Let me ask a few questions before we discuss how elemental analysis might fail.

1. Did you isolate a pale yellow powder?

2. Did it have a melting point of 84C?

3. Did the TLC show a spot that traveled halfway to the front when you used the Kansara solvent mixture?

The Kansara technique uses relatively mild nitration conditions (compare to Guzman et al. http://bmjopen.bmj.com/content/suppl/2013/06/17/bmjopen-2013-002672.DC1/bmjopen-2013-002672supp.pdf). Guzman used fuming nitric acid that is more that twice as concentrated as the 8N used by Kansara (red fuming is ~21.4 N). While the Guzman protocol was also kept cold, it yielded the dinitroibuprofen molecule. Of course, the ratio of nitric acid to ibuprofen in the Guzman protocol is ~10:1 while it is 1.3:1 in the Kansara method.

Did you use 8 N nitric acid in your synthesis of did you use the more commonly found 68-70% (~16 N) in your synthesis?

Clearly, if you formed the dintroibuprofen, you would see less carbon than expected. But the first question is whether you had any nitroibuprofen present as indicated by color, MP and Rf.

The procedure followed by you which was reported by Kansara and other coworkers.pl see first whether all physical and analytical data. matches or not..seethe purity of  ibuprufen, and then caarry  out  reaction in mild condition. and also contact kansara asked them what difficulty they face and how they overcome that and also confrm the structure by physical and analytical data.

Under the conditions of nitration, the molecule might undergo decarboxylation.

I did'nt try it before, but I guess, so.

It's better to start by esterifying ibuprofen, then undergo nitration using mild condition, and by using 1.5 equivalents of the nitrating agent or mixture, temp not to be higher than 50, follow up by TLC relative to ester and acid. Nitro product will be mainly a compound that has nitro gp at C3. You have to expect that another isomer might be detected (the 2-nitro derivative) because neither the hydroxycarbonyl nor  the alkoxycarbonyl as an alpha substituent on sp3 carbon at C1 will inhibit totally the substitution at position C2  of the ring. Otherwise you've to prepare tertiary butoxycarbonyl to prevent almost, totally the substitution in this position.

Good luck

Whatever I have answerd earlier,I mean to say that you can protect carbpxylic acid and then follow it in milder condition so that oxiidation  and decarboxylation wiill be avoided .However Kanasa have carried out without protection of carboxyl group thats the reason I said confirmed with spectral data.

I am not any expert in brufene derivatization and its physical properties /e.g. solubility in non polar solvents which as I guess is rather low/. But you could try to mono-nitrate it in CH2Cl2 to which a stoichometric amount of 90 to 95 % HNO3 was added. The temperature  should be  equal to boiling point of dichlormethane. Distill off the reaction mixture via rotavap, the dry residuum should be nitrobruphene, if I am not wrong.

I am not surprised, some journals and authors do not produce results that can be repeated. Run a Carbon NMR and check if the COOH group is present after your nitration. .

thank you

secondly, I isolate isolate a pale yellow powder with M.P=84

What do you think of this PDF?

http://www.academicjournals.org/article/article1379511415_Lotfy%20et%20al.pdf

also , I isolate the sme product , but the analysis is not agree .

do you have any papers to prepare nitro, amino , and bromo - Ibuprofen ?

If you do not need large quantities of the nitro-ibuprofen why not try one equivalent nitronium tetrafluoroborate in acetonitrile or tetramethylene sulfone. It should be noted that phenylacetic acids are prone to  decarboxylation in highly acidic media. so the methyl ester or better the ethyl ester, may prove to be a better substrate.

You asked a few different questions. Let me see if I can answer them all.

1. Did you make the 3-nitroibuprofen? I think that you probably did. The MP seems good. But to verify it, I would suggest either a C13 or a proton NMR. As you saw. Lotfy provides NMR data but no MP. The NMR data should be very conclusive as to structure.

2. Why is the elemental analysis off? There are two recurrent reasons for elemental analyses that don't match expectations (besides having the wrong material). The first is that the sample is contaminated. The things that could be contaminating your sample are water (water bound in crystals is sometimes difficult to drive off) or salt. Since the salt adds mass but no C, H or N, it appears as "oxygen". This might push your C down (N gets pushed down as well but you are assuming 1 nitrogen atom per molecules and then calculating the number of carbon atoms. The second major cause of errors is mis-weighing. If the sample is mis-weighed before decomposition, the C, H and N values drop and the missing mass is considered to be oxygen. I would suggest that you check first with NMR and if those results look good, you dry your sample for a longer time before weighing and having it analyzed.

3. Can you synthesize bromo-, nitro- and amino-ibuprofen. Yes. Although the papers are very little on the synthetic protocols. See http://pubs.acs.org/doi/abs/10.1021/jo00314a053 to see the aliphatic bromination of ibuprofen. See http://www.sciencedirect.com/science/article/pii/S0960894X0301223X for ways to generate 3-nitro, 3-bromo and 3-amino ibuprofen. Note that the paper synthesizes the amides of ibuprofen but you can hydrolyze the amides to the acids relatively simply.

I hope that this helps.

thank you 

but, The paper don't contain the procedure to generate 3-nitro, 3-bromo and 3-amino ibuprofen

The Gupta paper provides the bare minimum of synthetic information in the caption to scheme 3:. Bromination of ibuprofen. Reaction conditions: (a) HNO3, H2SO4, 0C, 0.5 h; (b) CH3OH, H2SO4, rt, overnight; (c) NH4Cl, Fe powder, CH3OH, reflux, 14 h; (d) NaNO2, CuBr, 0 C, 1 h; (e) NaOH, CH3OH, reflux, 5 h.

The first step is the nitration of ibuprofen. I think that you have already succeeded with this despite your elemental analysis. I suggest that you use NMR to check the structure. I think that your elemental analysis is off because you submitted a sample contaminated with solvent (possibly water) and/or you submitted the sodium salt.

Step b makes the methyl ester of nitroibuprofen and the nitrogroup is then reduced with iron. I would look at the reduction of nitrobenzoic acid to aminobenzoic acid for specific steps. Step d changes the amino group to a diazo group which is displaced by Br-. Step e is just an alkaline hydrolysis to the free acid (actually free anion).

Fortunately, the first author of the article, Kushol Gupta is a member of ResearchGate and you can probably contact him directly for additional information.

Just ignore the elemental analysis; they are great for proving purity but absolutely useless in determining the identity of a structure particularly in organic chemistry.

Fractionally recrystallise your product with HPLC grade solvents a few times, thoroughly dry it and resubmit the elemental analysis until it matches.

For establishing identity, use NMR and high res mass spectrometry.

Melting points, Elemental analysis, and sometimes even IR-spectra are significantly limited by the presence of minor impurities.

Hossam,

I agree with Eric Harris. As I said earlier, ignore the elemental analysis--it is probably off due to contaminants or water of crystallization. Your MP is exactly what others reported. For complete confirmation get an NMR (C or H) for confirmation. MS is relatively straightforward and should give confirmation.

when I prepared nitro ibuprofen , and dried for long time (7 day), the color of the product converted from yellow(before drying ) to deep yellow (brown yellow )color ,why?

The possible cause of the color change to braunish might result from the presence of tiny residua of reagent used for nitration /yielding oxidation products of nitro ibuprofene/, or some intermdiates. Pity, you did not  give the conditions of drying /drying medium, temperature, presence of oxigen and UV radiation..../. Long term thermal treatment /in case of drying in thermostat/ might partly modify your product, as well.

During Nitration check whether single side chain or both the side chains get oxidised to get acid .Take IR & PMR , Mass to confirm the structure..

i think in this experiment you face the problem in faulty  nitrating media which cause reduction in carbon number.so you should check the structure(spectro) at different interval during the reaction to find out the reducing step..