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. Sharp, The Penguin Dictionary of Chemistry , 2nd ed. London: Penguin Books, Wade, Jr. Martha Windholz ed. Benzene 3D Download 3D Benzene was first isolated by Michael Faraday in , from the whale oil used in gaslights; he also determined that it had an empirical formula of CH.
Amygdalin Laetrile 3D Download 3D Amygdalin is a disaccharide consisting of two glucose molecules in which a carbon bearing a cyano CN group and a benzene ring is attached to one of the oxygen atoms on a glucose. Hyacinthin 3D Download 3D Hyacinthin is commonly used in perfumes; it is responsible for the floral scent found in hyacinth oddly enough.
Cinnamaldehyde 3D Download 3D Cinnamaldehyde is found in oil of cinnamon, which is found in the bark of the cinnamon tree. Naphthalene 3D Download 3D Naphthalene is a white crystalline solid, derived from coal tar, with a characteristic odor of mothballs — which is not a coincidence, since naphthalene is frequently used in mothballs.
Pyridine 3D Download 3D In the pyridine molecule, one of the CH groups of the benzene ring is replaced with a nitrogen atom. Two drawings of this ion are below, followed by a 3D model. So what do we learn from the story of cyclooctatetraene and its dianion? Cyclooctatetraene has eight , and is not aromatic; its dianion has ten , and is aromatic. So clearly, six is not the only allowed number. In fact, the cation derived from cyclopropene, shown below, is unusually stable, and is considered aromatic.
As another example, with a larger n, consider a particular isomer of annulene -- the isomer with every third double bond cis. The following set of figures show three representations of its structure.
In fact, some programs will calculate a planar structure for it. Actual measurement shows that is very slightly distorted from planar, due to the repulsion of the hydrogens that are inside the ring. The following figure shows 3D representations of both "forms" of annulene; they have been rotated so you view them "edge on". The top structure is the planar form that one might naively expect; the other is the slightly distorted structure, which closely corresponds to what is actually observed.
The annulene name is used generically for cyclic molecules with alternating single and double bonds. The numeric prefix indicates the ring size. Benzene might be considered as 6-annulene, and cyclooctatetraene as 8-annulene. Note that all annulenes have the general formula C x H x where x must be an even number , and that the term does not in itself imply aromatic character. Aromatic compounds are more stable than we might expect when we see a structure showing single and double bonds.
We start our study of aromaticity with the classic case of benzene. But as we continue, we find examples of aromatic compounds that contain heteroatoms, charges, and rings of different sizes. Aromaticity requires a planar loop of electrons in overlapping p orbitals.
Definition First, why is aromaticity an issue? The p orbital on the ch2 outside of the ring is not part of the cyclic array of p orbitals. However this is not enough to make it truly aromatic. Hi, for cyclopentadienyl carbanion, a comment on rehybridization from sp3 typical in carbanion species to sp2 to get some benefits from resonance stabilization since it is adjacent to other sp2 hybridized carbons should be given.
Great job! Will there be any changes about aromaticity? The furan oxygen will be sp2. In the p orbital will be a lone pair of electrons, aligned with the other p-orbitals in the pi system. The other lone pair will be in the same plane of the molecule as the C-H bonds. If there is a system of 2 fused rings in which one is fully conjugated but other one has sp3 carbon , so will that structure be considered as aromatic since one ring is aromatic? Well, one ring is aromatic and the other is clearly not.
Think of it as a substituted benzene derivative. Think about the rules for aromaticity and work through it. Is it cyclic? Your email address will not be published.
Save my name, email, and website in this browser for the next time I comment. Notify me via e-mail if anyone answers my comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Previous Introduction To Aromaticity. Rules For Aromaticity: The 4 Key Factors In the last post we introduced the concept of aromaticity , a property of some unusually stable organic molecules such as benzene. So what are the rules?
How can we predict whether a molecule is aromatic or not? Four Key Rules For Aromaticity There turn out to be 4 conditions a molecule must meet in order for it to be aromatic. If any of these conditions are violated, no aromaticity is possible. First, it must be cyclic. Second, every atom in the ring must be conjugated. There are a few alternative ways to say the same thing.
For example, how do we count electrons in the benzene anion below left or pyridine? Do we count the lone pair electrons as Pi electrons, giving a total of 8?
Or do we ignore them? What about furan middle which has two lone pairs on oxgyen? What about pyrrole, with its lone pair on nitrogen, or imidazole, with two nitrogens? Since that carbon is not involved in any pi-bonding, the answer is yes. Summary: Rules For Aromaticity This post went through the four conditions a molecule must meet to be aromatic. Thanks to Matt Knowe for assistance with this post.
Check out these worked examples Notes Fun fact. Pyrrole actually reacts with acid on carbon, not nitrogen! In this examples below, cyclohexene is not actually a planar molecule although it may look it from the drawing , so it is not aromatic:. Aromatic molecules containing several fused joined rings are called polycylic aromatics or sometimes simply "polycyclics" for short.
Those polycyclic aromatics made up only of carbon and hydrogen are called polycyclic aromatic hydrocarbons or PAH's. Many PAH's are extremely potent carcinogens or mutagens. For example, the molecule shown on the right, benzo[ a ]pyrene is an exceedingly potent carcinogen found commonly in coal tar and soot, including tobacco smoke and diesel exhaust both of which are listed by IARC as "carcinogenic to humans".
PAH's can also cause other cancers through skin contact and ingestion and can have reproductive as well as possibly teratogenic effects. A compound that has an odor is not necessarily aromatic. Likewise, not all aromatic molecules have an odor. When the term aromatic is used on an SDS it is referring to the chemical definition of aromaticity, not the smell or odor of the compound.
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