1. Aromatic Hydrocarbons
*These molecular structures follow Huckel’s Rule…but we are not yet defining Huckel’s rule!
(hydrocarbon molecules containing benzene rings or similar structures*)
To make things easier to draw we typically eliminate the need to show the hydrogen in the structure (i.e. you assume what is needed to be there is there) as well as the carbons at every angle (this is called line angle notation), to get the two possible resonance structures in an easily written form:
What is a benzene ring?
The formula for benzene is C6H6. See if you can sketch the Lewis Structure for benzene. (Remember to count the valence electrons first and keep in mind that the term “ring” must have some significance!)
Since both structures “blended” represent a better way to describe benzene they can be “combined” and are typically shown like this ▼
Does your Lewis Diagram look like this? If not, fix it.
Does this molecule exhibit resonance?
Yes, because the alternating double bonds can be in the alternate positions!

2. Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
Using your model kit, construct a model of benzene.
Looking at your model, what is the EPA about each of the carbon atoms?
Does your model look like the picture below... If not fix it.
The EPA about each carbon is trigonal planar. Do you see that the molecule is “flat” or planar as a result?
This model exhibits only one of the resonance forms…but it’s the best we can do!

3. There are 3 derivational uses of benzene rings.
Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
There are 3 derivational uses of benzene rings.
1.) The hydrogen can be substituted for.
Replace a hydrogen atom (white) with a chlorine atom (green) on your benzene model.
This is what your molecule should now look like! It is named chlorobenzene, C6H5Cl

4. There are 3 derivational uses of benzene rings.
Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
There are 3 derivational uses of benzene rings.
1.) The hydrogen can be substituted for.
This is also known as orthodichlorobenzene. (the ortho- prefix indicates that the substituted parts are right next to each other on the ring)
Now replace a second hydrogen atom (white) with another chlorine atom (green) on your benzene model right next to the first one.
This is what your molecule should now look like! It is named 1,2 dichlorobenzene.
Can you tell why the 1,2 designation is used at the beginning of the name?
…because the chlorines are attached to the first two carbons (you can begin numbering the carbons where ever you like but always use the smallest numbers possible in the name….therefore we start with 1)

5. There are 3 derivational uses of benzene rings.
Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
There are 3 derivational uses of benzene rings.
1.) The hydrogen can be substituted for.
Move the second chlorine atom that you placed on the benzene molecule such that it is now a second position away from the first.
This is also known as metadichlorobenzene. (the meta- prefix indicates that the substituted parts are two positions apart from each other on the ring
This is what your molecule should now look like! It is named 1,3 dichlorobenzene.

6. There are 3 derivational uses of benzene rings.
Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
There are 3 derivational uses of benzene rings.
1.) The hydrogen can be substituted for.
Move the second chlorine atom that you placed on the benzene molecule such that it is now a third position away from the first.
This is also known as paradichlorobenzene. (the para- prefix indicates that the substituted parts are two positions apart from each other other on the ring
Paradichlorobenzene is the main component of mothballs….
This is what your molecule should now look like! It is named 1,4 dichlorobenzene.

7. There are 3 derivational uses of benzene rings.
Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
There are 3 derivational uses of benzene rings.
1.) The hydrogen can be substituted for.
Now, replace one of the chlorines on your model with a methyl radical (CH3 group) and the other with a hydrogen
This is also known as toluene. Toluene is just a common name.
Toluene is a liquid that is often used as paint thinner... you can buy it at Home Depot.
This is what your molecule should now look like! It is named methylbenzene.

8. There are 3 derivational uses of benzene rings.
Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
There are 3 derivational uses of benzene rings.
2.) Benzene can be used as a group attached to another molecule as a radical.
Continue to look at the toluene model you have.
The benzene radical is known as the phenyl group and so the name of toluene could also be phenyl methane
◄Methane with the fourth hydrogen replaced by the benzene radical (C6H5-)
Can you see this as a benzene ring attached to a methane molecule in which a hydrogen is replaced by a benzene ring.

9. There are 3 derivational uses of benzene rings.
Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
There are 3 derivational uses of benzene rings.
3.) Benzene molecules can be attached to eachother...they are called “fused rings”
Remove the methyl group from the ring and attach a second ring to the first…such that the formula for the molecule turns out to be C10H8
This molecule is the simplest of the fused rings and is called napthalene.
You may be struggling a bit to get this…..but compare what you have to the model shown here

10. Aromatic Hydrocarbons
(hydrocarbon molecules containing benzene rings or similar structures*)
Now that you are familiar with some of the aromatic hydrocarbons, ask the substitute for your Homework Worksheet. If you have time, you may begin it….if not, do it at home! In any event make sure you put your computer back and plug it in.