Aromatic Compounds

What is aromaticity

• The Huckle Rule - compounds that contain (4n+2) pi electrons are considered to be aromatic (n is an integer)

• Anti-aromatic compounds contain 4n pi electrons

• Name comes from the strong aromas of these compounds

Benzene

• Aromatic hydrocarbon

• C6H6

• 6 pi electrons

• Fully conjugated

• Resonance stabilized

Structure of Benzene

• All of the following are accepted representations of benzene

Molecular Orbitals of Benzene

the benzene system has three molecular orbitals which accommodate the six electrons, two in each

Resonance Stabilization of Benzene

• Cyclohexane – saturated • Benzene – unsaturated

– Hydrogenation values• Cyclohexene 28.6 kcal/mole• Cyclohexadiene55.4 kcal/mole• Cyclohexatriene 49.8 kcal/mole

– Expected around 84-86 kcal/mole

• Therefore: resonance stabilization energy~30-36 kcal/mole

Experimental Evidence

• Halogenation of benzene produces no reaction without a catalyst

• Most substitution reactions of benzene are endothermic (absorb energy)

Monosubstitued Benzenes

• Toluene

• Bromobenzene

• Chlorobenzene

• Phenol

• Nitrobenzene

• Alkylbenzenes

Toluene

• Toluene is a methylated benzene

Halogenated Benzenes

Cl

Phenol Structure

Nitrobenzene and Alkyl benzenes

Monosubstitued benzenes with a NO2 or alkyl group substituent

Benzene Activation vs. Deactivation

• Activation - Once a substituent has been added to the benzene ring it can cause the molecule to be more reactive than the benzene molecule alone.

• Deactivation – when a substituent on a benzene ring causes the molecule to be less reactive than benzene alone

Groups That Activate or Deactivate Benzene

Disubstituted Benzene

When a second substituent adds to a benzene ring the designations are:

Ortho – Para DirectorsRing Activating

• Alkyl groups• Halogens• OH• OCH3

• OR• NH2

• NHR• NR2

Meta DirectorsRing Deactivators

• NO2

• HSO3

• COR

• CN

• COOH

• COOR

Reactions of Benzene

• Electrophilic Aromatic Substitution - EAS is an organic reaction in which an atom, usually hydrogen, attached to an aromatic system is replaced by an electrophile. The most important reactions of this type that take place are nitration, halogenation, sulfonation, and acylation and alkylation.

Electrophile

• an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile

• Most electrophiles are positively charged, have an atom which carries a partial positive charge, or have an atom which does not have an octet of electrons. .

Reactions of Benzene

Halogenation of Benzene

Other catalyst AlCl3

Halogenation Mechanism

Step 1

Halogenation (cont)

Step 2

The hydrogen is removed by the AlCl4- ion which was formed in the first stage. The aluminium chloride catalyst is re-generated in this second stage.

Nitration of Benzene

Sulfonation of Benzene

Alkylation of Benzene