Chemistry help - organic analysis

Hi! I've just started organic analysis, and I'm getting super confused by these infrared spectrums. I don't know how to put the image here but I am stuck on question 1aii and aiii LINK- https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FOrganic-I%2F3.6-Organic-Analysis%2FSet-F%2FInfrared%20Spectroscopy%203%20QP.pdf
I identify a peak which corresponds to alcohols for aii, for example, but this is wrong - why?
MARK SCHEME - https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FOrganic-I%2F3.6-Organic-Analysis%2FSet-F%2FInfrared%20Spectroscopy%203%20MS.pdf

Reply 1

t_revision

8

I think I've done that question before and got it wrong, so idk why the diagrams were so unclear. it could be that there's other bits on the diagram which correspond to the other things??

infrared is usually easy

Reply 2

UtterlyUseless69

12

Original post by mitostudent

Hi! I've just started organic analysis, and I'm getting super confused by these infrared spectrums. I don't know how to put the image here but I am stuck on question 1aii and aiii LINK- https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FOrganic-I%2F3.6-Organic-Analysis%2FSet-F%2FInfrared%20Spectroscopy%203%20QP.pdf
I identify a peak which corresponds to alcohols for aii, for example, but this is wrong - why?
MARK SCHEME - https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FOrganic-I%2F3.6-Organic-Analysis%2FSet-F%2FInfrared%20Spectroscopy%203%20MS.pdf

(a)(ii) - the answer is A (cyclohexane). Alcohol peaks tend to be broad and span from 3200 - 3600 ish cm^-1 if memory serves. This peak is very narrow and around 3000 ish cm^-1, which is generally more indicative of C-H bonds in alkyl groups. If you look at all the spectra, there is a sort of peak of this shape in the same region, which implies it is caused by some common feature of all of the molecules shown (C-H bonds).

(a)(iii) - the answer is D (cyclohexene), because there is a small peak at 1650 ish cm^-1. This is classic of an alkene.

Edit: for future reference, take a look at the spectrum in part (a)(iv). The broad thing to the left of the narrow peak I mentioned is the classic alcohol peak (3200 - 3600 ish cm^-1). This sort of peak shape and wavenumber range is what to look out for if you want to show that an alcohol group is present.

(edited 10 months ago)

Reply 3

tkms2009

5

Original post by mitostudent

Hi! I've just started organic analysis, and I'm getting super confused by these infrared spectrums. I don't know how to put the image here but I am stuck on question 1aii and aiii LINK- https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FOrganic-I%2F3.6-Organic-Analysis%2FSet-F%2FInfrared%20Spectroscopy%203%20QP.pdf
I identify a peak which corresponds to alcohols for aii, for example, but this is wrong - why?
MARK SCHEME - https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FOrganic-I%2F3.6-Organic-Analysis%2FSet-F%2FInfrared%20Spectroscopy%203%20MS.pdf

An IR spectrum is for the identification of functional groups in an unkown compound.
First you look for the strongest absorption peak of the spectrum which is the carbonyl absorption peak. You can tell that a(i) is cyclohexanone as its IR spectrum shows a strong peak around 1700cm-1.
For a(ii), there are no characteristic N-H, O-H, C=O, absorption peaks, only saturated C-H stretching peaks are observed. These peaks appear below 3000 cm-1. So this is cyclohexane. You can observe these absorption peaks in all the other 3 IR spectra as all these compounds contain saturated C-H bonds.
For unsaturated C-H stretching peaks, they usually appear above 3000cm-1. There is a peak around 3080cm-1, So a(iii) is a cyclohexene.
a(iv) shows the characteristic broad OH absorption peak around 3300-3500cm-1, so this is cyclohexanol.

Chemistry help - organic analysis

Quick Reply

Related discussions

Articles for you