I have been given three spectras (MS, IR, H NMR) to analyse, from them I have found that the compound is chlorobenzene (and I've confirmed online). The task I have gotten asks me to further justify my findings by using reactions and predicting and analysing the spectra of the products. However, I am quite unsure how I'm going to predict and analyse the spectra of the products from the reaction to prove my compound.
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3$\begingroup$ You could think of some characteristic reactions that chlorbenzene would undergo and then evaluate your hypothesis based on product spectra. For example there's one reaction through aryne intermediate. I wonder then if you could also test for chlorides in the reaction mixture. $\endgroup$– wuschiCommented Oct 11, 2015 at 11:11
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$\begingroup$ As comment on language: The (single) "spectrum", and in plural several "spectra" (well, Webster lists "spectrums" as an alternative of modern times, too). For future reference, please refrain from thinking /saying / writing three "spectras" which were two plurals in one word. $\endgroup$– ButtonwoodCommented Feb 25, 2017 at 0:32
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2 Answers
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The task you had being assigned is called identification by derivatization. It's one of the strongest proof for the confirmation of the structure of a molecule. If you have an hard-to-identify compound X, you can make it react with Y and then analyze the product of the reaction, X-Y, instead.
I once synthesized a polysubstituted aromatic compound and wasn't sure whether or not a iodine atom was present on the ring. So I performed a Heck reaction on the molecule obtaining the corresponding substituted olefin, which was a known compound, present in the literature and could be identified by its NMR spectra.
Chlorobenzene is quite an anonymous compound and its derivatization does not lead to significative compounds. Just an example to answer your question, you can perform a nitration to get o-chloronitrobenzene which is a solid and can be identified by its melting point.
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1$\begingroup$ +1 For the example in the end, nitration will give a mixture of o- and p- nitrochlorobenzenes. You might further nitrate this mixture or use stronger conditions in the initial reaction to get 2,4-dinitrochlorobenzene which is also a known compound. $\endgroup$– K_PCommented Oct 11, 2015 at 13:28
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The key is to unequivocally form something (I'm presuming these are thought experiments) so there can be no ambiguity. The aryne example to get only one product is a good one. Could you also use deuterium compounds e.g. form an organometallic and quench in one case with water (to give benzene) and in the other with D2O as a comparison? Gentle reduction with H2 or D2 would have the same effect. Forcing conditions would give cyclohexane.
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$\begingroup$ Not bad in theory. In practice Cl-metal exchange doesn't generally work with chlorobenzene. The reduction would work but then you create benzene or d-benzene that would not be easy to isolate and characterise. $\endgroup$– K_PCommented Oct 11, 2015 at 18:56
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$\begingroup$ Mass spec, boiling point and NMR, proton and carbon $\endgroup$ Commented Oct 17, 2015 at 0:23