Structure of Alkanes

As the term "organic" implies, organic chemistry had its origins in the study of natural compounds extracted from living organisms. It was believed that these compounds contained a "vital force" that was responsible for life processes. This theory of "vitalism" held that organic compounds were somehow beyond the grasp of experimental science. Vitalism was disproved when Friederich Wohler accidentally created the organic compound urea by heating ammonium cyanate, which was classified as inorganic.

Since then, the definition of organic chemistry has been expanded to the study of compounds that contain carbon. Carbon is special for several reasons:

1. Carbon has a strong tendency to concatenate; that is, to form long chains of carbon atoms that provide an important skeletal framework on which to build molecules.
2. Carbon is tetravalent, forming four bonds with neighboring atoms. This allows carbon chains to take on significant branching and provides for a vast range of structural diversity.
3. Carbon forms strong Π-bonds. The presence of double and triple bonds expands the number of important bonding motifs available to organic molecules.

Indeed, almost all molecules of biological importance are built on such carbon frameworks. However, carbon-containing molecules are useful not only to biological systems but in industries as diverse as pharmaceutical medicine, food, clothing, communications, and heavy industry. The task of organic chemists is two-fold: to study organic molecules from a theoretical perspective and to learn new strategies for the synthesis and application of complex molecules in these industries.

The simplest organic molecules are hydrocarbons, compounds that contain only carbon and hydrogen. Two broad classes of hydrocarbons are aliphatic hydrocarbons and aromatic hydrocarbons. Aromatic hydrocarbons contain benzene-like structures, and we'll see in upcoming chapters that such compounds exhibit special chemistry. Aliphatic hydrocarbons don't contain benzene rings and typically consist of carbon chains connected by single, double, and triple bonds. Aliphatic hydrocarbons that contain only single bonds are alkanes. Hydrocarbons that contain double bonds are alkenes and those with triple bonds are alkynes.

We begin our study of organic molecules with alkanes, chains of carbon atoms held by single bonds. Even simple alkanes exhibit the structural diversity mentioned previously: an alkane can be unbranched or branched, and it can also loop back on itself to form a cyclic alkane. Cyclic alkanes will be considered separately later in this chapter. All acyclic alkanes (unbranched and branched) have the characteristic molecular formula C_nH₍2n + 2), where n is the number of carbon atoms in the chain. gives the molecular formulas and Lewis structure for the unbranched, or n-alkanes (n stands for normal). Notice that each n-alkane differs from the next one in the series by a (-CH2-), or methylene group.

As you can see from the structures in the figure above, drawing out full Lewis structures even for simple organic molecules can be quite tedious. Several shorthand notations are used by organic chemists to designate molecules more complex than methane and ethane. A condensed structural formula omits the single bonds to hydrogens. Sometimes even the carbon-carbon bonds are omitted. For example, hexane can be drawn using the following condensed structures.

Since all organic structures contain a hydrocarbon-like framework, it is often convenient to omit the carbon and hydrogen atoms altogether. Such representations are called line-angle structures or skeletal structures. Only lines representing C-C bonds are shown, while it is understood that a carbon atom lies at each vertex and endpoint. Carbon-hydrogen bonds are omitted. The number of implicit hydrogen atoms attached to each carbon is determined by subtracting the number of bonds on the carbon atom from four.