25.1. Hydrocarbons http://www.ck12.org
FIGURE 25.2
Methane is the simplest hydrocarbon. It
is shown here as (A) a structural formula,
(B) a ball-and-stick model, and (C) a
space-filling model.This version more clearly represents the geometry of the molecule, but it exaggerates the distances between atoms
and is much more difficult to draw than the structural formula. On the right is a space-filling model that gives a more
accurate representation of the space being occupied by each of the individual atoms. However, it can be difficult to
see all of the atoms in larger, more complex molecules when they are presented in this way. For the most part, this
text will use structural formulas when discussing organic molecules.
Alkanes are an example of a class of hydrocarbons called saturated hydrocarbons. Asaturated hydrocarbonis a
hydrocarbon that contains no double or triple bonds. It is referred to as saturated because it contains the maximum
number of hydrogen atoms that can be added to a given carbon skeleton.
Straight-Chain Alkanes
After methane, the next simplest alkane is called ethane (C 2 H 6 ). The two carbon atoms in ethane are connected by
a single covalent bond, and each carbon is also able to bond with three hydrogen atoms. As more carbon atoms
are added to the chain, the alkane series progresses to longer and larger compounds. Structural formulas for ethane,
propane (C 3 H 8 ), and butane (C 4 H 10 ) are shown below.
These alkanes are called straight-chain alkanes because the carbon atoms are connected to one another in one
continuous chain with no branching. Naming and writing structural and molecular formulas for the straight-chain
alkanes is straightforward. The name of each alkane consists of a prefix that specifies the number of carbon atoms
and the ending –ane. The molecular formula follows the pattern of CnH 2 n+ 2 where n is the number of carbons in the
chain. Listed below (Table25.1) are the first ten members of the alkane series.
TABLE25.1: First Ten Members of the Alkane Series
Name Molecular Formula Condensed Structural
FormulaBoiling Point (°C)Methane CH 4 CH 4 −161.0
Ethane C 2 H 6 CH 3 CH 3 −88.5
Propane C 3 H 8 CH 3 CH 2 CH 3 −42.0
Butane C 4 H 10 CH 3 CH 2 CH 2 CH 3 0.5
Pentane C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 36.0
Hexane C 6 H 14 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 68.7