For instance, phenol , which has an —OH group attached to the benzene ring, is very water-soluble. Because of the stability imparted by the delocalized pi-electrons in the ring, aromatic molecules do not undergo many of the reactions which are typical for alkenes, but there are a number of interesting and useful substitution reactions that they undergo, in which hydrogen atoms on the ring are replaced with other functional groups. Molecules which do not contain benzene rings are referred to aliphatic "fat-like" compounds.
Aromatic molecules with simple alkyl groups as substituents are named as derivatives of benzene. For instance, a benzene with an ethyl group attached to one of the carbons in the ring is simply called "ethylbenzene. A number of aromatic molecules are known by common names; for instance, benzene with a —CH 3 group attached is called " toluene "; benzene with an —NH 2 group attached is called " aniline "; a benzene with a —CO 2 H group is called "benzoic acid," etc.
For more complicated substituents, the benzene ring is named as a substituent, in which case it is called "phenyl. When there are two substituents on the benzene ring, numbers can be used to identify the position of the substituents, but an older system of nomenclature is often used instead, in which the prefixes ortho -, meta -, and para - often abbreviated as o -, m -, and p - are used to indicate the relative placement of the substituents:.
For three or more substituents, numbers must be used to indicate the placement of substituents. 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. Eilhardt Mitscherlich synthesized benzene in , and showed it to have a molecular formula of C 6 H 6. Many other compounds with similar properties to benzene were discovered in the s, all having a low ratio of hydrogen to carbon.
This usually indicates the presence of carbon-carbon double bonds, but these compounds did not undergo the reactions typical of alkenes under ordinary lab conditions. Since many of these molecules had pleasant aromas, they were called "aromatic compounds. The term "aromatic" in organic chemistry now means that the molecule contains benzene, or its structural relatives.
Benzene itself is a clear, colorless, highly flammable liquid, which boils at It was used extensively as a solvent for many organic reactions, but it is toxic by ingestion and inhalation, and may cause bone marrow problems or leukemia with prolonged exposure.
Toluene 3D Download 3D Toluene or methylbenzene is a commonly used organic solvent; it is less carcinogenic than benzene because the methyl group is easily oxidized, and can be converted to products that can be eliminated from the body with relative ease.
Toluene was first isolated from the gum of the Toluifera balsamum tree of South America, which produces a balsam that has been used in cough syrups and perfumes. Xylene 3D ortho -xylene ortho -dimethylbenzene 1,2-dimethylbenzene Download 3D meta -xylene meta -dimethylbenzene 1,3-dimethylbenzene Download 3D para -xylene para -dimethylbenzene 1,4-dimethylbenzene Download 3D There are three structural isomers of the xylenes , which have two methyl groups on the benzene ring.
The methyl groups may be on adjacent carbons ortho , they may be on carbons separated by one other carbon meta , or they may be on opposite sides of the benzene ring para.
Xylol is a common industrial solvent which contains a mixture of all three xylene isomers. Toluene and the xylenes are found in high-performance BTX gasoline. Aniline 3D Download 3D Aniline is a colorless, oily liquid.
It is easily converted into a number of other useful organic compounds, and is also a component of many dyes and pharmaceutical compounds. Phenol 3D Download 3D Phenol is a mild acid in aqueous solution where it is known as carbolic acid , and was originally used as a disinfectant in hospitals.
For more information on phenol and its derivatives, see the page on Phenols. Benzoic acid and its salts sodium benzoate, potassium benzoate, etc.
For more information of benzoic acid and its derivatives, see the page on Carboxylic Acids. Styrene is used in the manufacture of polystyrene and Styrofoam see the section on Addition Polymers. Benzaldehyde 3D Download 3D Benzaldehyde consists of a benzene ring connected to an aldehyde group a carbon-oxygen double bond bearing a hydrogen, —CHO.
Benzaldehyde is partially responsible for the odor of almonds and cherries, and is also found in peach and apricot pits. The natural source of benzaldehyde is a molecule called amygdalin. 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. The combination of a cyano group with a OH or OR group on a single carbon is called a cyanohydrin; cyanohydrins easily break down to release the cyanide ion, CN -.
Amygdalin occurs in almonds, cherries, and peach and apricot pits. When apricot or peach pits are ground up, the enzyme emulsin breaks down amygdalin, releasing benzaldehyde and hydrogen cyanide, HCN, which is a deadly poison. This preparation was used by the ancient Egyptians and Romans in making various poisons.
Under the name laetrile, amygdalin was used as an anti-cancer drug. It was never approved for sale in the United States, but was a popular "alternative medicine" for use in chemotherapy in other countries.
However, more research showed that not only is laetrile ineffective against cancer, it is extremely toxic, due to the release of hydrogen cyanide. Hyacinthin is commonly used in perfumes; it is responsible for the floral scent found in hyacinth oddly enough. Cinnamaldehyde is found in oil of cinnamon, which is found in the bark of the cinnamon tree.
Cinnamon is a carminative — it stimulates the release of gases such as hydrogen sulfide, methane, and hydrogen from the stomach and intestines. These gases can have some interesting social consequences depending on which direction they exit from the digestive tract, but I'll leave that to your imagination.
Anethole 3D Download 3D Anethole is found in oil of aniseed, fennel, and tarragon. Benzyl acetate 3D Download 3D Benzyl acetate is one of the compounds found in oil of jasmine, which is a common ingredient in perfumes and toiletries.
Phenylethanol 3D Download 3D Phenylethanol is found in oil of citronella, geraniums, and with geraniol is partially responsible for the odor or roses. Polycyclic aromatic hydrocarbons consist of two or more benzene rings fused together.
They are produced when organic compounds are heated to high temperatures, and are present in tobacco smoke, car exhaust, and sometimes in heavily browned foods. 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. Naphthalene consists of two benzene rings that are fused together; the resulting molecule is still aromatic, and undergoes the reactions that are typical of benzene itself.
Anthracene 3D Download 3D Anthracene is a white crystalline solid which exhibits a blue fluorescence under ultraviolet light. Phenanthrene 3D Download 3D Phenanthrene is a structural isomer of anthracene, in which the three benzene rings are fused to make an angle.
The lone pairs of electrons on the nitrogen atoms, combined with the rigidity of the aromatic ring system, makes ortho -phenanthroline useful in forming metal complexes. Dibenz[ a,h ]anthracene 3D Download 3D Dibenz[ a,h ]anthracene is a carcinogenic compound found in tobacco smoke and automobile exhaust. Pyrene 3D Download 3D Pyrene is also a suspected carcinogen. Benzo[ a ]pyrene 3D Download 3D Benzo[ a ]pyrene is a carcinogenic compound found in tobacco smoke and automobile exhaust.
Molecules whose Lewis structures can be drawn as a ring with alternating single and double bonds are known as annulenes. Annulenes are named by prefixing the number of carbon atoms in the rings in brackets before the word annulene : hence, benzene can be named as [6] annulene.
Thus, molecules with 2, 6, 10, 14, 18, etc. These compounds have chemical properties that are very similar to those of benzene, and many are found in important biological systems. Upon waking was inspired to deduce the ring structure of benzene.
Benzene presents a problem, as to account for all the bonds in the molecule, there must be alternating double carbon bonds: Benzene Molecular Structure. This is explained by electron delocalization. In order to picture this, we must consider the position of electrons in the bonds of benzene.
One representation is that the structure of the benzene molecule exists as a superposition of the forms below, rather than either form individually. This type of structure is called a resonance hybrid of the benzene molecule. In reality, neither form really exists. Delocalisation must be explained using a higher level of theory than single and double bonds. This second bond has electrons orbiting in paths above and below the plane of the ring at each bonded carbon atom.
The following diagram shows the positions of these p-orbitals in the benzene molecule:. This delocalisation of electrons is known as aromaticity , and gives benzene great stability. This is the fundamental property of aromatic chemicals which differentiates them from non aromatics. To reflect the delocalised nature of the bonding, the benzene molecule may be depicted as a circle inside a hexagon in chemical structure diagrams. As with most diagrams of molecular structures, the Hydrogen atoms are frequently omitted.
Substituents Many important chemicals are essentially benzene, with one or more of the hydrogen atoms replaced with some other functionality , eg:. The derivative tetrahydrofuran THF is very widely used as a solvent and reagent. It is one of the components of the coal tars given off when coal in converted to coke. Up until World War II, this source of benzene was sufficient to meet world demand for the chemical. However, in the 's, increasing demand for benzene, especially for the growing plastics necessitated the production of benzene from petroleum.
Today, most benzene comes from the petrochemical industry, with only a small fraction being produced from coal. 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 electrons are electrons that are not confined to the bond between two atoms, but are instead allowed to move between three or more.
The delocalization of the electrons in benzene can best be shown by showing benzene with a ring inside the hexagon, with the hydrogen atoms understood. 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. There are many derivatives of benzene. The hydrogen atoms can be replaced by many different substituents. Aromatic compounds more readily undergo substitution reactions than addition reactions; replacement of one of the hydrogen atoms with another substituent will leave the delocalized double bonds intact.
The following are typical examples of substituted benzene derivatives:. Toluene and xylene are important solvents and raw materials in the chemical industry. Styrene is used to produce the polymer polystyrene.
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