http://www.ck12.org Chapter 25. Organic Chemistry
Carbon
At one time in history, it was thought that only living things were capable of synthesizing the carbon-containing
compounds present in cells. For that reason, the term organic was applied to those compounds. Eventually, it was
proven that carbon-containing compounds could be synthesized from inorganic substances, but the term organic has
remained. Currently,most compounds that contain covalent bonds between carbon and at least one other atomare
consideredorganic compounds. However, for historical reasons, some oxides of carbon are considered inorganic,
such as the carbonate ion (CO 32 −) and carbon dioxide (CO 2 ). Organic chemistry is the study of organic compounds.
Organic chemistry is a vast and complex subject. There are over a million known organic compounds, which is
far more than the number of inorganic compounds. The reason lies with the uniqueness of carbon’s structure and
bonding. Carbon has four valence electrons, so it makes four separate covalent bonds when it forms compounds.
Carbon has the ability to make strong single, double, or triple covalent bonds with other carbon atoms, so long chains
of carbon or ringed structures can be produced. Carbon also readily makes covalent bonds with other elements,
primarily hydrogen, oxygen, nitrogen, the halogens, and several other nonmetals. Illustrated below (Figure25.1)
are the ball-and-stick models of two organic compounds.
FIGURE 25.1
(A) Stearic acid is composed of many car-
bon (black) and hydrogen (white) atoms,
along with two oxygen (red) atoms. (B)
Methionine is composed of carbon, hydro-
gen, oxygen, nitrogen (blue), and sulfur
(yellow) atoms.Hydrocarbons
Ahydrocarbonis an organic compound that is made up of only carbon and hydrogen. A hydrocarbon is the
simplest kind of organic molecule and is the basis for all other more complex organic compounds. Hydrocarbons can
be divided into two broad categories.Aliphatic hydrocarbonsare hydrocarbons that do not contain any aromatic
rings, whilearomatic hydrocarbonscontain one or more aromatic rings. We will define and discuss aromatic rings
in the next lesson. For now, we turn our attention to various types of aliphatic hydrocarbons.
Alkanes
Analkaneis a hydrocarbon in which there are only single covalent bonds. The simplest alkane is methane, with the
molecular formula CH 4. The central carbon atom makes four single covalent bonds to hydrogen atoms.
Shown above (Figure25.2) are three different depictions of the methane molecule, each of which is useful for
conveying different types of information. On the left is a structural formula, which was introduced in the chapter
Covalent Bonding. A structural formal clearly shows the bonding pattern of the entire molecule. The disadvantage
to a structural formula is that it shows a molecule in only two dimensions and does not give an indication of the
shape or bond angles. Recall that the methane molecule is tetrahedral, with H-C-H bond angles of 109.5° and sp^3
hybridization around the central carbon. In the middle of the figure above (Figure25.2) is a ball-and-stick model.