Organic molecules are those molecules that contain the atoms carbon and hydrogen. Since the carbon atom has four electrons in its outer shell, it must share four additional electrons by cova-lently bonding with other atoms to fill its outer shell with eight electrons. The unique bonding requirements of carbon enable it to join with other carbon atoms to form chains and rings while still allowing the carbon atoms to bond with hydrogen and other atoms.
Most organic molecules in the body contain hydrocarbon chains and rings, as well as other atoms bonded to carbon. Two adjacent carbon atoms in a chain or ring may share one or two pairs of electrons. If the two carbon atoms share one pair of electrons, they are said to have a single covalent bond; this leaves each carbon atom free to bond with as many as three other atoms. If the two carbon atoms share two pairs of electrons, they have a double covalent bond, and each carbon atom can bond with a maximum of only two additional atoms (fig. 2.8).
The ends of some hydrocarbons are joined together to form rings. In the shorthand structural formulas for these molecules, the carbon atoms are not shown but are understood to be located at the corners of the ring. Some of these cyclic molecules have a double bond between two adjacent carbon atoms. Benzene and related molecules are shown as a six-sided ring with alternating double bonds. Such compounds are called aromatic. Since all of the carbons in an aromatic ring are equivalent, double bonds can be shown between any two adjacent carbons in the ring (fig. 2.9), or even as a circle within the hexagonal structure of carbons.
The hydrocarbon chain or ring of many organic molecules provides a relatively inactive molecular "backbone" to which more reactive groups of atoms are attached. Known as functional groups
■ Figure 2.8 Single and double covalent bonds. Two carbon atoms may be joined by a single covalent bond (left) or a double covalent bond (right). In both cases, each carbon atom shares four pairs of electrons (has four bonds) to complete the eight electrons required to fill its outer shell.
■ Figure 2.9 Different shapes of hydrocarbon molecules.
Hydrocarbon molecules can be (a) linear, (b) cyclic, or (c) have aromatic rings.
of the molecule, these reactive groups usually contain atoms of oxygen, nitrogen, phosphorus, or sulfur. They are largely responsible for the unique chemical properties of the molecule (fig. 2.10).
Classes of organic molecules can be named according to their functional groups. Ketones, for example, have a carbonyl group within the carbon chain. An organic molecule is an alcohol if it has a hydroxyl group bound to a hydrocarbon chain. All organic acids (acetic acid, citric acids, lactic acid, and others) have a carboxyl group (fig. 2.11).
A carboxyl group can be abbreviated COOH. This group is an acid because it can donate its proton (H+) to the solution. Ionization of the OH part of COOH forms COO- and H+ (fig. 2.12). The ionized organic acid is designated with the suffix -ate. For example, when the carboxyl group of lactic acid ionizes, the molecule is called lactate. Since both ionized and unionized forms of the molecule exist together in a solution (the proportion of each depends on the pH of the solution), one can correctly refer to the molecule as either lactic acid or lactate.
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