Question

Addition of \(m\)-xylene to the strongly acidic solvent \(\mathrm{HF} / \mathrm{SbF}_{5}\) at \(-45^{\circ} \mathrm{C}\) gives a new species, which shows ' \(\mathrm{H}\)-NMR resonances at \(82.88(3 \mathrm{H}), 3.00(3 \mathrm{H}), 4.67(2 \mathrm{H})\), \(7.93(1 \mathrm{H}), 7.83(1 \mathrm{H})\), and \(8.68(1 \mathrm{H})\). Assign a structure to the species giving this spectrum.

Short answer

Question: Determine the structure of the new species formed by the addition of m-xylene to a strongly acidic solvent (HF/SbF5) at -45°C, given the following NMR data: Chemical shift, 𝛿(ppm) - Integration ratio - Number of protons (H) 2.88 - 3H, 3.00 - 3H, 4.67 - 2H, 7.93 - 1H, 7.83 - 1H, and 8.68 - 1H. Answer: The structure of the new species is a protonated m-xylene at the position between the two methyl groups on the aromatic ring.

Step-by-step solution

Extract the NMR data from the problem

We need to break down the given data into their respective values: Chemical shift, 𝛿(ppm) - Integration ratio - Number of protons (H) 2.88 - 3H, 3.00 - 3H, 4.67 - 2H, 7.93 - 1H, 7.83 - 1H, and 8.68 - 1H.

Analyze the data for clues about the structure

The NMR data can give us clues about the environment of the protons in the molecule: - Chemical shifts of around 2-3 ppm are usually indicative of aliphatic protons (CH3 and CH2 groups). - Chemical shifts of around 4-5 ppm are typically found in protons attached to electron-withdrawing groups or heteroatoms. - Chemical shifts of around 7-9 ppm are characteristic of aromatic protons.

Assign the protons to their respective environments

Based on the chemical shift values and knowledge of m-xylene's structure, we can make the following assignments: - 2.88 ppm (3H): The methyl group in the m-xylene attached to the aromatic ring. - 3.00 ppm (3H): The other methyl group in the m-xylene attached to the aromatic ring. - 4.67 ppm (2H): This is likely to be the protons on a carbon adjacent to an electron-withdrawing group. - 7.93 ppm, 7.83 ppm, and 8.68 ppm: The aromatic protons in the new species.

Propose a structure for the new species

Since the reaction is taking place in a strongly acidic medium, this can facilitate the protonation of the aromatic ring in m-xylene, leading to the formation of a carbocation. The resulting structure that aligns with the given NMR data is that a benzyl cation is formed on the position between the two methyl groups, with one of the methyl groups having one proton replaced by the acidic proton from the solvent (HF). This explains the chemical shift of 4.67 ppm for the 2 protons adjacent to an electron-withdrawing group (the carbocation on the aromatic ring). The other three aromatic proton chemical shifts are also assigned accordingly.

Confirm the structure

The proposed structure should now account for all the NMR data provided: - Aliphatic protons: 2.88 ppm (3H) and 3.00 ppm (3H) for the two methyl groups. - Protons adjacent to electron-withdrawing group: 4.67 ppm (2H) for the methylene group attached to the carbocation. - Aromatic protons: 7.93 ppm (1H), 7.83 ppm (1H), and 8.68 ppm (1H). Thus, the structure of the new species is a protonated m-xylene at the position between the two methyl groups on the aromatic ring.