Volatile compounds that contain sulfur smell particularly bad. This probably has an evolutionary basis. Consider the chemistry of spoiled food. Bacteria break down the macro nutrients, but the process is often incomplete. Small chemical fragments waft into the air and tell the world that this food is spoiled, and is probably not safe to eat. The smell is repulsive, and even if we are hungry we're going to look elsewhere for sustenance. Of course some animals, like vultures and other scavengers, are attracted to the smell. They can safely eat carrion if it isn't too old.
When threatened, a vulture will regurgitate his partially digested meal towards his attacker, and that is usually enough to deter. (You thought the meat smelled bad before he ate it!) If the predator continues to advance despite the noxious slime, the vulture, with his crop empty, is light enough to fly away. Better to lose a meal then become one.
The simplest example of a sulfurous compound comes from a rotten egg, which is again, something you shouldn't eat. When oxygen levels are low, as is the case in the interior of an egg, certain bacteria break down amino acids for energy, but the metabolism is incomplete. One of the byproducts is H2S, betraying the presence of bacteria in the egg. The same reaction occurs in your colon after eating cruciferous vegetables or other foods containing sulfur. Some compounds, such as fiber and oligosaccharides, are indigestible to us. Food rarely goes to waste in nature, hence certain intestinal bacteria derive energy from these foods under anoxic conditions and release gases, also known as flatulence. The principle gases are carbon dioxide, methane, and hydrogen, which have no smell, but if the food contains sulfur, then hydrogen sulfide is produced, along with trace amounts of other sulfurous compounds such as CH3SH, imparting a rather nasty smell.
An alcohol is a chain of carbons with hydrogens all around and OH at the end. In the same way, a thiol, originally called a mercaptan, is a chain of carbons with hydrogens all around and SH at the end. In other words, oxygen is replaced with sulfur. These compounds were originally called mercaptans because they are able to capture mercury, a rare and useful property. Here are some examples of simple thiols, still called mercaptans in their chemical nomenclature.
These thiols in combination are familiar to us as the smell of a skunk. In fact they are detectable at concentrations of 10 parts per billion. Even sweat contains tiny amounts of thiols, as does mouse urine, but skunks have parlayed sulfur chemistry into an impressive chemical defense. Charles Darwin wrote this in his Voyage of the Beagle.
"Conscious of its power, it roams by day about the open plain, and fears neither dog nor man. If a dog is urged to the attack, its courage is instantly checked by a few drops of the fetid oil, which brings on violent sickness and running at the nose. Whatever is once polluted by it, is for ever useless."
A skunk carries 15 cc of spray, enough for 5 or 6 shots, whereupon it takes ten days to replenish his supply. After a skunk wards off a predator, you might bravely approach, but I can picture the skunk saying this.
I know what you're thinking. Did he fire six shots or only five? Well, to tell you the truth, in all this excitement, I've kinda lost track myself. But being as these are highly evolved anal glands, the most powerful scent glands in the world, and would blow your nose clean off, you've got to ask yourself one question: "Do I feel lucky?" Well, do ya, punk?
Our dog tangled with a skunk one fine morning, and spread the smell throughout the house before we could catch her. Despite a thorough cleaning regimen, the smell remained within the house for weeks, and clung to the dog's thick fur in traces for nearly three months. On that fateful morning the smell was so strong I didn't recognize it as skunk. I thought she was deliberately splashed with a caustic chlorinated solvent, and I was concerned for her safety. "Should we take her to the vet?" I asked. She seemed physically unharmed, though in obvious distress. I stepped back a few paces and then it smelled like burning rubber. What kind of liquid smells like burning rubber? Finally I realized I was experiencing skunk spray at point blank range. Many compounds have different subjective smells at different concentrations. For example, beta-ionone smells like violets at low concentrations, and like cedar at higher concentrations. We recognize thiols as "skunk" at relatively low concentrations, the spray having dispersed throughout the air. It high concentrations it is almost unrecognizable, and you wonder, as you gasp for air, if you are under chemical attack - and technically you are. We washed our clothes, linens, upholstery, and our furry friend with gentle oxidizing agents, which took the edge off, but the light odor of skunk persisted in our home for quite some time.
In northern Mexico, Bear Grylls caught a skunk in one of his makeshift traps, and could smell his quarry 100 meters away. As he approached the animal, he had the same reaction I had. "It smells like 100 burning tires all at once. The stench is so bad I can barely breathe." He killed it, cooked it, and ate it, commenting, "Somebody's got your steak, rubbed it in dog feces, and then popped it in your mouth."
Thiols have practical applications, particularly as odorants in combustible fuels. Natural gas consists of methane and ethane, and has virtually no smell. A gas leak could go undetected until somebody lights a match or turns on an electrical appliance. The slightest spark can start a fire. In 1937, a natural gas explosion in an elementary school in New London, Texas killed 295, and injured 300 more. A gas leak had gone unnoticed for days until a spark from an electric sander ignited the mixture of gas and air, reducing the entire schoolhouse to rubble. Soon thereafter, the Texas legislature required odorants in natural gas, though some companies were already doing so. Other states and countries quickly adopted similar legislation. The ideal additive seems to be t-butyl mercaptan, C4H9SH. This is not a straight chain molecule like butyl mercaptan shown above. Instead, a central carbon has three methyl groups and an SH attached. Think of it as a tripod with a carbon in the middle, three methyls (CH3) for feet, and an SH on top. The substance is a clear liquid that boils at 65 degrees C, or 149 degrees F. Like other low molecular weight thiols, it has a hideous smell, detectable in parts per billion. Gas only needs this compound in parts per million, whence even a small leak, spread throughout a large room, is noticeable long before the gas reaches flashover concentrations. But there is another requirement: any odorant must be rendered nontoxic and nonoffensive through combustion. When completely burned, a thiol produces water, carbon dioxide, and sulfur dioxide. The latter has a lit-match smell, and is somewhat toxic in parts per million, but is harmless and undetectable in parts per billion. Thus gas stoves and hot water heaters vent their combustion products into the home, and this causes no trouble, assuming the house is properly ventilated. You may notice a trace of thiol when the appliance first lights, but after that all is well. The smell of the thiol, that would indicate a gas leak, is a thousand times stronger than the smell of sulfur dioxide. Butyl mercaptan is just what the doctor ordered.