Dissolving toluene in a mixture of HF–SbF5 generates species B which has a temperature dependent 1H — Physical Chemistry — Kinetics Chemistry Question
Toluene in a Superacid Solution
Dissolving toluene in a mixture of HF–SbF5 generates species B which has a temperature dependent 1H NMR spectrum (60 MHz) shown below.
CH3
HF-SbF5 → B
[VISUAL]
The upper figure shows the entire spectrum at –97 °C with the following parameters (chemical shifts are given in the ppm scale, δ: 9.38 (d, 2H), 8.40 (d, 2H), 5.05 (m, 2H), 3.30 (t, 3H).
[VISUAL]
The lower figure shows the signals from the upper figure in the range of 5 –10 ppm as the temperature is raised.
Provide a structure for B consistent with the –97 °C spectrum.
Model Answer
Structure of B is the para-protonated toluene arenium ion (benzenium ion with a CH3 group at C4 and a CH2 group at C1):
[VISUAL]
Assign each of the peaks in the –97 °C spectrum to the corresponding proton(s) in your structure for B.
Model Answer
See 19.1 above.
Provide structures and/or chemical equations that explain why the spectrum changes with increasing temperature. Label your structures.
Model Answer
All of the aromatics protons and those of the CH2 group exchange through a series of rapid 1,2 shifts providing the species shown below:
[VISUAL]
Labeled as species M, N, O, P, Q, R.
On the basis of the data provided and theoretical considerations, predict qualitatively the relative stabilities of your structures.
Model Answer
M < O, Q < N, R < P
The peak at 3.30 ppm in the –97 °C spectrum corresponds to a methyl group. Why is it a triplet (J = 4.4Hz)?
Model Answer
The methyl group signal is split by long-range coupling to the CH2 group. Note that the 9.38 peak is a doublet and thus is coupled only to the 8.40 peaks; its coupling to the methyl group is not resolved.
[VISUAL]