Which of protons, 1 to 4, in the following molecule gives a triplet signal in a 1H NMR spectrum?

Question

BlackTom AI · Accepted Answer

The molecule shown has four labeled protons/sets: 1 is a terminal CH3 on the far left, 2 is a CH with a methyl substituent, 3 is a CH2 adjacent to the carbonyl, and 4 is a terminal CH3 on the right end.

Option a (1): The leftmost CH3 (proton set 1) is attached to a CH (carbon 2). This CH (2) has a single hydrogen, so the CH3-1 protons couple to that one neighboring proton, giving a doublet rather than a triplet. They do not see two equivalent neighbors to give a triplet, so this is not correct.

Option b (2): Proton set 2 is a CH with one hydrogen. Its splitting comes from neighboring protons on carbons adjacent to it. On one side there is CH3-1 (3 protons) and on the other side there is CH2-3 (which has its own protons). The resulting pattern would be more complex than a simple triplet, not a clean triplet, so this option is not correct.

Option c (3): Proton set 3 is the CH2 adjacent to the carbonyl and to CH3-4. The CH2-3 protons are coupled to the three protons of CH3-4, which would produce a quartet for CH2-3, not a triplet. Additionally, CH2-3 is not coupled to exactly two equivalent protons; it couples to three, giving a quartet, so this option is incorrect.

Option d (4): Proton set 4 is the terminal CH3 on the right, which is adjacent to CH2-3 (which has two hydrogens). The three protons on CH3-4 split by the two equivalent protons on CH2-3 produce a triplet. This is the classic ethenic/alkyl “CH3–CH2–” pattern where the methyl next to a methylene appears as a triplet. Therefore, proton set 4 yields a triplet.

In summary, only the protons labeled 4 experience coupling to two equivalent neighboring protons, giving a triplet in the 1H NMR spectrum, while the others do not provide a simple triplet due to different neighboring proton counts and coupling patterns.

Which of protons, 1 to 4, in the following molecule gives a triplet signal in a 1H NMR spectrum?

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