Saturday, May 9, 2009

Cyclic alkanes and conformations

Naming cyclic alkanes

First you would count the longest carbon chain. You can see that the "cyclic" ring has 8 carbons, so it's octane. Since it's a cyclic ring, you call it a "cyclo" alkane. Hence, the main part of the molecule would be called cyclooctane. The substituent is called a methyl group, so you get the name methylcyclooctane.

Now, the hard part of this is figuring out numberings. You can pick ANY carbon to be "1," so we'll conveniently choose the carbon that the methyl is on as methylcyclooctane. It wouldn't matter if the methyl group was on the top right carbon, because it would still be methylcyclooctane, because you could rotate that molecule to get the one above. Now, let's try two substituents.

We again know it's a cyclooctane, but the problem arises in how to number the substituents. We could name this 1-methyl-3-ethylcyclooctane, or 1-ethyl-3-methylcyclooctane. We name it 1-ethyl-3-methylcyclooctane because of the alphabetization rules. Also, priority is given to the longest carbon chain substituent in naming (so we wouldn't name it 3-ethyl-1-methylcyclooctane).

These are some ways to draw the same compound, cyclohexane. Cyclohexane is a pretty important molecule, and I guarantee you will see these again.

Basically the last two are cyclohexane drawn the way it really does look. The 2nd drawing is for "watered down" organic that is taught to AP Chemistry students. If you take a real orgo class (although I've only taken an intro course) you will learn all sorts of cool stuff like that.

Now, with the 4th cyclohexane, there are two positions for atoms.

Note that this is the same molecule as the top right cyclohexane, except I've drawn in all the hydrogens.

The lines going up and down are considered the "axial" position, while the ones going right and left are considered "equatorial." Now, guess which position is more stable. The equatorial is! If you have an axial position, there is steric repulsion between all the axial atoms.

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