic acid ( \(p\)-anisic acid).

Short answer

Answer: The observed peak at m/z 135 can be attributed to the loss of -OH from the carboxylic acid group, while the peak at m/z 107 can be attributed to the loss of -OCH3 from the methoxy group in 4-methoxybenzoic acid.

Step-by-step solution

Identify the structure of 4-methoxybenzoic acid

Draw the structure of 4-methoxybenzoic acid (p-anisic acid). It is an aromatic compound with a methoxy group at the para position, and a carboxylic acid group at the ortho position.

Calculate the molecular weight

Calculate the molecular weight of 4-methoxybenzoic acid. Its molecular formula is C8H8O3, which has a molecular weight of 152 g/mol.

Identify the fragmentation pathways

We need to identify the possible fragmentation pathways in the mass spectrum that can give rise to the observed peaks at m/z 135 and 107. The following fragmentation pathways could be responsible for these peaks: a) Loss of -OH from the carboxylic acid group: If the -OH group is lost from the carboxylic acid, the resulting structure would have a molecular weight of 135 g/mol. b) Loss of -OCH3 from the methoxy group: If the -OCH3 group is lost from the methoxy group, the resulting structure would have a molecular weight of 107 g/mol.

Relate the fragmentation pathways with the observed peaks

The observed peak at m/z 135 can be attributed to the loss of -OH from the carboxylic acid group, while the peak at m/z 107 can be attributed to the loss of -OCH3 from the methoxy group in 4-methoxybenzoic acid. Therefore, the fragmentation pathways described in Step 3 can account for the presence of peaks at m/z 135 and 107 in the mass spectrum of 4-methoxybenzoic acid.

Key concepts

4-Methoxybenzoic Acid Analysis

Analyzing the structure and characteristics of 4-methoxybenzoic acid, commonly known as p-anisic acid, is central to understanding its mass spectrum. This organic molecule comprises an aromatic benzene ring with a p-methoxy substituent and an ortho carboxylic acid functional group.

Understanding the molecular weight is a foundation to mass spectrometry studies; for 4-methoxybenzoic acid, it’s calculated based on its molecular formula, C8H8O3. With atomic weights of carbon (12), hydrogen (1), and oxygen (16), the compound has a total molecular weight of 152 g/mol. This value is crucial as it serves as the starting point in mass spectrometry to which all subsequent fragments are compared.

Mass Spectrum Interpretation

Interpreting the mass spectrum of a compound involves correlating observed peaks with potential molecular fragments. In the case of 4-methoxybenzoic acid, distinct peaks at mass-to-charge ratios (\(m / z\)) of 135 and 107 require careful analysis. Students should remember that the mass-to-charge ratio, in most organic molecules, typically reflects the mass of the fragment since the charge is usually +1.

The peak at an \(m / z\) of 152, corresponding to the molecular weight of the unfragmented molecule, is known as the molecular ion peak. The absence or presence of specific peaks allows us to infer the stability and common fragmentation patterns of the molecule, providing a path to identify the structure of the original compound and the smaller fragments it breaks down into during ionization.

Molecular Fragmentation Pathways

Understanding the molecular fragmentation pathways is key to deducing the structure of a compound from its mass spectrum. In mass spectrometry, a molecule can fragment in various ways once ionized, leading to peaks correlating to the mass of the fragments formed.

For 4-methoxybenzoic acid, we see two significant fragmentation pathways:

• A loss of the hydroxyl (-OH) group from the carboxylic acid forms a fragment with an \(m / z\) of 135.
• A methoxy group loss (-OCH3) results in a fragment with an \(m / z\) of 107.

These pathways reflect the molecule's tendency to lose neutral fragments. By considering these predictable fragmentation behaviors, the mass spectrum can be used to piece together a molecule's structural elements, assisting in the identification and characterization of organic compounds.