atom145 Posted January 30, 2017 Share Posted January 30, 2017 (edited) I'm trying to identify an unknown white crystal experimentally. It dissolves in NaOH (but not water or HCl), and bubbles are evolved when it's added to a solution of sodium bicarbonate so it's acidic. Its boiling point is between 105-130 degrees C. It decolourises bromine water, so it must be unsaturated. This suggests an alkene or alkyne, but I'm not familiar with acidic unsaturated compounds except fatty acids, and that isn't consistent with the boiling point. Does anyone have a clue what this could be? Until it decolourised bromine I was convinced I had benzoic acid on my hands, but since aromatic groups don't undergo addition that doesn't make sense. It doesn't smell of anything, and when I added it to a flame it smelt a bit like burnt rubber. Edited January 30, 2017 by atom145 Link to comment Share on other sites More sharing options...
hypervalent_iodine Posted January 30, 2017 Share Posted January 30, 2017 You don't really have enough information to say more than what two of its functional groups possibly are. One you have correctly identified as an alkene or alkyne, the other you're almost at. What part of a fatty acid makes it acidic? Have you been given a molar mass and / or molecular formula at all? Link to comment Share on other sites More sharing options...
atom145 Posted January 30, 2017 Author Share Posted January 30, 2017 (edited) The carboxyl group obviously. I don't have a molar mass or any formula. And I can't find an alken- or alkynoic acid with a melting point anything like that high. I've attached an IR and UV/vis spectrum. The UV/vis might be a bit dodgy though and I don't have access to the equipment any more. Any thoughts about further steps? I don't have access to the equipment for mass spectrometry or NMR and I'm not sure what other reactions or techniques I could use to help identify it. Edited January 30, 2017 by atom145 Link to comment Share on other sites More sharing options...
Sensei Posted January 30, 2017 Share Posted January 30, 2017 (edited) Organic compounds that contain Nitrogen, while heating without presence of Oxygen, can decompose to ammonia. You could try these tests also: bubbles are evolved when it's added to a solution of sodium bicarbonate so it's acidic. You should verify whether produced gas is CO2.. Pass it through solution of Ca(OH)2 https://en.wikipedia.org/wiki/Limewater Edited January 30, 2017 by Sensei Link to comment Share on other sites More sharing options...
hypervalent_iodine Posted January 30, 2017 Share Posted January 30, 2017 Your IR looks dodgy. Do you have access to mass spec of any sort (GCMS maybe)? NMR really is the gold standard. If you also don't have mass spec, you are going to struggle. Could you give more context on the problem? Is it an assignment of some sort? Sorry, just read that you don't have access to mass spec either. I can't see a practical way forward in properly identifying your compound without this or NMR. I would rerun your IR for starters, but it isn't going to tell you much that you don't already know. Just having the possible identity of two functional groups and a melting point isn't enough to tell you anything beyond that. Is elemental analysis possible? Again, context would be helpful. Is this homework? Is this a reaction product or a biological isolate of some sort? One more thing. Bromine can decolourise functional groups other than alkynes and alkenes. At a high school level, you would typically only be expected to consider the reaction with these two groups, however I am unclear on whether or not this is where the question comes from. If not, you need to consider things like aldehydes, anilines, phenols, etc. This is why I think you're going to be a bit stuck without mass spec and NMR. Link to comment Share on other sites More sharing options...
atom145 Posted January 31, 2017 Author Share Posted January 31, 2017 (edited) It's for a project in my final year of school that makes up part of my final grade. I'm confident it doesn't contain sulfur or a halogen, as my teacher (who chose the compound) implied as much, as he didn't want the fusion test to be necessary. And unfortunately I can't rerun the IR as I used a portable IR kit which the school only had for a day. As far as I understand, because the sample was solid when I ran the IR, the left hand side of the graph would have been suppressed. Are there any carbohydrate compounds which fit these results you could suggest? Even just more tests I could use to gather data? I've got some free time in the next few days due to exam leave and was hoping to get any remaining tests done. Edited January 31, 2017 by atom145 Link to comment Share on other sites More sharing options...
hypervalent_iodine Posted January 31, 2017 Share Posted January 31, 2017 It's for a project in my final year of school that makes up part of my final grade. I'm confident it doesn't contain sulfur or a halogen, as my teacher (who chose the compound) implied as much, as he didn't want the fusion test to be necessary. And unfortunately I can't rerun the IR as I used a portable IR kit which the school only had for a day. As far as I understand, because the sample was solid when I ran the IR, the left hand side of the graph would have been suppressed. Are there any carbohydrate compounds which fit these results you could suggest? Even just more tests I could use to gather data? I've got some free time in the next few days due to exam leave and was hoping to get any remaining tests done. Your sample being a solid shouldn't make the IR look like that. The issue with the >1500 cm-1 region is that this is where you would normally look for functional group peaks, and it's so broad and lumpy that you can't see anything. The fingerprint region isn't much better, which makes it a bit useless. Do you know what kind of IR you used? If your compound were a sugar, it would most likely be a solid, not a liquid. To me, your situation is such that I can't see how you're going to come to any sort of answer without some more analytical techniques. Your IR and UV are both inconclusive due to both data sets being of poor quality. You have what I would consider one definite functional group (the acid). The other test you did is really inconclusive, but that depends on what you've learned to date about bromine water tests (have you only learned about their interaction with alkenes and alkynes?). You know it's a liquid at room temp. That, sadly, tells you almost nothing. You can do some other chemical tests for other functional groups, but these still won't allow you to have anywhere near a complete picture. For example, you still wouldn't know the number of carbons, their arrangement, where the functional groups are, if there are more than one of them... In all, I have to believe that there's something we're missing here. How are other students completing this assignment? What exactly is your teacher expecting of you? Edit: sorry, I misread your OP. Is your compound a solid? How come you have a boiling point range for it and not a melting point? Given that it wasn't water or acid soluble, you can still discount it being a sugar. Link to comment Share on other sites More sharing options...
atom145 Posted January 31, 2017 Author Share Posted January 31, 2017 (edited) Yeah, I meant melting point and for some reason stupidly mistyped. It's solid at room temperature and I'm the only one doing it as we have to choose our own project (it's for Advanced Higher Chemistry in the Scottish system). There's no set goal since you choose your own (I chose this from a list of projects done in previous years) but I think the point is to apply chemistry at Advanced Higher level to postulate a sensible structure - the examiner won't know what the compound really was. I have learnt a bit about electrophilic substitution of halogens but not in the context of a bromine water test. I suspect the IR and UV equipment wasn't in particularly good condition- it was a portable kit that goes round every school in southern Scotland, and it didn't help that I had to rush through them. Thank you so much for all the time you've spent! I really appreciate it. Edited January 31, 2017 by atom145 Link to comment Share on other sites More sharing options...
Sensei Posted January 31, 2017 Share Posted January 31, 2017 (edited) Yeah, I meant melting point and for some reason stupidly mistyped. It's solid at room temperature In other words in original post there should be: "Its melting point is between 105-130 degrees C.".. (Why so large dispersion?) You could try to search for it, on the list of unsaturated fatty acids: https://en.wikipedia.org/wiki/List_of_unsaturated_fatty_acids How about its density? (do you have weight with +-0.01g or +-0.001g precision?) How about solubility of compound in various solvents (organic/non-organic/water) ? If your compound were a sugar, it would most likely be a solid, not a liquid. If it would be sugar, he could try caramelization.. https://en.wikipedia.org/wiki/Caramelization (to read by OP) Edited January 31, 2017 by Sensei Link to comment Share on other sites More sharing options...
hypervalent_iodine Posted January 31, 2017 Share Posted January 31, 2017 Yeah, I meant melting point and for some reason stupidly mistyped. It's solid at room temperature and I'm the only one doing it as we have to choose our own project (it's for Advanced Higher Chemistry in the Scottish system). There's no set goal since you choose your own (I chose this from a list of projects done in previous years) but I think the point is to apply chemistry at Advanced Higher level to postulate a sensible structure - the examiner won't know what the compound really was. I have learnt a bit about electrophilic substitution of halogens but not in the context of a bromine water test. I suspect the IR and UV equipment wasn't in particularly good condition- it was a portable kit that goes round every school in southern Scotland, and it didn't help that I had to rush through them. Thank you so much for all the time you've spent! I really appreciate it. In that case, you also have a problem with your melting point. 105-130 is an incredibly huge range, and would normally indicate that there is something wrong with your sample. It's a shame you don't have access to more information on your compound. In a second year course I used to tutor here at the university I work at, students would be given a similar assignment, but the Unknown compounds always came with a mass spec. I don't see how you come up with a structure without this or a formula. Repeat your melting point. You should get a range of no more than a few degrees. In other words in original post there should be: "Its melting point is between 105-130 degrees C.".. (Why so large dispersion?) You could try to search for it, on the list of unsaturated fatty acids: https://en.wikipedia.org/wiki/List_of_unsaturated_fatty_acids How about its density? (do you have weight with +-0.01g or +-0.001g precision?) How about solubility of compound in various solvents (organic/non-organic/water) ? If it would be sugar, he could try caramelization.. https://en.wikipedia.org/wiki/Caramelization (to read by OP) It's not a sugar. Link to comment Share on other sites More sharing options...
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