[lunaprau]

Hello,

Background: In the labs, we were nitrating bromobenzene, separating isomers (the first fraction of crystals obtained after recrystallisation should have been a completely pure 1-nitro-4-bromobenzene; the remaining liquid was separated by rotary evaporation to obtain the less pure second product) and doing an NMR of the second fraction, the less pure. Little to say, my lab skills failed yet again and I got a (relatively to other classmates) horrendous spectrum, adding to it my limited confidence in reading NMR spectra I would appreciate if someone could take a look at my workings and say if and where I'm wrong. //

(See pic of spectrum: [green peaks (B, C, E, F) belonging to meta-nitro bromobenzene; purple peaks (D, I) belonging to para-nitro bromobenzene; and blue peaks (G, H, J) belonging to ortho-nitro bromobenzene] and see table for my analysis).

My main point of worry is the meta isomer (is it even meta? It shouldn't be there if I haven't let the emperature reach too high). Why (in what I have labelled) is hydrogen 9 less deshielded than hydrogen 10? Hydrogen 9 should be deshielded by both NO2 and Br in ortho position, so very strongly. I was trying to follow guidance from this video: https://youtu.be/94D_gy0lG_0?t=756

[rolnor]

2D-nmr?

[pgk]

1). Don’t worry, it often happens. Despite what simplifyingly figuring in many textbooks, little amounts of meta- derivatives are almost always formed during ortho/para directing electrophilic aromatic substitutions.
2). This does not work like that. Ortho/para protons to electron donating groups are upfielded, in contrast to ortho/para protons to electron withdrawing groups that are downfielded.

[Babcock_Hall]

Have you ruled out that any peaks might be starting material or be the result of a second nitration?  I don't think that either possibility is all that likely, but one does not want to jump to conclusions, either.

[pgk]

Yes indeed! Impurities by further nitration, may also be present.

[lunaprau]

2D-nmr?

Unfortunately impossible at this point, this is an experiment from few weeks ago that I only have this data available from, and both uni and labs are closed due to the virus. Wish I could though!

Have you ruled out that any peaks might be starting material or be the result of a second nitration?

No, I haven't. All we did actually was cold conc. sulphuric and nitric acid with bromobenzene, kept below 65°, refluxed, added to ice-cold water, solid filtrated, washed with water, dried, recrystallised from hot ethanol (to give the first, apparently pure crop of crystals) and the liquid remaining after recrystallisation had solvent removed by rotary evaporation - this second product is what I have an NMR of. Quite likely that thos will be the impurities, not the meta isomer. In the video I was looking at the meta isomer appears to have wide-spread triplet, doublet, narrow triplet and a doublet, whereas in my spectrum it is at most a very narrow triplet and three doublets (looking from larger to lower chemical shift). Maybe rather than forcing the observations to fit my meta theory it's better to say that it's probably the impurities form further nitration.

Thanks everyone for the help so far!