If you have ever pumped gas that claimed to "oxygenated" -- something that is common in most urban areas in the winter -- then you have used gasoline containing MTBE. MTBE is the acronym for methyl tertiary butyl ether, a fairly simple molecule that is created from methanol. Click here to see MTBE's chemical structure.
MTBE gets added to gasoline for two reasons:
- It boosts octane (see this Question of the Day for a discussion of octane).
- It is an oxygenate, meaning that it adds oxygen to the reaction when it burns (see this Question of the Day for a discussion of oxidizers). Ideally, an oxygenate reduces the amount of unburned hydrocarbons and carbon monoxide in the exhaust.
MTBE started getting added to gasoline in a big way after the Clean Air Act of 1990 went into effect. Gasoline can contain as much as 10% to 15% MTBE.
The main problem with MTBE is that it is thought to be carcinogenic, and it mixes easily with water. If gasoline containing MTBE leaks from an underground tank at a gas station, it can get into groundwater and contaminate wells. Of course, MTBE isn't the only thing getting into the groundwater when a tank leaks -- so is gasoline and a host of other gasoline additives, but in recent years, MTBE has been singled out.
According to this page at the EPA:
The most likely thing to replace MTBE in gasoline is ethanol -- normal alcohol. It is somewhat more expensive than MTBE, but it is not a cancer threat.