Benzene has the empirical formula CH. Its molecular formula has been shown to be C 6 H 6, which implies four degrees of unsaturation in the molecule.

Several incorrect structures of benzene have been proposed in the past: Of these, only Claus benzene has not been synthesized. All of the other structures represent unstable substances which isomerize to benzene in very exothermic reactions.

The correct molecular structure of benzene is:

Benzene is relatively inert. Among the reactions it does undergo is the reaction with bromine in the presence of catalytic amounts of FeBr 3 : The addition product is not formed, as might be expected.

Further reaction with bromine leads to a mixture of isomeric di-substituted products:

15-1

Naming the Benzenes

Benzene and its derivatives were originally called “aromatic compounds” because of their strong aromas.

Benzene is considered the “parent” aromatic molecule.

The structure of benzene is written as a pair of resonance structures or as a regular hexagon containing an inscribed circle:

Monosubstituted benzenes are often named by adding a prefix to the word benzene:

Disubstituted benzenes are named using the prefixes: •1,2- (ortho-, or o-) •1,3- (meta-, or m-) •1,4- (para-, or p-) The substituents are then listed in alphabetical order:

For tri- and more highly substituted benzenes, the ring carbons are numbered to give the substituents the lowest set of numbers, as in cyclohexane nomenclature:

The following benzene derivatives will be used often in this course:

The naming of three substituted benzene systems do not follow IUPAC nomenclature in the Chemical Abstracts indexing preferences: phenol, benzaldehyde and benzoic acid.

Ring substituted compounds of these substances are named by numbering the ring positions or using the prefixes o-, p- and m-.

The carbon carrying the substituent giving the compound its base name is given the number 1.

Several other common names have been accepted by IUPAC, including:

A substituted benzene is called an arene.

An arene, when used as a substituent, is called an aryl group (Ar).

The parent aryl substituent is phenyl, C 6 H 5 -. The group C 6 H 5 CH 2 - is called phenylmethyl (benzyl).

15-2

Structure and Resonance Energy of Benzene: a First Look at Aromaticity

At room temperature, benzene is inert to acids, H 2 , Br 2 usually add to conjugated alkenes.

and KMnO 4 , reagents that The cyclic 6-electron arrangement of double bonds imparts a special stability in the form of a large resonance energy.

The benzene ring contains six equally overlapping p orbitals.

Experimentally, benzene can be shown to be a completely symmetrical hexagon; there are no alternating single (long) and double (short) bonds.

The electronic structure of the benzene ring consists of 6 sp 2 carbon atoms, with each p-orbital overlapping the p-orbitals of its nearest neighbors.

Benzene is especially stable: heats of hydrogenation.

It is useful to compare the heats of hydrogenation of benzene, 1,3-cyclohexadiene and cyclohexene. In each case, cyclohexane is the final product.

The heat of hydrogenation of benzene can be experimentally measured using special catalysts and is measured to be 29.6 kcal mol -1 less than the calculated heat for the hypothetical molecule, 1,3,5-cyclohexatriene.