Why does the signal for the hydrogen bonded to oxygen in ethanol appear as a triplet in pure ethanol and as a singlet in ethanol that contain a trace amount of acid? (a) Spin exchange causes spin decoupling. (b) Electron exchange causes spin decoupling. (c) Proton exchange causes spin decoupling. (d) Chemical exchange causes spin decoupling.?

Question

Answer

B

Answer

Proton Exchange in Ethanol:
In pure ethanol (CH3CH2OH), the hydrogen atom bonded to oxygen typically appears as a triplet in the NMR spectrum due to the presence of hydrogen bonding. However, when a trace amount of acid is added to the ethanol, the signal for this hydrogen atom shifts to a singlet.

Reason for Signal Change:
- Proton Exchange: The presence of a trace amount of acid in ethanol facilitates proton exchange between the hydrogen atoms in the ethanol molecules. This exchange process leads to the averaging of the chemical environments experienced by the hydrogen atoms, resulting in the appearance of a singlet signal in the NMR spectrum.
- Chemical Exchange: The exchange of protons between the ethanol molecules causes a chemical exchange process, where the hydrogen atoms are constantly switching positions. This results in the loss of coupling between the hydrogen atoms, leading to the observation of a singlet signal in the NMR spectrum.

Impact of Acid Addition:
- Spin Decoupling: The proton exchange induced by the presence of acid disrupts the spin-spin coupling between the hydrogen atoms, leading to the simplification of the NMR signal for the hydrogen bonded to oxygen. This phenomenon is known as spin decoupling.
Therefore, the presence of a trace amount of acid in ethanol causes a singlet signal to appear for the hydrogen bonded to oxygen, as proton exchange and chemical exchange processes lead to spin decoupling in the NMR spectrum.

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