Aromatic Compounds Unit 10

Structure of Benzene Kekule Structure: Kekule (1866) bravely proposed that benzene had a cyclic structure with three C—C alternating double bonds and three C—C single bonds.

Benzene Structure There is a p-orbital on each atom. The molecule is planar. There is an odd number of electron pairs in the -system. Benzene has much more stability than predicted by the simple resonance delocalized structure—it does not react and is much more stable than normal cyclo-alkenes.

Aromatic Nomenclature Many aromatic hydrocarbons are known by their common names. Aromatic hydrocarbons are cyclic hydrocarbons that have some particular features. Historically, benzene and its derivatives had pleasant aromas.

Reactions of Aromatic Compounds In aromatic compounds, unlike in alkenes and alkynes, each pair of -electrons does not sit between two atoms. Rather, the electrons are delocalized; this stabilizes aromatic compounds.

When a catalyst is added to the benzene bromination reaction, reaction does occur, but the reaction is not an addition, but rather a substitution (a ring hydrogen is substituted for a ring bromine). All three double bonds are retained in the product. Reactions of aromatic compounds often require a catalyst. Friedel-Crafts Reaction

Heterocyclic Aromatic Compounds Pyridine Nitrogen, oxygen and sulfur are the most common heteroatoms found in aromatic compounds. Pyridine is a nitrogen containing analogue of benzene. The lone pair of electrons are not involved in the aromatic system and stick out away from the molecule. The lone pair make pyridine capable of acting as a base.

Heterocyclic Aromatic Compounds Pyrrole Pyrrole is a 5-membered heterocycle which is also aromatic. It contains an N—H unit and four C—H units, with 2 double bonds.

Heterocyclic Aromatic Compounds Imidazole Pyrimidine Pyrimidine is a 6-membered heterocycle with two nitrogens in a 1,3 arrangement. Imidazole is a 5-membered ring with two nitrogens that is also aromatic.

Heterocyclic Aromatic Compounds Thiophene Furan Furan is a 5-membered ring similar to pyrrole but has an oxygen in place of the N—H. Thiophene is the sulfur analogue.

Polynuclear Aromatic Compounds These compounds (often called PAH’s or PNA’s) are composed of two or more fused benzene rings, with naphthalene being the simplest.

Nomenclature of Benzene Derivatives Aromatic compounds have been used widely for the last 100 years and most are referred to almost exclusively by their common names. Some are referred to as simple derivatives of benzene.

Isomers of a Disubstituted Benzene If we replace two of the hydrogens on benzene with chlorines, we have dichlorobenzene, C6H4Cl2. The chlorine atoms can be arranged in three different ways:

Isomers of a Disubstituted Benzene If the chlorines are on adjacent carbons, we have ortho- dichlorobenzene. If the chlorines are separated by one carbon, we have meta- dichlorobenzene. If the chlorines are opposite each other, we have para- dichlorobenzene.

Structure of Aromatic Compounds Two substituents on a benzene ring could have three possible relationships: ortho-: On adjacent carbons. meta-: With one carbon between them. para-: On opposite sides of ring.

Common Names for Disubstituted Benzenes H 3 O m -xylene mesitylene o -toluic acid p -cresol

Common Names of Benzene Derivatives

Three or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1.

Phenyl vs Benzyl Groups phenyl bromide C H 2 B r benzyl bromide Benzyl Phenyl When the benzene group is simply a constituent, it is called a Phenyl group and is often abbreviated Ph or ϕ . Do not confuse the benzyl group with the phenyl group—note the additional carbon attached to the benzene structure.

Physical Properties of Aromatic Compounds Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. Density: More dense than nonaromatics, less dense than water. Solubility: Generally insoluble in water.