When roasting vegetables in a oven, after a while there is a nice caramel smell. After that, if they start burning, there is a smokey smell, and if they continue to burn, the smokey smell gets more and more acrid. The smell really sticks in the back of your throat/sinuses, and induces gagging if it's strong enough. What chemicals make up the gasses and particles that cause that acrid smell?
Take a look at this link about Maillard reactions:
And what occurs if continue burning: pyrolisis