Organic Nomenclature

When you name an organic compound, step one is generally finding the main "parent" compound name that serves as the foundation on which all things are built. To do this, you find the longest continuous carbon chain in the molecule. Once you find it, the number of carbons sets the base name (or stem). Any side chains or functional groups on this chain will be named according to the name of the group and the number position in the main carbon chain. For example, maybe you find a 5-carbon chain as the longest in a molecule. Also, the 3rd carbon in the chain has a chlorine as a group (an alkyl halide is the classification now). The name would be 3-chloropentane.

Cl

Notice how in this case, you can number from either end of the C-chain and you will get 3 as the position of the chloro-group. Now look at the two chloropentane structures below. Are they different?

Cl     Cl

The answer is no, they are not different. Why? Well sure, it looks like the one on the left is 2-chloropentane and the one on the right is 4-chloropentane. However, you need to remember to always number the C-chain so that you get the smallest possible number for the first substituted group. This means that the second structure (the one on the right) is also 2-chloropentane. You just have to number the chain starting on the right end instead of the left end. So those two structures are technically identical and therefore have the same name.

Ok, now I'm going to add another C to the structure above and have two Cl's as well.

Cl Cl

Can you name it? It is still only 5 carbons in length because the extra C was added as a branching point on the 3rd C. But we now have two chloro groups. We will have to identify the chloro-positions now as numbers 2 and 4 on the pentane chain. We will also use prefixes to have multiplicity in groups. The prefixes we use are:

2 = di-    3 = tri-    4 = tetra-    5 = penta-    6 = hexa- and so on...

So the name of the previous compound is :

2,4-dichloro-3-methylpentane

And one more thing... when you have multiple side groups with different names, you put them in the name in alphabetical order. This is why I have the chloro's before the methyl.

Cycloalkanes

If you take any alkane of 3 or more C's and bond the end carbon back to the first one, you've got a cycloalkane. In order to join them you have to remove 2 H's to accommodate the new C-C bond. So the generic formula for a cycloalkane is CnH2n. Here are the first four basic cycloalkanes:

More Group Names (side groups or chains)

So I've used only chloro– and methyl– as groups so far on this page. There are lots and lots more. I'll list a bunch of common ones here that I think you should know and use.

group namesymbol
←left
symbol
right→
fluoroF––F
chloroCl––Cl
bromoBr––Br
iodoI––I
methylCH3–CH3
ethylCH3CH2–CH2CH3
phenylC6H5–C6H5
nitroO2N–*–NO2
aminoH2N–*–NH2
hydroxyHO–*–OH
cyanoNC–*–CN

* these are not always shown flipped like they are here. Often they look just like the right view but with the connecting line on the right side of the symbol. That is: NO2–, NH2–, OH–, and CN– .



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