Gasoline is not really a single substance that suddenly appears, ready-to-use in your car, as one of the things obtained from the fractional distillation of crude oil. It is, rather, a mixture of some of those things. Moreover, the components found in “gasoline” have been processed further, following that distillation.
To understand this, you should know that the term “gasoline” is really an old trade name for a mixture of fuels that car engines would run on. Back in the old days, it was all the rage to name products with made-up words that ended in “line” like Vasoline and Gas-o-line. Chuck Berry had a hit with “May-bo-lene” but that isn’t the same thing. Anyway, the “trademark” wasn’t really enforced, and the word gasoline became a generic term. The word gasoline isn’t really used much outside North America, anyway.
So, what are those things that are mixed to make this substance called gasoline? The short answer, chemically-speaking, is that it is made up of various hydrocarbons, specifically those distillates which have carbon chains between 4 and 12 carbons. I didn’t want to digress into such a technical discussion of the subject (even though I do understand it a little) because... well, because you are not likely to read it. But some sort of overview is necessary, so here goes:
The stuff contained in crude oil are hydrocarbons. That is, they are made up of combinations of the chemical elements called hydrogen and carbon. In the types of hyrocarbons we are speaking of, the carbon atoms are arranged all in a line (called a chain) with the hydrogen atoms bonded (by electrons) to the carbon atoms. The name of the chemical substance depends upon the number of carbon atoms in the “carbon chain” of that substance. The carbon chains can be quite long but here are the first 10:
1 carbon = methane
2 carbons = ethane
3 carbons = propane
4 carbons = butane
5 carbons = pentane
6 carbons = hexane
7 carbons = heptane
8 carbons = octane
9 carbons = nonane
10 carbons = decane
A drawing of methane, showing the electron bonds simply as lines would look like this:
A drawing of octane would look like the picture at the top of this post.
The lightest fraction that you get when you distill crude oil, is methane. Methane, with only one carbon atom, has the lowest boiling temperature and so it rises clear to the top of the tower before it is cooled enough to become a liquid again. The boiling temperature of methane and the other petroleum gases (up to 4 carbons) is so low it is hard to keep it liquid on a very hot day. That’s why the propane for your barbecue is sold under pressure to keep it liquified (Liquified Petroleum Gas, or LPG) and that’s why the tank has a safety valve so the gas can escape if the tank gets too hot or too bounced around. Otherwise, the tank would explode.
At the other end of the spectrum are the solids at the bottom of the tower, such as paraffin and tar. Things like these have the highest boiling temperatures - over a thousand degrees F. - and have really long chains of 70 or more carbons. I’m not going to get into “aromatics” here in this post. Just leave your mental image of this set on long straight chains.
In addition to mixing up some of the components that are distilled from crude oil and making gasoline from them, it is also possible to break up the chains of some of the longer chained hydrocarbons and use the resulting parts - shorter chains - to make more gasoline rather than just be satisfied with the amount of gasoline you normally get out of a barrel of crude oil (about 40% of crude oil is gasoline.)
A “catalyst” is some substance which acts to facilitate a chemical reaction. Facilitate usually means “to speed up” the process. Catalysts themselves don’t undergo any permanent chemical change. They only help the chemical change along or make it occur in the first place. Your body, for example, makes specialized enzymes which act as catalysts to speed up the digestion (breakdown of long/complex chemical chains) of some foods.
When long hydrocarbon chains are broken into smaller chains, this is known as “cracking” the chains. When a catalyst is applied for the purpose of speeding up the breakdown process, the procedure is called “cat cracking." Much of the work that goes on at an oil refinery, post-distillation, is concerned with squeezing more gasoline out of a barrel of oil - cat cracking longer-chained components down so they can be combined to make more gasoline. (Or diesel, or jet fuel, or whatever the market is currently demanding more of.)
Next: Why octane? - don’t knock it!