Nomenclature or the naming of Alkanes
I briefly touched on the nomenclature of Alkanes when I was talking about continuous, linear alkanes, but this was very simplified. This post will show you how to name any type of alkane or draw an alkane from any name. Again these will just be linear, continuous alkanes not cyclic or aromatic structures.
In organic chemistry in general this is the way you name compounds:
The prefix comes from the position/type of constituent in the molecule (1-ethyl, etc). The parent comes from the number of carbons in the molecule (meth-, Hex-, etc). The suffix comes from the class of compounds (-ane, -ene, -one, etc).
Above are listed some of the most common constituents used, but there are more including cyclic groups. Constituents are groups that branch off of a molecule. They’re named in a similar way (Cyclohexyl for cyclohexane, cyclopropyl for cyclopropane, etc)
To name compounds follow these steps:
1. Find the longest continual chain of carbon atoms. Use the continuous-chain alkane name with the same number of carbons as the parent (if longest chain of carbons is 5 then its pentane for the parent name). If there are two chains of equal length, choose the one with more branch points.
2. Number the carbon atoms of the chain starting from the end closest to a branch point. The goal is to have the lowest possible numbered prefixes corresponding to the branch point(s) of the molecule. We can call these branch points of the molecule substituents; they’re not part of the linear chain of carbon atoms in the molecule and ‘stick out’.
3. Name the substituents according to the alkane with the same number of carbon atoms but instead of writing out ___-ane change the parent name to -yl.
4. The number of the carbon atom to which a constituent is attached and the substituent name are placed before the parent name of the molecule. The number and substituent name are separated by a hyphen (-). Make sure that the branched substituents have the lowest possible numbers. If there is more than one branch point you may have to number carbon atoms from the other side of a molecule.
Here’s a simple example:
The name tells us this molecule is a 5 carbon Alkane (Pentane) with a single methyl group attached to the second carbon.
It should look something like this:
When there are two or more substituents and they’re identical groups use multiplying prefixes like di-, tri-, tetra- and penta-. Each substituent must have have a prefix number and each number separated by commas.
Heptane = 7 Carbon chain. The name also indicates there are 3 ethyl (CH2CH3) groups attached to the molecule at carbon positions 3 and 4. Note that there are two constituents attached at Carbon 3.
Now to draw it (excuse the crappy ascii molecule):
If there are more than one constituent and they’re different types than they’re listed alphabetically and separated by a hyphen.
I’ll give one final example on how to name structures from their molecule:
The longest chain is 5 Carbons = Pentane
There are two constituents at the Carbon positions of 2 and 3 (not 3 and 4 because we want the lowest numbers).
The constituents are an ethyl group at Carbon 3 and a methyl group at Carbon 2.
So the name is:
Happy naming everybody!
Sources: CHEM 2409 (Organic Chemistry I) notes from BCIT.