Question

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

Short answer

Answer: The fragment at m/z 135 is formed due to the loss of a hydroxyl molecule (OH) from the p-anisic acid molecule. This occurs because the O-H bond in the carboxylic acid group is susceptible to breaking due to the high electronegativity of the oxygen atom. The fragment at m/z 107 is formed due to the loss of a carboxylic acid group (COOH) from the p-anisic acid molecule. This occurs due to the cleavage of the C-C bond between the carboxylic acid group and the benzene ring under the influence of high-energy electrons.

Step-by-step solution

Identify the fragment at m/z 135

To identify a fragment with an m/z value of 135 in the mass spectrum of p-anisic acid, we will subtract the molecular weight of the possible fragment from the molecular weight of p-anisic acid (152). 152 - 135 = 17 The missing mass corresponds to the loss of an OH (hydroxyl) molecule from the parent molecule. This indicates that the fragment is formed by the loss of a hydroxyl group. The remaining fragment would have a molecular formula of \(\mathrm{C}_{8}\mathrm{H}_{7}\mathrm{O}_{2}\).

Identify the fragment at m/z 107

To identify the fragment with an m/z ratio of 107, again, subtract the molecular weight of the possible fragment from the molecular weight of the parent molecule. 152 - 107 = 45 The missing mass corresponds to the loss of a COOH group from the p-anisic acid. This indicates that the fragment is formed by the loss of the carboxylic acid group. The remaining fragment would have a molecular formula of \(\mathrm{C}_{7}\mathrm{H}_{7}\mathrm{O}\).

Explanation of the formation of the fragments

In mass spectrometry, molecules undergo fragmentation due to the high-energy impact of electrons. These fragments can have different structures and stabilities. For m/z 135: The fragment formed is due to the cleavage of the O-H bond in the carboxylic acid group and the loss of a hydroxyl (OH) molecule. This fragmentation is relatively common because the oxygen atom is highly electronegative, leaving the O-H bond susceptible to breaking. For m/z 107: The fragment formed is due to the cleavage of the C-C bond between the carboxylic acid group and the benzene ring, leading to the loss of the COOH group. This fragmentation pattern corresponds to the removal of a carboxylic acid group found in aromatic carboxylic acids. In conclusion, the peaks at m/z 135 and 107 in the mass spectrum of 4-methoxybenzoic acid (p-anisic acid) are formed due to the fragmentation of the parent molecule under the influence of high-energy electrons. The fragment at m/z 135 is formed due to the loss of a hydroxyl molecule (OH), while the fragment at m/z 107 is formed due to the loss of a carboxylic acid group (COOH).