The splitting pattern for a compound with molecular formula C3H5OCl gave a quartet and a triplet. Which is the most likely structure for this compound?

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To solve, we first identify the key signal pattern: a quartet and a triplet in a 1H NMR spectrum.
A quartet typically arises from a CH2 group that couples with three equivalent hydrogens on a neighboring CH3 group, while a triplet arises from a CH3 group coupling with two hydrogens on a neighboring CH2 group. The combination of a quartet and a triplet is characteristic of an ethyl group (CH3-CH2-) in a simple system, where the CH2 gives the quartet and the CH3 gives the triplet.
Next, we consider the molecular formula C3H5OCl. A propanoyl chloride (propionyl chloride) has the structure CH3-CH2-CO-Cl, which fits the formula: three carbons, five hydrogens, one oxygen, and one chlorine. In this molecule, the CH2 adjacent to the carbonyl group is the one that would appear as a quartet, and the terminal CH3 would appear as a triplet, matching the observed splitting pattern.
Now, evaluate the options conceptually:
- Option a: If this depicted a structure lacking the ethyl arrangement next to a carbonyl chloride (or showing an incorrect connectivity), the expected quartet and triplet pattern would not be consistent with the ethyl group next to COCl.
- Option b: If this structure placed the substituents in a way that disrupts the CH3-CH2-COCl environment (for example, a different functional group or incorrect carbon skeleton), the splitting would not reflect the ethyl group adjacent to the acyl chloride.
- Option c: This corresponds to propanoyl chloride (CH3-CH2-COCl), which aligns perfectly with the C3H5OCl formula and yields the CH2 quartet and CH3 triplet pattern described.
- Option d: If it showed a structure that precludes the ethyl group reporting as CH3-CH2-, the expected coupling pattern would be misrepresented, contradicting the observed quartet/triplet.
- Option e: If this option proposed an alternative arrangement that does not place the ethyl unit next to the carbonyl chloride, the splitting would mismatch the quartet/triplet assignment.
In summary, the quartet corresponds to the CH2 group next to the CH3, and the triplet corresponds to the terminal CH3, exactly as in propanoyl chloride (CH3-CH2-COCl). Therefore, option c correctly represents the most plausible structure given the formula and the observed splitting pattern.

CHM1052 - MUM S2 2025 CHM1052 2022 practice exam 1

The splitting pattern for a compound with molecular formula C3H5OCl gave a quartet and a triplet. Which is the most likely structure for this compound?

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