Benzene and Aromaticity

Overview

Description

Benzene is an aromatic compound that has two equivalent and significant resonance structures. This resonance creates stability, and benzene does not react in electrophilic addition reactions like other alkenes or conjugated polyenes. Aromatic compounds must be cyclic, have a continuous circle of p orbitals, have planar geometry, and have a Hückel number of electrons. If a molecule fulfills the first three rules but does not have a Hückel number, it is antiaromatic. If a molecule is not cyclic or planar or does not have a continuous circle of p orbitals, it is nonaromatic. Infrared spectroscopy, proton (1H) nuclear magnetic resonance, and carbon (13C) nuclear magnetic resonance are useful tools in identifying and analyzing benzene rings and other aromatics.

At A Glance

  • Benzene exists as a resonance hybrid. Because of delocalization of electrons over the π\pi system, benzene is much more stable than other isolated and conjugated polyenes. Because of its stability, benzene does not react in electrophilic addition reactions like other alkenes or conjugated polyenes.
  • Monosubstituted benzenes are named using the substituent followed by benzene without a number to indicate position. Disubstituted benzenes can be named with ortho, meta, and para or numbers. Polysubstituted benzenes must use numbers for location.
  • To be aromatic, a compound must be cyclic, have a continuous circle of p orbitals, have planar geometry, and have a Hückel number of electrons. Hückel's rule states that compounds having 2, 6, 10, 14, … (4n+24n+2) π\pi electrons (and satisfying the first three rules) are aromatic.
  • Antiaromatic compounds are cyclic, have a continuous circle of p orbitals, have planar geometry, and do not have a Hückel number of electrons. Nonaromatic compounds either are not cyclic, are not planar, or do not have a continuous circle of p orbitals. Annulenes, ions, heterocyclics, and polycyclics can all be aromatic if they fulfill all of the rules of aromaticity.
  • Aromatic compounds have two characteristic stretches in infrared spectroscopy, 3,000 to 3,100 cm–1 for carbon-hydrogen bonds and 1,450 to 1,650 cm–1 for carbon-carbon double bonds. In 1H NMR, aromatic protons usually appear between 6.5 and 8 ppm. In 13C NMR, aromatic carbons appear between 100 and 150 ppm.