1. (1.2) Cyclic and Aromatic Hydrocarbons
- Classification & Naming

2. Cyclic Hydrocarbons
Hydrocarbon ring structures are called cyclic hydrocarbons.
2 ends of a hydrocarbon chain join, thus we need to remove 2 H atoms from each end.
Ex. Steroids, cholesterol
steroid

3. Cyclic Hydrocarbons: Alkanes
consider hexane vs. cyclohexane
cyclohexane has 2 less H atoms
general formula for cylcoalkanes is:
CnH2n
Note:
This is the same general formula for an alkene

4. Drawing & Naming Cyclic Hydrocarbons: Alkanes
To draw: draw in a ring like shape Ex: cyclohexane
To name:
use the same rules as for alkanes
use prefix “cyclo” before the parent chain to indicate the ring structure
Ex: cyclopentane

5. Drawing & Naming Cyclic Alkanes
1) Branched cyclic alkanes
If there is a branch, name it first.
Ex. methylcyclohexane

6. Drawing & Naming Cyclic Alkanes
2) Two or more branches on a ring structure
Numbers used to indicate the position of the branches must be the lowest possible.
Any C in the ring can be #1 and we want lowest possible #, therefore if we have one branch we call that C #1
Branches are named in alphabetical order.
Ex. 1-ethyl-3-methylcyclohexane
Note:
We can count the C’s in either direction of the ring.

7. Drawing & Naming Cyclic Alkenes
For the double bond
Its position number is given priority for lowest number
Branches are numbered in order to give the lowest position numbers.
e.x. 3-methylcyclohexene

8. Drawing & Naming Cyclic Alkynes
Name and draw the same as
cycloalkenes, but use“-yne” ending

9. Aromatic Compounds and Benzene
Aromatic compounds contain benzene.
Benzene, C6H6 , is represented as a six carbon ring with 3 double bonds.
Two possible structures can be drawn to show benzene in this form.

10. Benzene Structure
The structures for benzene can also be written as a single structure where the alternating double bonds are written as a circle within the ring. Benzene structure

11. Aromatic Compounds in Nature and Health
Many aromatic compounds are common in nature and in medicine.

12. Naming Aromatic Compounds
Single Branch – method 1
Aromatic compounds are named with benzene as the parent chain.
One side group is named in front of the name benzene.
methylbenzene
(toluene)
chlorobenzene

13. Naming Aromatic Compounds
Single Branch – method 2
Alternatively, aromatic compounds may be named as a branch of a large molecule.
The benzene ring is an alkyl group called phenyl .
phenylmethane
2-phenylpropane

14. Naming Aromatic Compounds
2) Two or more branches – method 1
When two groups are attached to benzene, the ring is numbered to give the lower numbers to the side groups (counting in either direction).
e.x.1,2-dimethylbenzene

15. Naming Aromatic Compounds
2) Two or more branches - method 2
prefixes are also used
ortho (1,2-), meta (1,3-) and para (1,4-)
1,2-dimethylbenzene ,3-dichlorobenzene 1-chloro-4-methylbenzene
(ortho-dimethylbenzene) (meta-dichlorobenzene) (para-chloromethylbenzene)
o-dimethylbenzene m-dichlorobenzene p-chloromethylbenzene

16. Some Common Names
Some substituted benzene rings also use a common name.
Then naming with additional side groups uses the ortho-, meta-, para- system.

17. Learning Check 1 Select the names for each structure:
1. chlorocyclohexane
2. chlorobenzene
3. 1-chlorobenzene
1. meta-methyltoluene
2. meta-dimethylbenzene
3. 1,3-dimethylbenzene

18. Solution 1 Select the names for each structure: 2. chlorobenzene
1. meta-methyltoluene
2. meta-dimethylbenzene
3. 1,3-dimethylbenzene

19. Learning Check 2
Write the structural formulas for each of the following: a) 1,3-dichlorobenzene b) ortho-chlorotoluene

20. Solution 2
Write the structural formulas for each of the following: a) 1,3-dichlorobenzene b) ortho-chlorotoluene