http://www.ck12.org Chapter 25. Organic Chemistry
Recall that when two valid Lewis structures can be drawn for a single molecule, the true structure is the average of
those structures. In benzene, the true bonding between carbon atoms is neither a single nor a double bond. Rather,
all of the bonds are a hybrid of a single and double bond. In benzene, the pi bonding electrons are free to move
completely around the ring. Delocalized electronsare electrons that are not confined to the bond between two
atoms but are instead able to move between three or more atoms. The delocalization of the electrons in benzene is
often shown by drawing benzene with a ring inside the hexagon. The hydrogen atoms on each carbon are implied.
Delocalization of the electrons makes for a more stable molecule than a similar molecule that does not have
delocalized electrons. Benzene is a more stable and less reactive compound than straight-chain hexenes. Because
the carbons aresp^2 hybridized, the ideal C-C-C bond angles are 120°, which is equal to the internal bond angles of a
planar hexagon. As a result, benzene is completely planar and does not pucker like cyclohexane. Benzene rings are
common in a great many natural substances and biomolecules. The figure below (Figure25.12) shows the structural
formulas for vanillin, which is the primary component of vanilla extract, and naphthalene, which is commonly used
in mothballs.
Nomenclature of Aromatic Compounds
The simplest aromatic compounds are benzene rings with one substituent replacing one of the hydrogen atoms. If
this substituent is an alkyl group, it is named first, followed by “benzene.” For example, the molecule shown below
(Figure25.13) is called ethylbenzene.
Substituents do not need to be alkyl groups (Figure25.14). If a chlorine atom were substituted for a hydrogen
atom, the name would be chlorobenzene. An –NH 2 group is called an amino group, so the corresponding molecule
is called aminobenzene, often referred to as aniline. An –NO 2 group is called a nitro group, so the third example
below is nitrobenzene.
If more than one substituent is present, their location relative to each other can be indicated by numbering the
positions on the benzene ring (Figure25.15).
The number of the carbon then precedes the name of the substituent in the name, with the numbers separated by a
comma. As with branched alkanes, the lowest possible numbers are used, and additional prefixes are used if more