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Topic: IR Spectroscopy  (Read 7811 times)

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Offline Vitalspace

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IR Spectroscopy
« on: October 20, 2009, 07:10:59 AM »
4-methyl-2-pentanone and 3-methylpentanal are constitutional isomers of each other. Explain how you could tell them apart using infrared spectroscopy.

I know that 4-methyl-2-pentanone has a ketone and 3-methylpentanal has an aldehyde.

I am still a bit lost on this...

Thanks!


Offline TheDoctor

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Re: IR Spectroscopy
« Reply #1 on: October 20, 2009, 08:17:15 AM »
To be honest this is not the spectroscopy I would use to tell them apart myself, NMR would be much more useful in this regard. While ketones and aldehydes do have subtle differences in their carbonyl stretching frequencies depending on conjugation effects.

In this case it is very difficult to tell. For instance, a fully saturated ketone has a frequency of 1720 cm-1, whereas the aldehyde will be more likely around 1725 cm-1. The rest of the bond types are also the same (C-H's and C-C's) which won't tell you anything either sadly.

Offline jj74

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Re: IR Spectroscopy
« Reply #2 on: October 20, 2009, 08:23:58 AM »
I think that in the aldheyde there's a kind of C-H bond which is absent in the ketone (think about the carbon hybridization) and give a characteristic absorbtion band at 2800 cm -1 circa (but I agree that IR is not the best way)
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Offline TheDoctor

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Re: IR Spectroscopy
« Reply #3 on: October 20, 2009, 08:30:11 AM »
Indeed this is true, the C-H bond of the aldehyde may a possible way to see the difference by IR (a good idea!), although it may be hidden under your methylene, and methyl peaks which also show up around 2800-2900 cm-1.

I'd certainly not feel comfortable using that as a means of identification.

Shantanu Gaius

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Re: IR Spectroscopy
« Reply #4 on: October 23, 2009, 08:04:31 AM »
The IR spectrum is the fingerprint of your sample because all the peaks associated with it are unique. You can compare this spectrum to similar samples using pattern recognition and various calibration techniques to see whether your original sample meets certain criteria.

Offline doc30

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Re: IR Spectroscopy
« Reply #5 on: December 03, 2009, 09:39:28 AM »
IR can readily identify these materials and the differences are easy to see. Look for the presence of the aldehyde C-H band around 2700 cm-1. Aldehydes are lower than C-H stretching bands, at least for the two simple molecules you indicated.

NMR is an excellent tool for identification, but FT-IR is simpler to use, cheaper and far more common in labs. Spectral library searching, if available, increases the certainty of identification by orders of magnitude compared to eyeballing the spectra. If you use vapor phase, the resolution and resulting specificity is even higher.

I just attached a comparison of the spectra of the two molecules in question.

Offline xiaoruan

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Re: IR Spectroscopy
« Reply #6 on: December 03, 2009, 11:34:15 AM »
4-methyl-2-pentanone and 3-methylpentanal are constitutional isomers of each other. Explain how you could tell them apart using infrared spectroscopy.

I know that 4-methyl-2-pentanone has a ketone and 3-methylpentanal has an aldehyde .

I am still a bit lost on this...

Thanks!
IR could determine  related functional group.the two compounds both have carbonyl group(1680-1720cm-1), while aldehyde has two characteristic absorption peaks in functional group filed(272o cm -1 and 2820 cm-1 belong to H of aldehyde group)


Offline alchmist

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Re: IR Spectroscopy
« Reply #7 on: December 04, 2009, 12:32:50 AM »
You could also do a tollens test to tell them apart  ;)

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