Benzene & Aromatic Compounds

Structure = Cyclic planner Introduction Name= Benzene Chemical formula= C6H6 Structure = Cyclic planner Hybridization= SP2 Bond angles= 120o Bond Length= 1.39 A

1) Benzene can be written as a six- membered ring with alternating single and doublebonds (Kekulé structure). 2) Benzene reacts with substitution reactions not with addition reactions

The Aromatic Character When to say this compound is aromatic? 1) If it I cyclic 2) If it has π and σ alternating bonds 3) If it is planar

4) Reacts with substitution not addition reactions 5) Contain (4n+2) π electrons This is called Huckel’s Rule Where n= 0,1,2,3,……. When n=0 π=2 n=1 π=6 n= 2 π=10 n= 3 π=14

Examples of aromatic compounds Examples of non aromatic compounds

Excercise Apply Huckel,s rule to the following

Not aromatic (does not follow Huckel,s rule Not aromatic (not planar)

Stability of Benzene Benzene is more stable than cyclohexatriene by 36 kcal\mol : Resonance energy

Nomenclature of Aromatic Compounds 1. Monosubstituted Benzenes a. IUPAC name

b. Common Names Of Monosubstituted Benzenes

-C6H5, Ph- Φ Phenyl group Benzyl group

Disubstituted Benzene All disubstituted benzenes (two groups are attached to benzene), can give rise to three possible positional isomers.

2) When the substituents are different, they are arranged alphabetically

If one of the substituents is part of a common named compound, then the disubstituted benzene is named as a derivative of the parent compound

Some disubstituted benzenes have common names

Electrophilic Aromatic Substitution Reactions

Summary of the electrophilic substitution reactions

Side-Chain Reactions of Aromatic Compounds a. Halogenation of an Alkyl Side Chain b. Oxidation of an Alkyl Side Chain

Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted Benzenes Meta directors Ortho , para directors -NO2 -SO3H -COOH, -COOR -CHO, -COR -CN -OH, -OR -NH2, -NHR, -NR2 -C6H5 -CH3, -R (alkyl) -F, -Cl, -Br, -I

Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted Benzenes Alkyl groups and groups with lone pairs (electron donating groups) direct new groups to ortho-, para-positions and speed-up the reaction (i.e. o & p directors and activating groups). Halogens direct new groups to ortho-, para- positions but they slow down the reaction (i.e. halogens are o & p directors and deactivating groups). Electron withdrawing groups such as nitro, nitrile, and carbonyl direct new groups to the meta-position and slow the reaction down (i.e. i.e. m directors and deactivating groups).