Molecules: You'd better learn to live with them.
While taking organic chemistry, most people end up learning about octane ratings. If gasoline burns too fast, engine knock occurs. A number of factors influence this, the easiest of which to appreciate is branching. By definition, so-called "isooctane" (the structure of which I didn't even realize was the below until today) has an octane rating of 100.
n-heptane has one of 0. 87% isooctane, 13% n-heptane would give you an octane rating the same as regular gas (but wouldn't make very good gasoline, for reasons to be discussed another time...
High octane fuels won't give you better mileage or "more power." It's a shame octane is such a cool word because I see why you people mistake it for meaning "awesomeness." Ironically, many high-octane fuels like alcohols have high octane but lower energy density - they are effectively already partially combusted!
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Comments
I'm not trying to win a trip to the greatest science city in the world with this comment about the lack of comments. I've been checking back, convinced that SOMEONE would have SOMETHING interesting to say about octane. What a fool I've been. Oh well, neat structure, and I await further discussion of heptane. rb
Posted by: arby | September 18, 2007 9:18 AM
In my instrumental analysis class we are testing various gasolines for isooctane content, so this MotD is timely. In the varieties we have tested, there seems to be a lot less isooctane than I thought there would be. Of course, my students haven't handed in their work yet, so I'm can't say for sure.
rb, I hope that was interesting.... maybe not.
Posted by: Chemgeek | September 18, 2007 8:35 PM
Interesting. Remember benzene gives 101 octane, and ethanol gives 113 octane, so there are plenty of non-branched hydrocarbons that give high octane ratings. Butane will give you 91 (and your probably 87 octane gas samples you're injecting into the GC are outgassing butane from the minute you take them from the pump).
Posted by: Molecule of the Day | September 18, 2007 9:16 PM
Ah, well, that's better. Thanks, rb
Posted by: arby | September 18, 2007 9:55 PM
The use of n-heptane as zero rather than n-octane may seem perverse, but there is a nice piece of biology involved. The Jeffrey Pine produces a turpentine that is essentially pure n-heptane, and this was a simple and cheap source for the test. n-octane actually has an octane number of -10
Posted by: Alfred Russel Wallace | September 19, 2007 4:14 PM
General Jimmy Doolittle championed the development and production of 100 octane aviation gasoline in the 1930s. It contains (or did at the time) 50% isooctane.
Posted by: Jim | October 6, 2007 5:18 AM