Organic Chemistry

70 Benzene Structure

Benzene is a hexagonal ring of six carbon atoms connected to each other through one p-
orbital per carbon. Its chemical formula is C 6 H 6 , and its structure is a hexagonal ring
of carbons sharing symmetrical bonds, with all six hydrogen atoms protruding outwards
from the carbon ring, but in the same plane as the ring. The p-orbital system contains 6
electrons, and one way to distribute the electrons yields the following structure:

However, another resonance form of benzene is possible, where the single bonds of the first
structure are replaced with double bonds, and the double bonds with single bonds. These
two resonance forms are co-dominant in benzene. (Other forms, such as a structure with a π
bond connecting opposite carbons, are possible but negligible.) Thus, each bond in benzene
has been experimentally shown to be of equal length and strength, and each is accounted
as approximately a ”1.5” bond instead of either a single or double bond alone.

Electron density is shared between carbons, in effect yielding neither a single nor a double
bond, but a sort of one-and-a-half bond between each of the six carbons. Benzene has a
density of negative charge both above and below the plane formed by the ring structure.
Although benzene is very stable and does not tend to react energetically with most sub-
stances, electrophilic compounds may be attracted to this localized electron density and
such substances may form a bond with the aromatic benzene ring.

An electron delocalisation ring can be used to show in a single picture both dominant
resonance forms of benzene:

70.1 Benzene Properties.

Benzene is a colorless, flammable liquid with a sweet aroma and carcinogenic effects. The
aromatic properties of benzene make it far different from other alkenes in many ways.

70.1.1 Benzene Reactions

Main article:Aromatic reactions^1

Unlike alkenes, aromatic compounds such as benzene undergo substitution reactions instead
of addition reactions. The most common reaction for benzene to undergo is electrophilic

1 Chapter 80 on page 271