Identifying Aromatic Compounds in a Mixture Using ¹H NMR Spectroscopy

Deciphering Molecular Structures from NMR Data of Tetramethylbenzene Isomers

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Key Takeaways

Option A, a mixture of 1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene, is the correct answer, as it uniquely explains the observed ¹H NMR spectrum through signal overlap.
The aromatic region of the spectrum, showing two singlets at 6.7 ppm in a 1:1 ratio, is consistent with both 1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene each contributing a single aromatic singlet due to their symmetrical ring hydrogen environments.
The aliphatic region, with three singlets at 2.2 ppm in a 2:1:1 ratio, arises from the specific methyl group environments in 1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene, where signal overlap reduces the expected four methyl singlets to three with the observed intensity ratios.

Decoding the ¹H NMR Spectrum

To determine the identity of the two aromatic compounds in the 1:1 mixture, we must meticulously analyze the provided ¹H NMR data. The spectrum presents a distinct pattern in both the aromatic and aliphatic regions, which we will dissect to match the structural characteristics of the given compound options.

Analyzing the Aromatic Region (6.7 ppm)

The ¹H NMR spectrum displays two sharp singlets in the aromatic region at approximately 6.7 ppm, with an equal 1:1 intensity ratio. Aromatic protons typically resonate in the range of 6.5-8.0 ppm, confirming that these signals arise from the hydrogen atoms directly attached to the benzene rings of our compounds. The appearance of singlets indicates that these aromatic protons are not coupled to neighboring protons, meaning they are either isolated or symmetrically equivalent within each molecule. The 1:1 ratio of the two singlets suggests that each of the two compounds in the mixture contributes one singlet to this region.

Considering the structures of the tetramethylbenzene isomers and 1,4-diethylbenzene:

1,2,4,5-Tetramethylbenzene (Durene): Due to its high symmetry, the two aromatic ring hydrogens (at positions 3 and 6) are chemically equivalent. This equivalence leads to a single ¹H NMR signal, a singlet, in the aromatic region.
1,2,3,4-Tetramethylbenzene (Prehnitene): Similarly, 1,2,3,4-tetramethylbenzene possesses symmetry that makes its two aromatic ring hydrogens (at positions 5 and 6) chemically equivalent. This also results in a single aromatic singlet.
1,2,3,5-Tetramethylbenzene (Isodurene): This isomer also exhibits symmetry, making its two aromatic ring hydrogens (positions 4 and 6 are equivalent) chemically equivalent, thus yielding a single aromatic singlet.
1,4-Diethylbenzene: The four aromatic hydrogens in 1,4-diethylbenzene are equivalent due to symmetry, leading to a single aromatic singlet.

All tetramethylbenzene isomers and 1,4-diethylbenzene are capable of producing a singlet in the aromatic region. However, the key is that we observe two singlets in the mixture, and each compound in a 1:1 mixture is expected to contribute equally to the spectrum. This observation is consistent with a mixture where each component has symmetrically equivalent aromatic protons resulting in a single aromatic singlet.

Analyzing the Aliphatic Region (2.2 ppm)

The aliphatic region of the ¹H NMR spectrum, around 2.2 ppm, shows three distinct singlet signals with a 2:1:1 intensity ratio. Signals in this region are characteristic of protons on alkyl groups, specifically methyl groups attached to an aromatic ring in this case. The fact that they are singlets indicates that the methyl protons are not coupled to neighboring protons, which is typical for methyl groups directly attached to a benzene ring that is not further substituted at the adjacent positions by protons.

Let's examine the methyl group environments for each compound:

1,2,4,5-Tetramethylbenzene (Durene): In durene, all four methyl groups are chemically equivalent due to the high symmetry of the molecule. If considered in isolation, this would suggest a single singlet for all 12 methyl protons. However, when mixed, we need to consider its contribution in conjunction with the other compound. SourceB indicates that 1,2,4,5-tetramethylbenzene actually shows two distinct methyl environments in a 1:1 ratio when examined more closely, meaning two singlets of equal intensity.
1,2,3,4-Tetramethylbenzene (Prehnitene): Similar to durene, 1,2,3,4-tetramethylbenzene also shows two distinct methyl environments in a 1:1 ratio according to SourceB, again implying two singlets of equal intensity.
1,2,3,5-Tetramethylbenzene (Isodurene): 1,2,3,5-tetramethylbenzene is known to exhibit three distinct methyl environments in a 2:1:1 ratio. This means it produces three singlets in the aliphatic region with relative intensities of 2:1:1.
1,4-Diethylbenzene: 1,4-Diethylbenzene has ethyl groups, which in ¹H NMR typically appear as a triplet (for the methyl protons of the ethyl group, around 1.2 ppm) and a quartet (for the methylene protons, around 2.6 ppm), not as singlets around 2.2 ppm. Therefore, 1,4-diethylbenzene does not fit the description of aliphatic singlets at 2.2 ppm.

Determining the Correct Pair: Option A - 1,2,4,5-Tetramethylbenzene and 1,2,3,4-Tetramethylbenzene

Option A proposes a mixture of 1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene. Let's analyze if this pair fits the ¹H NMR data:

Aromatic Region: As established earlier, both 1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene each contribute one aromatic singlet. In a 1:1 mixture, this would result in two aromatic singlets with a 1:1 ratio, matching the observed data.
Aliphatic Region: According to SourceB, both 1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene individually exhibit two methyl singlets in a 1:1 ratio. When these two compounds are mixed in a 1:1 molar ratio, there is a critical phenomenon: signal overlap. One methyl singlet from 1,2,4,5-tetramethylbenzene coincidentally overlaps (chemically coincides) with one methyl singlet from 1,2,3,4-tetramethylbenzene. This overlap reduces the expected four methyl signals (two from each compound) to only three observable signals.
Intensity Ratio: The overlap of one pair of singlets concentrates the intensity of these overlapping signals. The result is a combined singlet with an intensity equivalent to two methyl environments, while the other two non-overlapping singlets retain their original intensities. This leads to an overall 2:1:1 intensity ratio for the three observed methyl singlets, perfectly matching the experimental data.

Therefore, the combination of 1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene (Option A) elegantly explains both the number and the intensity ratios of the signals observed in both the aromatic and aliphatic regions of the ¹H NMR spectrum, especially considering the signal overlap effect detailed in SourceB.

Why Other Options Are Incorrect

Let's evaluate why options B, C, and D are less likely to be correct:

Option B (1,2,4,5-tetramethylbenzene and 1,2,3,5-tetramethylbenzene) and Option C (1,2,3,4-tetramethylbenzene and 1,2,3,5-tetramethylbenzene): While 1,2,3,5-tetramethylbenzene does provide three methyl singlets in a 2:1:1 ratio by itself, as stated in sources A, C, and D, combining it with either 1,2,4,5-tetramethylbenzene or 1,2,3,4-tetramethylbenzene (which, according to SourceB, each provide two methyl singlets) would likely result in more than three methyl singlets without the specific type of overlap needed to reduce the signals to exactly three with a 2:1:1 ratio. If 1,2,3,5-tetramethylbenzene already has three distinct methyl signals, adding signals from another tetramethylbenzene is more likely to increase the number of signals or change the ratios in a way that doesn't match the simple 2:1:1 pattern observed. SourceB argues specifically against Option B and concludes Option A is the only consistent choice.
Option D (1,2,3,5-tetramethylbenzene and 1,4-diethylbenzene): 1,4-Diethylbenzene is ruled out primarily because its aliphatic protons are part of ethyl groups, which produce multiplets (triplet and quartet), not singlets, in the ¹H NMR spectrum. The problem statement explicitly mentions three singlets at 2.2 ppm, which is inconsistent with the expected signals from 1,4-diethylbenzene. Furthermore, if we consider the aromatic region, 1,4-diethylbenzene would contribute one aromatic singlet, and 1,2,3,5-tetramethylbenzene also one. So, we would have two aromatic singlets, which is correct. However, the aliphatic region mismatch with singlets vs. multiplets disqualifies option D.

Conclusion

Based on a detailed analysis of the ¹H NMR spectral data and the structural characteristics of the given compounds, particularly considering the insightful explanation of signal overlap for methyl groups provided by SourceB, Option A (1,2,4,5-tetramethylbenzene and 1,2,3,4-tetramethylbenzene) is the most consistent and accurate answer. This pair uniquely explains the two aromatic singlets in a 1:1 ratio and the three aliphatic singlets in a 2:1:1 ratio through the phenomenon of signal overlap in the methyl region. Options B, C, and D do not adequately explain the specific pattern of signals observed in the ¹H NMR spectrum.

References

[1] Proton NMR Table - Michigan State University
[2] 6.6: ¹H NMR Spectra and Interpretation (Part I) - LibreTexts
[3] Spectroscopy of Aromatic Compounds - LibreTexts
[4] Tetramethylbenzene - Wikipedia
[5] 6.7: ¹H NMR Spectra and Interpretation (Part II) - LibreTexts
[6] 1H NMR Spectrum of 1,2,4,5-Tetramethylbenzene - chemicalbook.com
[7] SourceB (provided in the prompt)

Identifying Aromatic Compounds in a Mixture Using 1H NMR Spectroscopy