[dazed-and-confused]

Hi everyone,
For a school assignment, I have to identify an organic compound, based on an IR and HNMR graph. I was told the molecular weight of 140.5 (+ or - .2)
From my IR graph, I interpreted there to be C=O, C=C and a possible aldehyde group. The NMR graph has doublets at 7.5, 7.8 and a singlet around 9.95, with an integration (calculated) of 2: 2: 1. I was convinced of a para-substituted benzene ring, (based on the IR graph and doublets) with an aldehyde group (based on 9.95 shift) and electronegative element (explaining why the doublets have different shifts.) I used SDBS and found C7H5ClO has a molecular weight of 140.57. After drawing all possible isomers of C7H5ClO as well as C8H9CL, I found that 4-chloro-benzaldehyde (p-chloro-benzaldehyde) matched my interpretation of the NMR graph almost perfectly. I then compared my NMR graph with online NMR graphs, using various websites, and found an almost perfect correlation. However, the online IR spectrums do not match AT ALL. The fingerprint region is completely different, and the online spectrums tend to be quite flat from 4000 - 3000 apart from one peak, whereas mine dips slightly before going back up. I have looked at structures and graphs of over 50 molecules, and I can't find anything that matches. I'm really quite desperate. I have attached links to the graphs. Any suggestions would be very much appreciated.




Thanks in advance

Also, the transmittance is quite low (below 60%), I'm not sure if that indicates anything?
Also, the two aldehyde peaks at 2850 and 2750 are not very distinctive. Is that a concern?

[Babcock_Hall]

It would help if you provided the exact IR frequencies of the putative carbonyl stretch near 1700 cm^-1 and any peaks you see near 2800 cm^-1.

[dazed-and-confused]

It would help if you provided the exact IR frequencies of the putative carbonyl stretch near 1700 cm^-1 and any peaks you see near 2800 cm^-1.

I don't know the exact frequencies? I used a ruler to try and get an exact value but the scale was quite dodgy. I would say the carbonyl stretch is at exactly 1700 and there are peaks at 2750, 2900 and 3150.

[Dan]

IR looks ok for 4-chlorobenzaldehyde to me:

http://www.sigmaaldrich.com/spectra/ftir/FTIR000292.PDF

It looks to me like the peaks match up.

[dazed-and-confused]

IR looks ok for 4-chlorobenzaldehyde to me:

http://www.sigmaaldrich.com/spectra/ftir/FTIR000292.PDF

It looks to me like the peaks match up.

I'm sorry I'm not trying to undermine your opinion or anything (I'm in high school what would I know) but those graphs don't look similar at all? I was under the impression that different IR spectrums for the same compound would be almost identical?

[Arkcon]

I'm sorry I'm not trying to undermine your opinion or anything (I'm in high school what would I know) but those graphs don't look similar at all? I was under the impression that different IR spectrums for the same compound would be almost identical?

They should be, but often aren't.  Its hard for you to know unless you've done a bunch of them, but the differences you see are really just noise.  Either your sample holder isn't clean, or your instrument's optics are old, or you're just not as pure as you need to be to get perfect results.  Your results at high wavenumbers is a dead-giveaway.

[Babcock_Hall]

I wonder whether or not the roll around 3500 might be residual water.

@OP when an aldehyde is right next to a double bond or an aromatic ring, its frequency may be slightly lower than other aldehydes.  I think your frequency sounds about right.

[dazed-and-confused]

That sounds reasonable. Thanks everyone for your *delete me*

[aag]

Do you still have this assignment by any chance? because I have the same organic compound and really could use some help

[wildfyr]

This is a 3 year old topic. Is the discussion here not enough to satisfy you? Our chemists seem to have agreed on 4-chloro benzaldehyde

[pgk]

Indeed, it is 4-chlorobenzaldehyde but the IR spectrum above 1800 cm-1 has the typical appearance of a carboxylic acid (intramolecular H-bonding of dimers), which means that the sample is partially oxidized to 4-chlorobenzoic acid and you see their mixture, in the IR spectrum. Besides, 4-chlorobenzoic acid has a similar 1H-NMR spectrum with 4-chlorobenzaldehyde, meaning that 4-chlorobenzoic acid can be hidden behind the signals of 4-chlorobenzaldehyde.