Benzene and Its Derivatives
Chapter 4
•
-very "aromatic," hence the name.
-benzene the most common. Toxic, industrially important.
-hybrid ring structure, since 3 double bonds not always in the
same location. so, C-6 H-6 formula.
-Nomenclature involves both the use of benzene suffix as well as
common names. (see examples below)
-important application: Fused Aromatics and Carcinogens, such
as Benzo-a-pyrene and smoke
Kekule’s Structure of Benzene
•
• (A)The bonds in C6H6 are
something between single
and double, which gives it
different chemical
properties than doublebonded hydrocarbons. (B)
The six-sided symbol with
a circle represents the
benzene ring. Organic
compounds based on the
benzene ring are called
aromatic hydrocarbons
because of their aromatic
character.
Benzene
• Benzene
•
Benzene is one of the most fascinating molecules. The structure of this
molecule eluded chemists until 1865 when Friedrich August Kekulé
proposed that it consisted of a hexagonal ring with a carbon atom at each
vertex. Every student of Organic Chemistry has heard the story of how
the structure appeared to Kekulé in a dream in which he saw chains of
carbon atoms dancing in circles like a snake chasing its own tail.
Alas, benzene is both toxic and carcinogenic. In fact, it might be
considered "the mother of all carcinogens," as a large number of
carcinogens have structures that include benzene rings. (See the link
below for the explanation of this.) I recall my Organic Chemistry
professor joking about how he used to "practically bathe in benzene up to
the elbows" when he would use it in his research (presumably before it
was identified as a carcinogen). He predicted that this would probably
lead to his demise. He was right--he died due to leukemia several years
ago.
Aromatic Compounds and
Benzene
Aromatic compounds contain benzene.
Benzene, C6H6 , is represented as a six carbon ring
with 3 double bonds.
Two possible can be drawn to show benzene in this
form.
H
H
H
H
H
H
H
H
H
H
H
H
Aromatic Compounds
and the Structure of Benzene
• In the early days the word aromatics was used
to described many fragrant molecules isolated
from natural sources.
Today the term
aromatic is used to describe benzene like
molecules.
• Benzene is a flat, symmetrical molecule with
the molecular formula C6H6.
• It has alternating three carbon-carbon double
and three single bonds.
• Benzene’s relatively lack of chemical
reactivity is due to its structure.
• There are two possible structures with
alternating double and single bonds.
• Experimental evidence suggest that all six carboncarbon bonds in benzene are identical.
• The properties, including the above one, of benzene
can only be explained by assuming that the actual
structure of benzene is an average of the above two
possible equivalent structures-known as resonance.
• Simple aromatic compounds like benzene are nonpolar, insoluble in water, volatile, and flammable.
• Unlike alkenes, several aromatic hydrocarbons are
toxic. Benzene itself is implicated as a cancer
causing chemical.
Aromatic Compounds in Nature and
Health
Many aromatic compounds are common in nature and
in medicine.
CHO
COOH
COOCH3
CH3
CH3
CH3CHCH2
CHCOOH
OCH3
OH
Aspirin
Vanillin
Ibuprofen
Naming Aromatic Compounds
Aromatic compounds are named with benzene as the parent
chain. One side group is named in front of the name benzene.
- No number is needed for mono-substituted benzene since all
the ring positions are identical.
CH3
methylbenzene
(toluene)
Cl
chlorobenzene
Naming Aromatic Compounds
When two groups are attached to benzene, the ring is
numbered to give the lower numbers to the side groups.
The prefixes ortho (1,2), meta (1,3-) and para (1,4-) are
also used.
Cl
CH3
Cl
CH3
Cl
1,2-dimethylbenzene
1,3-dichlorobenzene
(ortho-dimethylbenzene)
(meta-dichlorobenzene)
CH3
1-chloro-4-methylbenzene
(para-chloromethylbenzene)
Some Common Names
Some substituted benzene rings also use a common name.
Then naming with additional more side groups uses the
ortho-, meta-, para- system.
CH3
OH
CH3
Cl
Toluene
(Methylbenzene)
meta-chlorotoluene
(meta-chloromethylbenzene)
phenol
(hydroxybenzene)
• Many substituted aromatic compounds have
common names in addition to their systematic
names.
Learning Check
Select the names for each structure:
Cl
a. Chlorocyclohexane
b. Chlorobenzene
c. 1-chlorobenzene
CH 3
CH 3
a. Meta-xylene
b. Meta-dimethylbenzene
c. 1,3-dimethylbenzene
Learning Check
Write the structural formulas for each of the following:
A. 1,3-dichlorobenzene
B. Ortho-chlorotoluene
New Attached Groups
• Phenyl
4-phenyl-1-butene
• Benzyl
Benzyl alcohol
• Nitro -NO2
Refer to your chart for order
of priority!
2,4,6-trinitrotoluene
DDT and Benzene website
DDT the first of the chlorinated organic insecticides, was originally prepared in 1873, but it was not until
1939 that Paul Muller of Geigy Pharmaceutical in Switzerland discovered the effectiveness of DDT as an
insecticide he was awarded the Nobel Prize in medicine and physiology in 1948 for this discovery).
The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its
effectiveness against the mosquito that spreads malaria and lice that carry typhus. The World Health
Organization estimates that during the period of its use approximately 25 million lives were saved. DDT
seemed to be the ideal insecticideit is cheap and of relatively low toxicity to mammals (oral LD50 is 300 to
500 mg/kg). However, problems related to extensive use of DDT began to appear in the late 1940s. Many
species of insects developed resistance to DDT, and DDT was also discovered to have a high toxicity toward
fish.
The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolized very
rapidly by animals; instead, it is deposited and stored in the fatty tissues. The biological half-life of DDT is
about eight years; that is, it takes about eight years for an animal to metabolize half of the amount it
assimilates. If ingestion continues at a steady rate, DDT builds up within the animal over time.
The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of the
world. The buildup of DDT in natural waters is a reverisble process: the EPA reported a 90% reduction of
DDT in Lake Michigan fish by 1978 as a result of the ban
•
DDT/Benzene continued…
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Benzene rings have two important properties:
1. Two or more benzene rings can themselves bind together; and,
2. Chlorine can replace hydrogen on the outside of the ring.
These principles explain the formation of the very toxic families of PCBs,
furans and dioxins. A pair of benzene rings joined together forms biphenyl:
If chlorine is present when benzene is burned (and there is plenty of chlorine
in plastics), hydrogen atoms can be released and chlorine atoms can replace
them. The result is poly-chlorinated biphenyls, knows as PCBs. Their
production was banned in the 1970's.
If oxygen (abbreviated "O") forms another link between the two benzene rings
the result is furans. If chlorine replaces hydrogen atoms, the furans are also
very toxic:
Sometimes benzene molecules bind together with two oxygen atoms, with the
resulting name of dioxin. Unlike furans, dioxins are symmetrical (the same at
the top and bottom).
Estrogen and aromatics
Reproductive Hormones and
Benzene-like carcinogens
website
• The importance of other possible factors such as
breast-feeding history and exposure to
environmental toxins, however, have been more
difficult to establish. A review of the literature on
these two possible factors, with commonly used
organochlorine pesticides as the environmental
toxins being examined reveals linkage between the
two, with far-reaching implications for women to
consider.
Estrogen and Chemicals
•
Too Much Estrogen...
•
Excessive levels of estrogen can also increase the risk of breast cancer. The female
hormone signals cells to proliferate, which is necessary and normal during conception
and fetal attachment to the womb. At other times, however, estrogen may tell normal
cells to grow uncontrollably.
•
Twenty years ago, nutrition educator Carlton Fredericks, Ph.D., noted that some women
produced five more estrogen than did other women, increasing their risk of breast
cancer. Other women may increase their risk through estrogen-replacement therapy.
•
But several years ago, researchers stumbled across something completely unexpected:
many common pesticides and plastics contain chemicals that mimic estrogen in the
body. These estrogenic chemicals, called xenoestrogens, end up in the food and can
greatly increase the risk of breast cancer. Four years ago, Mary Wolff, Ph.D., of the Mt.
Sinai School of Medicine in New York City, found that high levels of estrogenmimicking pesticides increased breast cancer risk in women by four times.
Estrogen continued…
•
Because hormones are so powerful, only small quantities can do a lot of
damage. In one experiment, researchers found that 2 to 5 parts per billion of
bisphenol-A (BPA), one type of xenoestrogen, are enough to trigger hormonal
changes.
•
Combinations of pesticides magnify the risk, and such combinations are
common in foods and the environment. Last year, John McLachan, Ph.D., an
expert on pesticides and hormones at Tulane University, New Orleans,
reported in the journal Science that combinations of just two estrogen-like
pesticides were 1,000 times more potent than they were by themselves. What
happens when you mix together dozens of common estrogen-mimicking
chemicals? It's anyone's guess - but it doesn't bode well.
•
According to Malins, these xenoestrogens, along with the estrogen a woman
normally produces, could add to the free radical burden on breast cancer cells.
Additional free radicals, he pointed out, might also be generated when the
body breaks down estrogens and xenoestrogens. All these free radicals would
greatly increase a person's "oxidative stress" - and the need for compensatory
antioxidant vitamins.
The connection between:
Hormones in the Human Body and Organic Aromatics
PCBs
(Polychlorinated Biphenyls)
• There are 209 varieties
of PCBs, known
individually as
congeners. A congener
may have between 1
and 10 chlorine atoms,
which may be located
at various positions on
the PCB molecule.
PCBs
• PCBs are mixtures of man-made
chemicals with similar chemical
structures. PCBs can range from oily
liquids to waxy solids. Due to their nonflammability, chemical stability, high
boiling point and electrical insulating
properties, PCBs were used in hundreds
of industrial and commercial applications
including electrical, heat transfer, and
hydraulic equipment; as plasticizers in
paints, plastics and rubber products; in
pigments, dyes and carbonless copy paper
and many other applications. More than
1.5 billion pounds of PCBs were
manufactured in the United States prior to
cessation of production in 1977.
Food Dyes
• Food dye website
• Food dye website from manufacturer
contain OH directly attached
to aromatic ring
-initially used as antispectics
-since this is burning to
healthy tissue, replacements
have been found to be used
in mouthwashes, etc.
-