Chapter 1.3 Aromatic Hydrocarbons Learning Goals: I will be able to… Properly name, write formulas, and draw structures for aromatic hydrocarbons Identify the reactants and products of substitution reaction of hydrocarbons Explain and describe the properties of aromatic hydrocarbon

Michael Farady (1791-1867) First person to isolate and identify benzene Early benzene uses: Decaffeinate coffee Antiknock additive in gasoline (inhibit pre-ignition) Solvent in chemical reactions Now we know benzene is a carcinogen (a substance capable of causing cancer in living tissue)!

Aromatic Hydrocarbons An aromatic hydrocarbon is an unsaturated cyclic hydrocarbon with a pattern of bonding that makes it chemically stable. Benzene, C6H6, is the simplest aromatic hydrocarbon.

Benzene Benzene’s bonds are each of equal length, which means that electrons responsible for the bonds are delocalized and are shared equally between all carbon atoms. The structure alternates between the two forms.

Naming Aromatic Hydrocarbons Case #1 Here, the benzene ring is considered to be the parent molecule with functional group or alkyl group attached.

Naming Aromatic Hydrocarbonds Case #2 Here, the benzene ring is a substituent on the hydrocarbon chain. In this case it is called a phenyl group.

Drawing Aromatic Hydrocarbons 1-bromo-2-chloro-4-ethylbenzene 4,6-diphenyl-oct-2-ene

Ortho, Meta, Para An older naming system uses the above prefixes to indicate the positions of substituents. You should be able to recognize them.

Practice Time! P. 30 #1, 2

Properties of Aromatic Hydrocarbons Many are liquid at room temperature Some are crystalline solid Symmetrical structure means they are nonpolar Previously used as a solvent in dry cleaning (replaced by methylbenzene AKA toluene)

When we look at reactivity… Generally Speaking… Alkanes  aromatics  alkenes reactivity increases

Reactions of Aromatic Compounds Benzene does not readily undergo addition reactions because of its alternating single/double bonds. It does undergo SUBSTITUTION reactions where H atoms are replaced by other atoms, ex. halogens. It behaves like an alkane.

Substitution Reactions of Benzene

Comparison of Reactions

Did You Learn? Aromatic hydrocarbons are a class of cyclic unsaturated hydrocarbons that have a ring structure and bonding that causes them to be chemically stable. Measurements show that the bonds in a benzene ring are all equal in length. Benzene is less reactive than alkenes. Since it participate in substitution reactions, benzene behaves more like an alkane.

HOMEWORK P. 31 #1 – 5