What is the structure of a hydrocarbon that shows a molecular ion m/z = 182 in mass spec with following H NMR?

Question

7.2 (sigma greek letter), 5H
2.9 (sigma greek letter), 2H


I have tried NUMEROUS times to answer this question -- help is MUCH needed..thank you.

**hydrocarbon (C and H only)

Answer 1

Hi Smiley!

Let me walk you through how I'm approaching this problem.

First we need to find the chemical formula of the compound. I'm assuming 182 m/z is the total mass of the compound - this would be a minor peak in the mass spec, but present. Otherwise, I don't know how you'd be expected to solve this problem.

Your NMR Hs add to 7. However, this cannot be correct, since 182 - 1(7)= 175, which would mean you would have 14.5 carbons - impossible unless you're talking about a crazy fused system. Let's try 14 Hs. That gives us (182 - 14)/12 = 14 carbons. So your molecular formula is C14H14 - that's manageable, but you're going to have some unsaturation, and that's OK. (If you multiplied 7 one more time, you'd have 21 Hs, which'd leave you with (182 - 21)/12 = 11.5 C....probably not likely.)

Now calculate your unsaturation number, which is the number of pi bonds or rings in your molecule. The equation to do this is X - (y/2) + (z/2) + 1, where X is the number of carbons, Y is the number of hydrogens + halogens, and Z is the number of nitrogens. For C14H14: 14 - (14/2) + 1 = 8. Hmmm - an aromatic ring system? (Each benzene has 4 degrees of unsaturation - one for being a ring, and three for having three double bonds.) If you look at your H NMR, 10 of your hydrogens have shifts comprable to benzene Hs.

Based on all the data you're given, the only structure I can come up with that matches everything we've found so far is 2 benzene rings connected by CH2CH2 (if I had to name this compound, though this naming is probably incorrect, I'd say 1-(2-phenylethyl)-benzene). Each benzene has 4 degrees of unsaturation, adding up to 8, there are 10 benzene Hs (5 on each ring), deshielded a great deal, and there are 4 Hs on two Cs in the middle that are moderately deshielded by their close proximity to the aromatic ring systems. The molecule exhibits a good deal of symmetry :).

Hope this helps....if this is not correct, keep trying various combinations using two benzene rings.

Answer 2

You have to specify some things

1. σ-scale is not commonly used. δ-scale is more commonly used so based on your data could you convert the values to the δ-scale?

2. You provide the number of H that correspond to each peak, but you don't mention anything about the multiplicity of the peaks (are they all singlets?)
It looks like what the previous answer said (1,2-diphenyl-ethane).
In that case you would get a singlet for -CH2- protons since they are equivalent due to symmetry. For the aromatic H you should actually get 3 peaks (1 for ortho, 1 for meta and 1 for para H), so you need to clarify if it is low resolution and single peak or what.
Also if the scale was actually δ, then 7.2 would correspond to aromatics and 2.9 could be Ar-C-H proton.