The carbon atoms are now joined in a
ring, rather than just a straight chain.
The naming works essentially the same as for the others, but now the
prefix ‘cyclo’ is added in front of
the main name.
 |
| Although it is possible to have cyclohydrocarbons with as few as 3 or 4 carbons, they are rarely seen because of their instability (due to the very small bond angles). It is also possible to have rings with 7 or more carbons, but they are not as commonly occurring as the rings with 5 or 6 carbons. |
NameIts!
Sometimes it is best to name a cyclohydrocarbon using the ring as the main chain that has substituents (first structure below). Other times, it makes more sense to treat the ring as a substituent on another main group (second structure below).
 |
| As usual, create a numbering system that provides the lowest possible numbers for all substituents. In this case, "1" will be at the bottom left methyl and numbering will continue around the ring in a clockwise manner. This gives the following name: 1,2,4 - trimethylcyclohexane. |
 |
| As usual, create a numbering system that provides the lowest possible numbers for all substituents. In this case "1" will be on the right and continue left down the length of the chain. This gives the following name: 5-ethyl-4-methyl-2-cyclopentyloctane. Notice that the "cyclo" is not used for alphabetization, but rather the "p" in pentyl. |
Aromatic
Hydrocarbons
The most common aromatic group is benzene (to whom we were first introduced when discussing graphite in the Structures & Properties unit - which seems like a hundred years ago 😕).
NameIts!
Sometimes
it is best to name a benzene-containing compound using the ring as the main chain
that has substituents (first structure below). Other times, it makes
more sense to treat the benzene as a substituent on another main group
(second structure below). When benzene is a substituent , we use the
name "-phenyl."
 |
| As usual, create a numbering system that provides the lowest possible numbers for all substituents.In this case, "1" will be on the upper right methyl group and numbering will continue around the benzene ring in a clockwise manner. This gives the following name: 4-ethyl-1,2-dimethylbenzene. |
 |
| As usual, create a numbering system that provides the lowest possible numbers for all substituents.In this case, "1" will be on the main chain carbon to which the benzene ring is attached and continue left down the length of the chain. This gives the following name: 6-ethyl-1-phenyloctane. |
Drawing
a Structure from a Formula
We can draw a structure from a formula. Start drawing by looking at the end of the
name and working forward.
DrawIts!
1-cyclohexyl-4,5,7-trimethyl-1-phenyl-trans-non-2-ene
- First draw a nine carbon chain and number it.
- Add the double bond at the second carbon, being sure it has trans conformation.
- Add the phenyl at "1."
- Add three methyl groups at "4", "5" and "7" respectively.
- Add cyclohexyl group at "1."
This gives:
Ortho, Meta & Para Benzene
Substitution
Benzene is so often substituted with two substituents that chemists have come up with a special naming system for this situation:
- 1,2 substitution - ortho
- 1,3 substitution - meta
- 1,4 substitution - para
 |
| This could be appropriately named 1,2- dimethylbenzene or ortho-dimethylbenzene or o-dimethylbenzene. |
 |
| This could be appropriately named 1,3- dimethylbenzene or meta-dimethylbenzene or m-dimethylbenzene. |
 |
| This could be appropriately named 1,4- dimethylbenzene or para-dimethylbenzene or p-dimethylbenzene. |
Homework #1-5
Homework and Answer Keys can be found here.
