[prasad2154]

So I had a test previously, but the first problem was really odd. It was:

HOOC(CH₂)₃COOH reacting with P₂O₅ and in heat. I originally thought it was a decarboxylation with one of the carbonyl oxygens taking out the other carboxylic acid's hydrogen, then producing CO₂ and a 5-membered-ring hydrate with two -OH groups on a carbon, but it made no sense as to why there would be P₂O₅ if there wasn't any water being produced. I then just did a proton transfer from one of the COOH's to the other's OH group, then attacked the carbon with the Oxygen anion to push the water off as a leaving group, giving me a six-membered ring with one of the Oxygens being a part of the ring with the ketones on either side, with water being a byproduct.

Here is a picture of my solution: https://imgur.com/a/Ct30dIO
(I just realized I forgot to correctly draw the correct number of carbons (5) in my middle steps, but my final product does indeed have the correct 6 carbons, and I don't feel like re-uploading the picture again)

I know this seems like a simply problem, but it was on a test with a bunch of more complicated problems like Wolff-Kishner and stuff, so I was really confused. I wasn't able to think of another example in class in which we had used a dehydrating agent in relation to carboxylic acids other than decarboxylation, but the carbonyl wasn't a beta-carbonyl so I wasn't sure if sterics would be an issue. In addition, why would there be a dehydrating agent if there's no water forming?

[phth]

Always number your carbons especially on an exam. You're right about P2O5 taking the oxygen atom. a mnemonic for P2O5 is the part of matches that ignites them upon friction. Check out :https://chemistry.stackexchange.com/questions/35154/mechanism-of-carboxylic-acid-and-amide-dehydration-with-phosphorus-pentoxide for the mechanism. Does it make sense that P2O5 acts as a base? The pKa of the sp3 oxygen is much lower (harder to protonate) than the sp2 oxygen of the carbonyl. Memorize pKas practice mechanims with a pencil and paper, and you could check your work with google.