What kind of structure could give you a HNMR spectrum consisting of a singlet integrating to 5 protons?

Question

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.

Answer 1

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).

Answer 2

Your clean $\ce{^1H}$-NMR spectrum of a compound $(\ce{C11H14O2})$ with the integrations filled in as $(\ce{5H, 2H, 2H,2H, 3H})$ from down-field to up-field is most probably benzyl butanoate (Ref. 1). The assignment of the peaks are as follows:

• $\delta \ \ \pu{7.5 ppm}, \ \ce{5H}, \ s \ (ar)$: This resonance can be assigned as 5 aromatic protons. It should be a multiplet instead of singlet, but probably due to low-resolution spectrometer has produced (see the authentic spectrum below) a broad singlet (I assume).
• $\delta \ \ \pu{5.1 ppm}, \ \ce{2H}, \ s \ (bz)$: This resonance can be assigned as 2 benzyl protons, which is also next to carbonyl carbon.
• $\delta \ \ \pu{2.4 ppm}, \ \ce{2H}, \ t \ (3)$: This resonance can be assigned as 2 protons on the carbon next to carbonyl carbon.
• $\delta \ \ \pu{1.8 ppm}, \ \ce{2H}, \ quintet \ (2)$: This resonance can be assigned as 2 protons on the second carbon from the carbonyl carbon (see butanoate chain). You weren't sure about it is a quintet or quartet, probably due to low-resolution spectrometer. Nevertheless, i should be a quintet due to methyl and methylene groups on either side of that carbon.
• $\delta \ \ \pu{1.0 ppm}, \ \ce{3H}, \ t \ (1)$: This resonance can be assigned as 3 (methyl) protons on the third carbon from the carbonyl carbon (see terminal end of butanoate chain). It is a triplet because it next to methylene group.

The $400$-$\mathrm{MHZ}$ high resolution NMR spectrum of this compound is published in Ref. 1. The spectrum clearly shows the relevant peaks with approximately close chemical shifts, $\delta \ 7.28(m), 5.05(s), 2.20 (t), 1.58 (m),$ and $0.85(t)$, respectively:

References:

1. Ronald P. D’Amelia, Joseph Mancuso, and Nanette Wachter, "Application of Quantitative Proton Nuclear Magnetic Resonance Spectroscopy for the Compositional Analysis of Short Chain Fatty Acid Benzyl Ester Mixtures," World Journal of Chemical Education 2019, 7(3), 189-195 (doi:10.12691/wjce-7-3-1) (https://pubs.sciepub.com/wjce/7/3/1/index.html).