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Chapter 22: Problem 571

At room temperature, the nmr spectrum of 1 -ethylaziridine + CYCLOA shows the triplet-quartet of an ethyl group, and two other signals of equal peak area. When the temperature is raised to \(120^{\circ}\), the latter two signals merge into a single signal. How do you interpret these observations?

Short Answer

Expert verified
In the given NMR spectrum at room temperature, the two additional signals with equal peak area correspond to hydrogens in CYCLOA with similar but distinct chemical environments. This distinction could be due to conformation or chemical exchange processes. Upon raising the temperature to 120°C, the rate of the exchange process or conformational change increases, leading to an averaging of the chemical shifts of these hydrogens and causing the two signals to merge into a single signal. This suggests that the compound CYCLOA undergoes conformational change or has exchangeable protons whose chemical environments are affected by temperature.

Step by step solution

01

Recall the Ethyl Group Signals in NMR

The ethyl group in NMR typically gives rise to a triplet-quartet pattern, which we observe in the given spectrum. This pattern is composed of a triplet signal (corresponding to the methylene group, -CH2-) and a quartet signal (corresponding to the methyl group, -CH3). The splitting pattern occurs due to the scalar coupling between the adjacent hydrogens.
02

Identify the other two signals

The NMR spectrum shows two other signals of equal peak area besides the triplet-quartet of the ethyl group. These signals likely arise from the hydrogens in the unknown compound CYCLOA. Since these signals have equal peak area, we can infer that the corresponding hydrogens are chemically equivalent or have very similar chemical environments.
03

Analyze the effect of raising the temperature

Raising the temperature to 120°C causes the two equal peak area signals to merge into a single signal. This suggests that increasing temperature removes the differences in the chemical environment of the hydrogens that were previously observed as two separate signals. This effect can happen due to the increasing rate of chemical exchange or conformational change of CYCLOA at higher temperatures. As the temperature increases, the speed of the exchange process or the transition between different conformations also increase, leading to an average chemical shift for the corresponding hydrogens.
04

Interpret the observations

Based on the information above, we can interpret these observations as follows: At room temperature, the two additional signals in the spectrum belong to the hydrogens in CYCLOA and have similar but distinct chemical environments. These differences could be due to the conformation or chemical exchange process. When the temperature is raised to 120°C, the rate of the exchange process or conformational change increases, causing the two signals to appear as one due to averaging of their chemical shifts. This observation implies that the unknown compound CYCLOA undergoes conformational change or has exchangeable protons whose chemical environments are affected by temperature.

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