It is employed as a starting material for the production of detergents, drugs, dyes, insecticides, and plastics. Once widely used as an organic solvent, benzene is now known to have both short- and long-term toxic effects. The inhalation of large concentrations can cause nausea and even death due to respiratory or heart failure, while repeated exposure leads to a progressive disease in which the ability of the bone marrow to make new blood cells is eventually destroyed.
This results in a condition called aplastic anemia , in which there is a decrease in the numbers of both the red and white blood cells. Learning Objectives To describe the bonding in benzene and the way typical reactions of benzene differ from those of the alkenes.
Concept Review Exercises How do the typical reactions of benzene differ from those of the alkenes? Briefly describe the bonding in benzene. Answers Benzene is rather unreactive toward addition reactions compared to an alkene. Valence electrons are shared equally by all six carbon atoms that is, the electrons are delocalized. The six electrons are shared equally by all six carbon atoms.
Key Takeaway Aromatic hydrocarbons appear to be unsaturated, but they have a special type of bonding and do not undergo addition reactions. Exercises Draw the structure of benzene as if it had alternate single and double bonds. Methyl 2-pyridyl ketone 3D Download 3D Methyl 2-pyridyl ketone is one of the main components in the odor of popcorn.
One of the ironies of organic chemistry is that while pyridine itself smells like something that died a long time ago, hanging a ketone group off the pyridine ring produces the pleasant, mouth-watering odor of popcorn. Pyrazine 3D Download 3D In the pyrazine molecule, two CH groups on opposite sides of the benzene ring are replaced by nitrogen atoms.
Pyrazine itself has a strong, unpleasant odor, similar to that of pyridine, but substituted pyrazines contribute to many pleasant odors, such as the smell of bread crusts, rum, whisky, chocolate, and some vegetables and peppers.
Purine 3D Download 3D In the purine molecule, four nitrogen atoms are found in two fused rings. Purine derivatives, such as adenine and guanine, are found in DNA and RNA, as well as in many other important molecules, such as caffeine.
Imidazole 3D Download 3D Imidazole is important in the catalytic activity of many enzymes. Furan 3D Download 3D Furan , which has an oxygen atom in a ring with four carbon atoms, is also an aromatic molecule; a lone pair of electrons on the oxygen contributes two electrons in addition to the four electrons from the carbon-carbon double bonds, making a total of six p electrons in the ring system.
If the double bonds are reduced to single bonds by the addition of four hydrogen atoms, the molecule tetrahydrofuran is produced; this is a very commonly used organic solvent. Thiophene 3D Download 3D Thiophene has a sulfur atom in a ring with four carbon atoms, and, like furan, is aromatic because there are six p electrons in the ring four from the double bonds and two from a lone pair on sulfur.
Pyrrole 3D Download 3D Pyrrole is a another aromatic molecule, once again having six p electrons in the ring system. Molecules containing pyrrole occur in a number of important biological systems, ranging from photosynthesis to oxygen transport in the blood. See Porphine , Heme , and Chlorophyll below. I know you're out there, I can hear you breathing. Porphine 3D Download 3D Porphine contains four pyrrole molecules which have been joined to form a larger ring system.
This molecule has 18 p electrons in the ring system, and is therefore aromatic. Like other aromatic molecules, this ring system is especially stable. The ring system is flat, since all of the carbon and nitrogen atoms in the ring are sp 2 -hybridized, and therefore trigonal planar in shape. This leaves the molecule with a large open cavity in the center, with four nitrogen atoms pointing towards the center of the cavity. Porphine is the parent compound of a class of molecules called the porphyrins , in which various substituents replace the hydrogen atoms on the outside of the porphine ring.
The name "porphyrin" is derived from the fact that many of these substances form purple crystalline solids the Greek word for "purple" is "porphyros". Porphyrins occur ubiquitously in nature, especially in systems involving respiration see Heme and Chlorophyll below. The protein hemoglobin contains four subunits two "alpha" and two "beta" subunits , each of which complex a heme molecule; this protein is responsible for oxygen transport in the body. The iron atom in heme forms complexes with oxygen molecules, which are carried throughout the body by the hemoglobin in the red blood cells.
Chlorophyll 3D Chlorophyll Greek, chloros "green" and phyllon "leaf" is a porphyrin derivative found in green plants and cyanobacteria which allows these organisms to perform photosynthesis. Shown below are the structures of chlorophyll a and chlorophyll b ; the magnesium-free forms of these molecules are called pheophytin a and pheophytin b. The three-dimensional structure shown on the right is that of pheophytin a. A double bond in one of the pyrrole rings in the porphyrin macrocycle is reduced to a single bond in the structure below, this is the carbon-carbon bond to which the long side-chain, sometimes called the phytyl group, is attached ; these "dihydroporphyrins" are known as chlorins.
The energy from the sunlight that is absorbed by chlorophyll in the chloroplasts of green plants is used to drive the synthesis of carbohydrates such as glucose C 6 H 12 O 6 from carbon dioxide and water:.
This equation summarizes a lot of complex chemistry, which it is beyond the scope of this page to try to summarize. In the fall, the green color of many plants disappears as the chlorophyll starts to break down; this allows other pigments, such as carotenes, in the leaves to "show their colors," producing the vivid yellows and oranges associated with fall colors.
Buckminsterfullerene , or C 60 , is a soccer-ball shaped molecule consisting of 60 carbon atoms. The molecule was discovered by H. Kroto, R. Smalley, and R. Curl in the s in experiments involving graphite vaporized with lasers they were awarded the Nobel Prize in Chemistry for their discovery in The structure, a truncated icosahedron, having 12 pentagons and 20 hexagons, reminded them of the shape of the geodesic dome designed by the architect R.
Buckminster Fuller, and they named the molecule in his honor. Similar spherical-shaped carbon-only molecules, such as C 70 , are often referred to as fullerenes or "buckyballs.
The fullerenes are considered another allotrope stable structural form of carbon, in addition to graphite and diamond. In the s it was discovered that C 60 could be made in larger quantities by heating graphite in an inert atmosphere. Since then, these molecules have been intensely investigated.
Nanotubes are cylindrical versions of the fullerenes; they look something like a chain link fence rolled into a cylinder, with a dome-shaped cap on the end half of a buckyball. Nanotubes also known as "buckytubes" are extremely strong, as well as being very lightweight since they are made of nothing but carbon atoms.
These materials are being tested for potential use in many materials; some nanotubes also conduct electricity, leading to some potential applications in circuit design and electronics. C 70 3D Download 3D C 70 is another example of a fullerene.
It has a slightly elongated, oval shape. Atkins, Molecules , 2nd ed. Cambridge: Cambridge University Press, , p. Paula Yurkanis Bruice, Organic Chemistry , 4th ed.
Upper Saddle River: Prentice Hall, , p. Marye Anne Fox and James K. Whitesell, Organic Chemistry , 3rd ed. Sudbury: Jones and Bartlett Publishers, p. Maitland Jones, Jr. New York: W. Richard J. Lewis, Sr. New York: Van Nostrand Reinhold, Englewood Cliffs: Prentice Hall, , p. Royston M. Roberts, Serendipity: Accidental Discoveries in Science.
The number of the carbon location then precedes the name of the substituent in the overall name, with the numbers separated by a comma. As with branched alkanes, the system requires that the numbers be the lowest possible and that prefixes be used for more than one of the same substituent. If there are different substituents, the first in alphabetical order is given the lower number and listed first.
The structures below are called 1,2-dimethylbenzene and 1-ethylmethylbenzene. An alternate system for naming di-substituted benzene rings uses three different prefixes: ortho , meta , and para. If two groups are in the ortho position, they are on adjacent carbon atoms. The meta positioning refers to being in a 1,3 arrangement. The para positioning refers to being in a 1,4 arrangement. Shown below are the three possibilities for dimethylbenzene, also called xylene. That group is called a phenyl group and so the molecule below is called 2-phenylbutane.
Austin State University with contributing authors. Learning Objective Name and identify aromatic compounds. Nomenclature of Aromatic Compounds The simplest aromatic compounds are benzene rings with one substituent replacing one of the hydrogen atoms.
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