Somehow the brain classifies the thousands of smells we encounter every day. Finding out how this works may bring biologists not only intellectual satisfaction, but also money. There reigned in the cities a stench barely conceivable to us modern men and women. The streets stank of manure, the courtyards of urine, the stairwells of moldering wood and rat droppings, the kitchens of spoiled cabbage and mutton fat.
Aldehydes are simple organic compounds that also happen to be smelly .
Eleven-carbon aldehydes smell like citrus. Lop off one carbon atom and you have a soapy, waxy smell. Down around six carbons you get the overwhelming sensation of freshly mowed grass. Somehow the nose can differentiate these aldehydes, and it doesn't stop there. The orange, rose-like smell of octanol contrasts with the rancid, sweaty odor of its chemical relative octanoic acid.