The Inorganic Chemistry of Carbon
For more than 200 years, chemists have divided compounds into two categories. Those that were isolated from plants or animals were called organic, while those extracted from ores and minerals were inorganic. Organic chemistry is often defined as the chemistry of carbon. But this definition would include calcium carbonate (CaCO3) and graphite, which more closely resemble inorganic compounds. We will therefore define organic chemistry as the study of compounds, such as formic acid (HCO2H), methane (CH4), and vitamin C (C6H8O6), that contain both carbon and hydrogen. The chemistry of carbon is dominated by three factors.
1. Carbon forms unusually strong C-C single bonds, C=C double bonds, and carbon-carbon triple bonds. 2. The electronegativity of carbon (EN = 2.55) is too small to allow carbon to form C4- ions with most metals and too large for carbon to form C4+ ions when it reacts with nonmetals. Carbon therefore forms covalent bonds with many other elements. 3. Carbon forms strong double and triple bonds with a number of other nonmetals, including N, O, P, and S. Elemental Forms of Carbon: Graphite, Diamond, Coke, and Carbon Black Carbon occurs as a variety of allotropes. There are two crystalline forms diamond and graphiteand a number of amorphous (noncrystalline) forms, such as charcoal, coke, and carbon black. References to the characteristic hardness of diamond (from the Greek adamas, "invincible") date back at least 2600 years. It was not until 1797, however, that Smithson Tennant was able to show that diamonds consist solely of carbon. The properties of diamond are remarkable. It is among the least volatile substances known (MP = 3550oC, BP = 4827oC), it is also the hardest substance known, and it expands less on heating than any other material. The properties of diamond are a logical consequence of its structure. Carbon, with four valence electrons, forms covalent bonds to four neighboring carbon atoms arranged toward the corners of a tetrahedron, as shown in the figure below. Each of these sp3-hybridized atoms is then bound to four other carbon atoms, which form bonds to four other carbon atoms, and so on. As a result, a perfect diamond can be thought of as a single giant molecule. The strength of the individual C-C bonds and their arrangement in space give rise to the unusual properties of diamond.
In some ways, the properties of graphite are like those of diamond. Both compounds boil at 4827oC, for example. But graphite is also very different from diamond. Diamond (3.514 g/cm3) is much denser than graphite (2.26 g/cm3). Whereas diamond is the hardest substance known, graphite is one of the softest. Diamond is an excellent insulator, with little or no tendency to carry an electric current. Graphite is such a good conductor of electricity that graphite electrodes are used in electrical cells. The physical properties of graphite can be understood from the structure of the solid shown in the figure below.
Graphite consists of extended planes of sp2-hybridized carbon atoms in which each carbon is tightly bound to three other carbon atoms. (The strong bonds between carbon atoms within each plane explain the exceptionally high melting point and boiling point of graphite.) The distance between these planes of atoms, however, is very much larger than the distance between the atoms within the planes. Because the bonds between planes are weak, it is easy to deform the solid by allowing one plane of atoms to move relative to another. As a result, graphite is soft enough to be used in pencils and as a lubricant in motor oil. "Lead" pencils do not, incidentally, contain lead. (This is fortunate because many people chew pencils and lead compounds are toxic.) Lead pencils contain graphite, or "black lead" as it was once known, which is mixed with clay (20% to 60% by weight) and then baked to form a ceramic rod. Increasing the percentage of clay makes the pencil...
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