Organic Chemistry
Organic Chemistry
Perhaps a more accurate term than “organic chemistry” might be “carbon chemistry,” con- sidering that the major element in organic molecules is carbon. Carbon is a unique element with a structure of six protons, six neutrons, and six electrons that results in its being a most versatile element, and it is used to produce a multitude of organic (carbon) compounds. (See the section on carbon for more about this element.) Organic compounds have been used by humans, as well as all other living organisms, since the beginning of life on Earth. However, it was not until the late nineteenth century when scientists begin exploring the nature of all the organic chemicals that we began to use it every day. Organic chemistry is the study of two basic types of carbon compounds. One, biochemistry is the chemistry of compounds that make up living organisms and their products, such as sucrose (table sugar) with the formula of
C 12 H 22 O 11; and two, hydrocarbon chemistry includes the study of both natural and man-made compounds composed mainly of carbon and hydrogen, such as natural gas (methane) with the
formula CH 4 and propane cooking gas with the formula C 3 H 8 . The organic carbon compounds identified in the science of biochemistry are composed of some very large molecules. For instance, all carbohydrates (sugars, starches), fats/lipids (lard), proteins (meat), and nucleic acids (DNA) consist of organic compounds composed of many large molecules. Many of these compounds are constituents of living organisms. The products we use, such as fossil fuels (coal, oil, gasoline, natural gas) and some cosmetics, wax, soap, and alcohol, are hydrocarbons that are related to biochemistry only in that they contain carbon. Hydrogen and several other elements combine with carbon to form many interesting and useful compounds. Although this book deals mostly with inorganic (nonliving and mostly noncarbon) chemistry rather than organic chemistry, it also emphasizes everyday use of the chemical elements. This requires some understanding of organic chemistry and how and why carbon is such a major factor in many of our useful chemical products.
Over the years, many people contributed to the development of the field of organic chem- istry. To better understand how this science provides so many useful items for our daily use, it is necessary to be familiar with some of the nomenclature of organic chemistry. There are two basic types of hydrocarbon substances, namely, aliphatic and aromatic. There are three basic types of aliphatic hydrocarbon molecules defined by the number of bonds involved in straight linear-chained molecules. If the basic structure of a hydrocarbon molecule is a ring instead of
a straight chain, they are known as aromatic hydrocarbons, typified by the benzene ring. Aliphatic hydrocarbon molecules, normally called “hydrocarbons,” are divided into several major groups. Their molecules are mostly formed as straight or branched chains that form three major subgroups:
1. Alkanes (saturated straight or branched chains of carbon atoms)
For example, H 3 C —CH 2 —CH 3 (Propane), and CH3 H3C—CH—CH3 (Butane)
Atomic Structure | 21 The general formula for alkanes is C n H 2n+2 (where n = a small whole number such as 1, 2,
3). (Note: The term alkyl is often used to identify a paraffinic hydrocabon group of alkanes by dropping one hydrogen atom from the formula, for example, methyl CH 3 , or ethyl C 2 H 5 .)
2. Alkenes (unsaturated and very reactive olefins) 3. Alkynes (highly saturated acetylenes with triple bonds)
Hydrocarbon molecules that have only single bonds (C–C) are known as saturated hydrocarbons, whereas unsaturated hydrocarbon molecules have double or triple bonds (C=C or C≡C). A very logical system that assigns names to the structures of these types of hydrocarbons uses Greek prefixes to identify the number of carbon atoms in a particular type of hydrocarbon molecule (see Table 2.2).
The molecular formulas just shown for 10 alkane hydrocarbon molecules represent the pro- portions of carbon to hydrogen in each molecule. These formulas do not reveal much about their structures, but rather indicate the proportions of each element in their molecules. Each molecule may have several different structures while still having the same formula. Molecules with different structures but the same formulas are called isomers. For example, n-butane is
formed in a straight chain, but in an isomer of butane, the CH 3 branches off in the middle of the straight chain. Another example is ethane, whose isomeric structure can be depicted as H 3 CH 3 C–CH 3 . The name for the normal structure sometimes uses “n” in front of the name. In 1825 Michael Faraday (1791–1867) discovered an unknown substance that was pro- duced from heated whale oil. Later, Eilhardt Mitscherlich (1794–1863) isolated this new compound and named it benzene.
In the mid-19th century Fredrich von Kekule (1829–1886) determined that carbon was a tetravalent atom with a valence of four, capable of forming a number of different compounds including a great variety of organic molecules found in living tissues. It was Michael Faraday who determined that the molecule for the aromatic compound benzene contained six carbon atoms with a total of 24 bonding electrons, but benzene also had six hydrogen atoms but with
Table2.2:Tenalkanehydrocarbonmolecules. Chemicalname
Numberofisomers Methane
Numericalprefix
Molecularformula
C4H10
3 Hexane
pent(5)
C5H12
5 Heptane
hex(6)
C6H14
9 Octane
hept(7)
C7H16
18 Nonane
oct(8)
C8H18
? Decane
non(9)
C9H20
dec(10)
C10H22
22 | The History and Use of Our Earth’s Chemical Elements only one bonding electron each. When this combination of
24 carbon bonds and six hydrogen bonds were diagrammed, either as a straight line of carbon atoms with the hydrogen atoms attached or as a branching structure, the bonding arrangement just did not add up. Thus, 6 × 4 = 24 for car- bon, and 6 ×1 = 6 for hydrogen resulted in too few electrons to satisfy the octet rule for a linear or branched structure
such as C 6 H 6 . It is reported that Kekule solved this problem one night in a dream in which he saw a different configura- tion that resembled a snake eating its own tail. He woke up excited and, by working the rest of the night, came up with the structure of the benzene ring.
The ring consisted of each carbon atom sharing two of its four bonding electrons with another carbon atom, one valence electron with a partner on the other side on the ring, and one valence electron with a hydrogen atom outside the ring, resulting in the classical hexagonal benzene ring. This answered many questions and was a revolution for organic chemistry because it was now possible to substitute other atoms, molecules, or radicals for one or more of the six hydrogen atoms in one or more connected rings.
Figure2.5:Artist’sdepictionofFriedrich It was not until the 1930s that the complete picture of AugustvonKekule’sdreamofasnake
the benzene ring became clear. Linus Pauling (1891–1994) eatingitsowntail,adreamthataidedvon determined that the benzene rings are of two alternating
Kekuleinsolvingtheproblemofthestruc- structures wherein the double and single bonds between turefortheorganiccompoundbenzene,
the carbon atoms in the ring alternate positions between composedofaringofcarbonatoms.
the two side-by-side rings. Therefore, the molecule should
be thought of as resonance structure that is a hybrid of the two joined rings. Because of this hybrid structure, the benzene molecule is very stable and thus requires high temperatures and pressures while using catalysts to substitute atoms of other elements for the hydrogen on the outside of the ring. Many useful products are made by substituting various aliphatic groups for the hydrogen atoms of benzene. Two benzene rings may join to form aromatic compounds such as naphthalene. Through organic synthesis, put- ting together organic molecules, as with building blocks, millions of new and useful organic compounds are produced.