Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Benzene

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Benzene, colorless liquid with a characteristic odor and burning taste. The benzene molecule is a closed ring of six carbon atoms connected by bonds that resonate between single and double bonds; each carbon atom is also bound to a single hydrogen atom. It is insoluble in water, but miscible in all proportions with organic solvents. Benzene itself is an excellent solvent for certain elements, such as sulfur, phosphorus, and iodine; for gums, fats, waxes, and resins; and for most simple organic chemicals. It is one of the most commonly used solvents in the organic chemical laboratory. Benzene melts at 5.5° C (41.9° F), boils at 80.1° C (176.2° F), and has a specific gravity of 0.88 at 20° C (68° F). If inhaled in large quantities, benzene is poisonous. The vapors are explosive, and the liquid violently flammable. Many compounds, such as nitrobenzene, are obtained from benzene. Benzene is also used in the manufacture of drugs and in the production of important derivatives, such as aniline and phenol. Benzene and its derivatives are included in the important chemical group known as aromatic compounds.

Pure benzene burns with a smoky flame because of its high carbon content. When mixed with a large proportion of gasoline it makes a satisfactory fuel. In Europe benzene mixed with some toluene and other related compounds has long been added to motor fuels.

Benzene was discovered in 1825 by the English scientist Michael Faraday, and in 1842 it was made available in large quantities after it was found that coal tar contains benzene. A ton of coal, if coked in a by-product oven, yields about 7.6 liters (about 2 gallons) of benzene. Large quantities of benzene are now obtained from petroleum, either by direct extraction from certain types of crude oils or by chemical treatment of gasoline (hydroforming and cyclization).

The structure of the benzene molecule is of the utmost importance to the theory of organic chemistry. The first to formulate the resonating ring structure described above was the German chemist Friedrich August KekulĂ© von Stradonitz, in 1865. For various reasons, 20th-century theorists found difficulties with this picture and instead developed a “molecular orbital” picture of electrons orbiting the entire molecule rather than particular carbon atoms. In the 1980s, however, more advanced studies are returning to KekulĂ©'s picture, but with the electrons in deformed orbits around their particular atoms.

See also Organic Chemistry.