75 Aliphatic Hydrocarbons
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Alkanes
Alkanes are relatively unreactive hydrocarbons that contain no double or triple bonds in their carbon skeletons.
LEARNING OBJECTIVES
Identify the basic properties of alkanes
KEY TAKEAWAYS
Key Points
- Alkanes are saturated hydrocarbons, which means that their carbon backbones contain no double or triple bonds.
- Due to the valence configuration of carbon, there are a variety of alkane isomers, which are commonly separated into linear and branched structures.
- Alkanes are used in a number of industrial applications and are found in natural gas and petroleum.
Key Terms
- saturated hydrocarbon: A compound consisting entirely of hydrogen and carbon, composed entirely of single bonds.
- hydrocarbon: A compound consisting only of carbon and hydrogen atoms.
Alkanes, also called paraffins, are a class of hydrocarbons that are fully saturated with hydrogen. They contain no double or triple bonds in their carbon skeletons and, therefore, have the maximum number of carbon to hydrogen covalent bonds. This is in contrast to alkenes and alkynes, which contain double and triple bonds and are known as unsaturated hydrocarbons.
Structure of Alkanes
Alkanes have the general formula [latex]\text{C}_\text{n} \text{H}_\text{2n+2}[/latex]. For example, an alkane with 2 (n) carbon atoms, will have 6 (2n + 2) hydrogen atoms. Their adjacent atoms are connected with sigma bonds and form tetrahedral centers around the carbon atoms. As these bonds are all single bonds, there is free rotation around all connections. Each carbon atom has four bonds (either C-H or C-C bonds), and each hydrogen atom is joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the carbon skeleton or carbon backbone. The number of carbon atoms is used to define the size of the alkane (e.g., C2-alkane).
An alkyl group, generally abbreviated with the symbol R, is a functional group or side-chain that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms; for example, R could represent a methyl or ethyl group. An alkyl group is a piece of a molecule with the general formula [latex]( \text{CH}_3 )_\text{n}[/latex], where n is any integer. For example, a methyl group [latex]( \text{CH}_3 )[/latex] is a fragment of a methane molecule [latex]( \text{CH}_4 )[/latex]. In this example, n=1.
The simplest possible alkane is methane [latex]( \text{CH}_4 )[/latex]. Saturated oils and waxes are examples of larger alkanes where the number of carbons in the carbon backbone is greater than ten.
In linear alkanes, the carbon atoms are joined in a snake-like structure. In branched alkanes, the carbon backbone splits off in one or more directions. In cyclic alkanes, the carbon backbone is linked so as to form a loop. Cyclic and branched alkanes will be discussed in greater detail in subsequent sections.
Nomenclature for Alkanes
Alkanes are named with the suffix “-ane” following the hydrocarbon prefixes. The series contains m