[Catfok]

I have been given an unknown solid compound. It is bright yellow crystals.

MP - 78/80deg c
Insoluble in water, 5% HCl and 5%NaOH
In sulphuric acid a fine red ppt evolved however some of the yellow solid remained. It looked as though some product dissolved in the reaction because the liquid turned slightly transparent red as well.

From the IR I believe there is carbonyl group, and when burning the solid it gave off a black smoke which makes me think it is an aromatic as well. Halide testing was negative.

I am just having a little difficulty interpreting the IR spectra, and am not sure if I might have an ester/ketone/aldehyde. Or if I am completely missing the mark... I can't see an OH stretch so I don't believe it is a carboxylic acid.

[Consequentium]

I find it's helpful to list what you know in a table for these sorts of problems.

You state that you developed a red substance in concentrated (I'm assuming) sulfuric acid. Ask yourself rhetorically: Why did the color change?

Also, if you have access to the very popular Pavia Spectroscopy textbook, there are more advanced techniques for determining whether you have an aromatic compound by looking at the fingerprint region and a little after where you normally expect a carbonyl spike. If you don't have the textbook, there are numerous websites that give very specific (more than you'll ever need to know)  information about where certain absorption patterns occur.

You know you have a carbonyl. You know you don't have an alcohol. I would ask the following other questions:

• What are the IR characteristics of an aldehyde?
• What are the IR characteristics of an ester?
• Is it possible to see both in the same spectra?
• Are there functional groups that absorb in the same region that might confuse the issue?
• Is there functionality I haven't thought to look for?

I hope this helps. If you find this gives you more questions and you're still confused, you can at least know more specifically where you're struggling.