1.3 AROMATIC HYDROCARBONS

MICHAEL FARADAY (1791-1867) first person to isolate and identify benzene early benzene uses: - decaffeinate coffee - antiknock additive in gasoline - solvent in chem rxns now we know benzene is a carcinogen!

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. (b)

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 HYDROCARBONS ~ 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 ~ EXAMPLE 1 1-bromo-2-chloro-4-ethylbenzene

DRAWING AROMATIC HYDROCARBONS ~ EXAMPLE 2 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 substitution rxns substitution rxns addition rxns --->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: UV

HOMEWORK: p. 31 #1-6