How do you distinguish these peaks on IR data?

[snr.mmorris4.19]

How do you distinguish between an aldehyde peak and a ketone peak on infrared spectroscopy graphs?

[Owlbird83]

Hi, welcome to the forums!

You won't be able to distinguish between whether the peak from the carbonyl (C=O) is in an aldehyde or ketone just from the IR spec. Once you know there's a C=O in the molecule you'll need to use the other info they give you in the question such as HNMR spec to work out the arrangement of the atoms in the molecule.

[Bri MT]

Hi, welcome to the forums!

You won't be able to distinguish between whether the peak from the carbonyl (C=O) is in an aldehyde or ketone just from the IR spec. Once you know there's a C=O in the molecule you'll need to use the other info they give you in the question such as HNMR spec to work out the arrangement of the atoms in the molecule.

Just a note that QCE doesn't do NMR (I think that's pretty sad & I was surprised when I learnt it but it's how it is )

[keltingmeith]

How do you distinguish between an aldehyde peak and a ketone peak on infrared spectroscopy graphs?

I don't /like/ teaching this, because it's not always true. In general, if it's strong and ~1600-1800 wavenumbers, it's a carbonyl, and it's not worth your time going any further than that because LOTS of things will change what region it is in.

... But, some people will claim (and potentially this is what is aimed to be taught to you?) that you can predict what functional group carbonyl you have based on this list here. This isn't always the case by ANY stretch of the imagination, and this can often be wrong - so it's worth looking at other information you have available (such as mass spec, or crystallographic information provided to you). However, in a perfect world with perfect IR and simple molecules (which is likely the case in an exam), then you can use this list.

However, if you're asking this question for IA3, I would not recommend this approach - instead, you should consider suggesting/doing some wet tests, like Tollens' test. These tests CAN have false positives, so make sure you understand if your molecule is likely to give a false positive before trying this out, but it's a simple alternative you can couple with IR spectroscopy that will let you know if you truly do have a ketone or an aldehyde.

[snr.mmorris4.19]

I don't /like/ teaching this, because it's not always true. In general, if it's strong and ~1600-1800 wavenumbers, it's a carbonyl, and it's not worth your time going any further than that because LOTS of things will change what region it is in.

... But, some people will claim (and potentially this is what is aimed to be taught to you?) that you can predict what functional group carbonyl you have based on this list here. This isn't always the case by ANY stretch of the imagination, and this can often be wrong - so it's worth looking at other information you have available (such as mass spec, or crystallographic information provided to you). However, in a perfect world with perfect IR and simple molecules (which is likely the case in an exam), then you can use this list.

However, if you're asking this question for IA3, I would not recommend this approach - instead, you should consider suggesting/doing some wet tests, like Tollens' test. These tests CAN have false positives, so make sure you understand if your molecule is likely to give a false positive before trying this out, but it's a simple alternative you can couple with IR spectroscopy that will let you know if you truly do have a ketone or an aldehyde.

thankyou! very helpful information