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A clean spectrum was given to me with the integrations filled in. I just need to assign peaks.

I just don't see how it's possible to have a singlet integrate to 5 protons. I've considered symmetry, but still, 2 or 3 way symmetry wouldn't give me anything.

The chemical formula is $\ce{C11H14O2}$, with 5 degrees of unsaturation. The spectrum is as follows:

  • 5H, singlet, 7.5 ppm
  • 2H, singlet, 5.1 ppm
  • 2H, triplet, 2.4 ppm
  • 2H, quartet, (or sextet?) 1.8 ppm
  • 3H, triplet, 1 ppm

I'm just looking for a hint on what a 5H singlet could be.

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    $\begingroup$ Unlikely to be a singlet in the aromatic region- expand the spectrum and you will see overlapping signals $\endgroup$
    – long
    Sep 8, 2015 at 5:48
  • $\begingroup$ @long If the coupling between the peaks is stronger than the CS difference, they coalesce and cannot be resolved, unless you go to significantly higher fields. $\endgroup$
    – Karl
    Sep 4, 2019 at 20:58

1 Answer 1

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The 5H singlet is likely due to five protons on a monosubstituted benzene ring.

The peak comes in at 7.5 ppm which is the aromatic region, immediately telling you that they are hydrogens on an aromatic ring. Benzene rings are the most common of these.

But why are they all equivalent? They actually aren't, but their shifts are so close that they appear to be equivalent. Sometimes they will appear as different peaks (usually one peak for the 2 hydrogens close to the substituent and one peak for the 3 hydrogens away from it) if there are strong enough electron withdrawing groups on the substituent that are close to the ring. It is likely that the singlet you're looking at isn't clean cut and looks a bit like a multiplet (see this question).

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