3
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I count 7 equivalent absorptions total, but answer says 8. I don't see where this eighth absorption comes from?

Heres how I'm counting starting on right side of molecule:

1 is Carbox acid H

2 is the 1 H just left of the double bonded Oxygen

3 is the 3 H's of the methyl group next to the lone H

4 and 5 are the 2 sets of equivalent H's in the ring structure

6 is is the 2 H's to left of ring

7 is the 2 methlys on the isoproplyl (these are equivalent so same signal)

How do they get 8?

C13H12O2

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    $\begingroup$ 8 is the H on the central carbon of the isopropyl group $\endgroup$ – ron Mar 18 '16 at 22:31
  • $\begingroup$ thanks for the reply ron. ..I actually already accounted for the H at that location. The isoproply has 2 absorptions: the 2 equivalent methls + the H you just mentioned. So it still sums to 7 $\endgroup$ – trav95 Mar 18 '16 at 22:35
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    $\begingroup$ No you didn't... $\endgroup$ – Mithoron Mar 18 '16 at 22:37
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    $\begingroup$ oh wait.. ..you are 100% correct.. I totally looked over that.. Shouldve noticd the only 3 bonds there hence an H.. $\endgroup$ – trav95 Mar 18 '16 at 22:37
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    $\begingroup$ It's actually ibuprofen. And technically it's an isobutyl group not isopropyl. $\endgroup$ – orthocresol Mar 19 '16 at 4:53
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To answer this question properly one must understand that the field of the instrument is important. At 400 MHz, ibuprofen displays 7 distinct carbon-bound proton signals. Below is the spectrum from the SDBS website. Although the two methyls of the isobutyl chain are recorded [G] as a "singlet" at δ 0.889, they are diastereotopic and inherently different no matter how far they are removed from the asymmetry on the other side of the ring. If this spectrum were recorded on a 1,020 MHz 1H NMR, perhaps separation of the two methyl signals would occur. That the two methyls are different, imagine that one of them is a perdeutero methyl group. This compound is a diastereomer of the compound having the other methyl group as the perdeutero group. So there are actually 9 total distinct types of hydrogens (including the carboxyl hydrogen) that, given the right spectrometer, could appear in a spectrum of ibuprofen. It is a matter of resolution. [As a case in point, see the 1H NMR of cholesterol at the SDBS website and note that the two methyl groups at the end of the side chain have different chemical shifts even though the are separated from the nearest center of asymmetry by three methylene groups.]

Acknowledgment: http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

enter image description here

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There are 8 non-equivalent protons as shown. Find 8 non-equivalent protons

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  • $\begingroup$ adianadiadi how did you number the protons? i mean what software? I was trying to do this in chemdoodle but no success. i was totally missing #6 proton for some reason, but ths is pretty clear now thanks $\endgroup$ – trav95 Mar 19 '16 at 16:07
  • $\begingroup$ Numbering done manually. $\endgroup$ – adianadiadi Mar 20 '16 at 6:35

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