Download 8 Aromatic Compounds GOB Structures

12.8 Aromatic Compounds
In 1825, Michael Faraday
isolated a hydrocarbon called
benzene, which consists of a
six-carbon ring with
alternating double bonds and
the molecular formula C6H6.
The space-filling model for
benzene consists of a six-carbon
ring with six hydrogen atoms.
Learning Goal Describe the bonding in benzene; name
aromatic compounds, and draw their line-angle structural
formulas.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Benzene, an Aromatic Compound
Benzene is
• an aromatic compound.
• a ring of six C atoms, each bonded to one H atom.
• a flat ring structure drawn with three alternating double bonds.
• represented by two structures, because the electrons are shared
equally among all the C atoms.
• represented by a line-angle structural formula using a circle in the
center instead of the alternating double bonds.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Aromatic Compounds
Some common examples of aromatic compounds that we use
for flavor are anisole from anise, estragole from tarragon, and
thymol from thyme.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Benzene
• Each carbon atom uses three valence electrons to bond to
the hydrogen atom and two adjacent carbons. The fourth
valence electron was thought to be shared in a double bond
with an adjacent carbon.
• In 1865, August Kekulé proposed that the carbon atoms in
benzene were arranged in a flat ring with alternating single
and double bonds between the adjacent carbon atoms.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Aromatic Compounds
• Aromatic compounds containing a benzene ring and a
single substituent are named as benzene derivatives.
• Since the ring contains only one substituent, the ring is not
numbered.
• Some common names such as toluene, aniline, and phenol
are allowed by IUPAC rules.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Aromatic Compounds
• When a benzene ring is a substituent, — C6H5,
it is named as a phenyl group.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Aromatic Compounds
• When there are two or more substituents, the
benzene ring is numbered to give the lowest
numbers to the substituents.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Aromatic Compounds
• When a common name such as toluene, phenol, or aniline
can be used,
• the carbon atom attached to the methyl, hydroxyl, or
amine group is numbered as carbon 1.
• prefixes are used to show the position of the two
substituents.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Aromatic Compounds
• The common name xylene is used for the isomers of
dimethylbenzene.
• If three or more substituents are attached to the benzene
ring, they are numbered in the direction to give the lowest
set of numbers and then named alphabetically.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Common Aromatic Compounds
Toluene is used as a
reactant to make drugs,
dyes, and explosives
such as TNT
(trinitrotoluene).
The benzene ring is also
found in some amino
acids (the building blocks
of proteins).
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Common Aromatic Compounds
The benzene ring is found in pain relievers such as
aspirin, acetaminophen, and ibuprofen; and in
flavorings such as vanillin.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Properties of Aromatic Compounds
The flat, symmetrical structure of benzene allows for
interactions between the cyclic structures, which contributes
to the higher melting points and boiling points of benzene
and its derivatives.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Properties of Aromatic Compounds
• Aromatic compounds are less dense than water, although
they are usually more dense than other hydrocarbons.
• Halogenated benzene compounds are denser than water.
• Aromatic hydrocarbons are insoluble in water and are
used as solvents for other organic compounds.
• Only aromatic compounds containing strongly polar
functional groups such as — OH or — COOH will be
somewhat soluble in water.
• Benzene and other aromatic compounds are resistant to
reactions that break up the aromatic system, although
they are flammable.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Select the correct name for each compound.
1.
A. chlorocyclohexane
B. chlorobenzene
C. 1-chlorobenzene
2.
A. 1,2-dimethylbenzene
B. 1,4-dimethylbenzene
C. 1,3-dimethylbenzene
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Select the correct name for each compound.
1.
B. chlorobenzene
2.
C. 1,3-dimethylbenzene
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Concept Map
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.