ORGANIC CHEMISTRY - CHAPTER 15: BENZENE AND AROMATICITY

Important Concepts

1. Substituted benzenes are named by adding prefixes or suffixes to the word benzene. Disubstituted 

systems are labeled as 1,2-, 1,3-, and 1,4- or ortho, meta, and para, depending on the location of the substituents. Many benzene derivatives have common names, sometimes used as bases for naming their substituted analogs. As a substituent, an aromatic system is called aryl; the parent aryl substituent, C₆H₅, is called phenyl; its homolog C₆H₅CH₂ is named phenylmethyl (benzyl).

2. Benzene is not a cyclohexatriene but a delocalized cyclic system of six π electrons. It is a regular hexagon of six sp²-hybridized carbons. All six p orbitals overlap equally with their neighbors. Its unusually low heat of hydrogenation indicates a resonance energy or aromaticity of about 30 kcal mol^-1 (126 kJ mol^-1). The stability imparted by aromatic delocalization is also evident in the transition state of some reactions, such as the Diels-Alder cycloaddition and ozonolysis.

3. The special structure of benzene gives rise to unusual UV,  IR,  and NMR spectral data. ¹H NMR spectroscopy is particularly diagnostic because of the unusual deshielding of aromatic hydrogens by an induced ring current. Moreover,  the substitution pattern is revealed by examination of the o,  m, and p coupling constants.

4. The polycyclic benzenoid hydrocarbons are composed of linearly or angularly fused benzene rings. The simplest members of this class of compounds are naphthalene, anthracene, and phenanthrene.

5.  In these molecules, benzene rings share two (or more) carbon atoms, whose π  electrons are delocalized over the entire ring system. Thus, naphthalene shows some of the properties characteristic of the aromatic ring in benzene: The electronic spectra reveal extended conjugation, ¹H NMR exhibits deshielding ring-current effects, and there is little bond alternation.

6. Benzene is the smallest member of the class of aromatic cyclic polyenes following Hückel’s 

(4n + 2) rule. Most of the 4n π systems are relatively reactive anti- or nonaromatic species. Hückel’s rule also extends to aromatic charged systems, such as the cyclopentadienyl anion, cycloheptatrienyl cation, and cyclooctatetraene dianion.

7. The most important reaction of benzene is electrophilic aromatic substitution. The rate-determining step is addition by the electrophile to give a delocalized hexadienyl cation in which the aromatic character of the original benzene ring has been lost. Fast deprotonation restores the aromaticity of the (now substituted) benzene ring. Exothermic substitution is preferred over endothermic addition. The reaction can lead to halo- and nitrobenzenes, benzenesulfonic acids, and alkylated and acylated derivatives. When necessary, Lewis acid (chlorination, bromination, Friedel-Crafts reaction) or mineral acid (nitration, sulfonation) catalysts are applied. These enhance the electrophilic power of the reagents or generate strong, positively charged electrophiles.

8. Sulfonation of benzene is a reversible process. The sulfonic acid group is removed by heating 

with dilute aqueous acid.

9. Benzenesulfonic acids are precursors of benzenesulfonyl chlorides. The chlorides react with alco-hols to form sulfonic esters containing useful leaving groups and with amines to give sulfonamides, some of which are medicinally important.

10. In contrast with other electrophilic substitutions, including Friedel-Crafts acylations, Friedel-Crafts alkylations activate the aromatic ring to further electrophilic substitution, leading to product mixtures.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE