Please help me to identify the unknown! The IR is posted below, as are my notes
ID: 1000931 • Letter: P
Question
Please help me to identify the unknown! The IR is posted below, as are my notes and obervations. Please explain your thinking process and how you came up with what you did. I really want to understand how to figure this out.
Here are my notes for the liquid unknown based on my observations and some tests I ran;
-Clear liquid
-Strong odor
-Not soluble in water, NaOH, or HCl.
-Soluble in H2SO4. Turned a redish brown color when dissolved in H2SO4. Also gave off heat.
- Successful positive for the multiple bonds test. Added more than 20+ drops of bromines, and the solution still changed back to a cloudy white every time.
-Negative gerrous hydroxide test. Turned a swmapy green color.
Explanation / Answer
Important IR absorptions and conclusions are as follows.
1) 2 medium weak absorptions in the range 2700 – 2800 and 2800 – 2900 cm–1 indicate Aldehydic C-H stretch.
2) A very strong sharp absorption at 1700 cm–1 indicates carbonyl C=O stretch and hence Aldehydic group is much possible.
3) Sharpness of the C=O stretch tend to think the possible conjugating groups like phenyl ring/ C=C etc. But then the peak for C=O has a sharp peak at its shoulder or at about 1650 – 1640 cm –1 for conjugated C=C group.
But as Br2 test is +ve we can nor neglect C=C possibity.
4) Negative Gerrous hydroxide test rull out the possibility of –NO2 group and hence conjugating group must be either phenyl or C=C.
5) Aromatic sp2 C-H generally appear above 3000 cm–1 which is not obtained hence phenyl ring possibility rulled out.
6) No absorptions above 3000 and –ve Gerrous hydroxide test strongly rull out –OH and –NH possibility.
Hence possible structure will be just a conjugated aldehyde without any olefinic C-H i.e. C=C is tetrasubstituted and hence become IR inactive.
Any exact structure cannot be suggested.
1H-NMR and 13C-NMR will help in determining exact structure.
Hence the analysis of given IR spectra.
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