Question

Propylbenzene, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\), and isopropyl benzene, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\), are constitutional isomers with the molecular formula \(\mathrm{C}_{9} \mathrm{H}_{12}\). One of these compounds shows prominent peaks in its mass spectrum at \(m / z 120\) and 105 . The other shows prominent peaks at \(m / z 120\) and 91 . Which compound has which spectrum?

Short answer

Answer: The isomer with a prominent peak at \(m/z = 105\) is propylbenzene, and the isomer with a prominent peak at \(m/z = 91\) is isopropylbenzene.

Step-by-step solution

Identify the molecular ion peak

We know that both isomers have a molecular formula of \(\mathrm{C}_9\mathrm{H}_{12}\) and both have a prominent peak at \(m/z = 120\). This peak corresponds to the molecular ion peak since the molecular weight of \(\mathrm{C}_9\mathrm{H}_{12}\) is 120. So, the peak at \(m/z=120\) represents the whole molecular ion for both isomers.

Analyze the other prominent peaks

For the first compound, the other prominent peak is at \(m/z = 105\), and for the second compound, the other prominent peak is at \(m/z = 91\). These represent significant fragment ions produced during the mass spectrometry fragmentation. We will analyze the structures of propylbenzene and isopropylbenzene to determine which fragments they can form that correspond to these peaks.

Determine the possible fragments for propylbenzene and isopropylbenzene

For propylbenzene (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\)), a possible fragmentation could occur at the C-C bond between the benzene ring and the first carbon of the propyl group. This would produce a fragment ion with a mass of \(\mathrm{C}_{6}\mathrm{H}_{5}^{+}\), corresponding to the exact loss of a \(\mathrm{CH_{2}CH_{2}CH_{3}}\) from the molecular ion, therefore \(m/z = 120 - 15 = 105\). For isopropylbenzene (\(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\)), a possible fragmentation could occur at the C-C bond between the methyls groups and the benzene ring, which would produce a fragment ion with a mass of \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}^{+}\), corresponding to the exact loss of two \(\mathrm{CH_{3}}\) from the molecular ion, therefore \(m/z = 120 - 29 = 91\).

Assign the spectra to the compounds

Since the fragment with \(m/z = 105\) corresponds to the fragmentation of propylbenzene and the fragment with \(m/z = 91\) corresponds to the fragmentation of isopropylbenzene, we can conclude that the first compound (with peaks at \(m/z = 120\) and \(m/z = 105\)) is propylbenzene and the second compound (with peaks at \(m/z = 120\) and \(m/z = 91\)) is isopropylbenzene.

Key concepts

Molecular Ion Peak

In the fascinating world of mass spectrometry, the molecular ion peak is a crucial aspect to understand. The molecular ion peak, typically the highest m/z value in a mass spectrum, represents the entire molecule before it has fragmented into smaller parts. This peak holds the molecular weight of the original compound.

For example, in the case of propylbenzene and isopropylbenzene, both are constitutional isomers of the formula \(\mathrm{C}_9\mathrm{H}_{12}\). This is why they share a prominent molecular ion peak at \(m/z = 120\).

This peak is crucial because it tells us about the molecular mass. From here, we then seek out other peaks caused by fragmentation and identify which parts of the molecule have broken off, helping in revealing the structure of the compound.

Fragmentation Patterns

When a molecule undergoes mass spectrometry, it doesn't just result in a single peak. It shatters into fragments, each creating its own unique peak. These fragmentation patterns depend heavily on the molecule's structure.

For propylbenzene, the fragmentation shows a peak at \(m/z = 105\), indicating the loss of a \(\mathrm{CH_{3}CH_{2}CH_{2}}\) fragment. This particular fragment ion, \(\mathrm{C}_{6}\mathrm{H}_{5}^{+}\), suggests a cleavage of a bond between the benzene ring and the rest of the propyl group.

In contrast, isopropylbenzene fragments a bit differently. Its spectrum reveals a peak at \(m/z = 91\), indicating the loss of two \(\mathrm{CH_{3}}\) groups resulting in the fragment \(\mathrm{C}_{6}\mathrm{H}_{5\mathrm{CH}^{+}}\). This loss points to cleavage near the branching point of the isopropyl group.

Understanding these fragmentation patterns is vital for determining the structure and identity of a molecule in mass spectrometry.

Constitutional Isomers

Constitutional isomers, also known as structural isomers, are molecules with the same molecular formula but different structural arrangements of atoms. This means that they have the same number of each type of atom but the atoms are connected differently.

In the example of propylbenzene and isopropylbenzene, both are \(\mathrm{C}_{9}\mathrm{H}_{12}\) isomers. Yet, their structures are different: in propylbenzene, the propyl group is straight, whereas, in isopropylbenzene, it contains a branching making it a more compact structure.

This structural difference affects their behavior in mass spectrometry, resulting in distinct fragmentation patterns and helping us identify each isomer. Recognizing constitutional isomers is key in chemistry because it reflects how structural differences can influence chemical properties and reactions.