I recently completed an unknowns lab. I was given an unknown, and list of possible aldehydes and ketones. I am certain at this point as to what it is (4-hydroxy-3-methoxybenzaldehyde aka vanillin), but I cannot explain why it gave a negative result for both tollens and schiffs test when ran.
Evidance I have for it being vanillin: 1st, the strong scent of vanilla. 2nd: melting point of 81.9-83.6C (Lit. 82C), it's semicarbizone derivative MP was 224.7-225.5C, and it's 2,4-dinitrohydrozone derivative MP was 267.5-269.1C (Literature for both was 230C and 271C respectfully). IR also matches very well with literature IR.
Info on tollens test: 1ml 5% aqueous silver nitrate w/ 0.5ml NaOH, Added about 1.5-2mL 10% ammonia, mixedm abd dukyted to 5mL w/ DI H2O. Test tubes also sat a couple minutes with NaOH. started with 0.5mL tollens soln and 10-20mg vanillin. Heated, saw nothing. Added a little more tollens soln and more vanillin, still saw nothing. It was in warm water bath for about 30 minutes. At the end, it was perhaps slightly darkened, but certainly no silver mirror. Positive control had turned black+silver. Another positive was the same. Also something I found interesting: Our positive control, which worked fine, was 4-methoxybenzaldehyde. Not too unsimilar from vanillin, but a very different result.
Schiffs test: I didnt run this personally, but what I know is this: it initially turned yellow, and when heat was applied, it turned pink/salmon, but reverted to yellow when cooled.
All the data points very strongly to it being vanillin, but I cannot help but wonder why these tests would come out negative.
Thanks for any input or ideas.