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How to identify that a unknown compound is benzophenone?

My working:-

First I added 2,4- DNP(2,4-Dinitrophenyl hydrazine) to the compound. Yellow-orange crystals were formed which indicated presence of carbonyl group. Then, I added Tollen's reagent to the compound. It did not gave silver mirror which means aldehyde group is absent. So, it must be a ketone. So, I added m-Dinitrobenzene to the compound. It did not gave violet colour. Alternatively, I added Sodium nitroprusside to the compound. It did not gave a red color. Both the test is for confirmation of ketone compound.

I asked my teacher what is the unknown compound. He said that it is benzophenone. Also, in the practical book where all the chemical test were mentioned, it was mentioned as a side-note that benzophenone does not give the above mentioned test. So, what is the chemical test for identification of benzophenone?

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  • $\begingroup$ You can use iodoform test after Tollens test. $\endgroup$ – Aditya Dev Dec 22 '15 at 13:46
  • $\begingroup$ @AdityaDev, benzophenone will not give iodoform test because it does not contain ketomethyl group ($\ce{-CH3CO}$). $\endgroup$ – Nilay Ghosh Feb 17 '16 at 7:12
  • $\begingroup$ oops. i thought benzophenone was CH3-CO-ph. my mistake. $\endgroup$ – Aditya Dev Feb 17 '16 at 7:16
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After identification of ketone, to identify benzophenone you need to find the range of melting point using melting point apparatus which you should get in range of $48.3\ \mathrm{^\circ C}$ to $50.6\ \mathrm{^\circ C}$.

Then you can find IR of unknown(benzophenone).

Source: Prezi

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experiment To 50 mg of benzophenone taken in a dry test tube add 1 gm of naphthalene mix and heat until melts. Add a small piece of sodium metal and heat again. observation The mixture turns green due to the free radical formation which is in delocalisation with two phenyl rings

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  • $\begingroup$ Could you provide a source from which you found the experiment? $\endgroup$ – Nilay Ghosh Apr 29 '16 at 13:16
  • $\begingroup$ @koteswararao D How do you say that the radical will be formed in the benzophenone? A radical anion formation in naphthalene with sodium is known. $\endgroup$ – Eashaan Godbole May 24 '17 at 2:00

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