25.1. Hydrocarbons – The Backbone of Organic Chemistry http://www.ck12.org
Triple bonds can be identified in a similar way, except that the suffix-yneis used instead of-ene(to indicate that we
are dealing with an alkyneinstead of an alkene).
Cyclic Hydrocarbons
Many organic compounds are cyclic in structure. The compound cyclohexane involves a ring of six carbon atoms,
each of which is also bonded to two hydrogen atoms. Figure25.8 shows a few different representations of the
cyclohexane molecule.
FIGURE 25.8
Ways of representing the structure of cy-
clohexane.The structure on the right gives the complete picture, where all atoms are explicitly drawn. The middle structure
shows a flat representation of the molecule based on the standard shorthand rules for drawing organic structures. The
folded structure on the left highlights an important point about organic chemistry –the three-dimensional structure
of a molecule is not always portrayed accurately by flat drawings. The true structure of the cyclohexane molecule
has a puckered shape that looks more like the structure on the left than the flat hexagon in the center. The preferred
three-dimensional conformations of organic molecules often play an important role in how the molecule reacts. The
following structures illustrate some of the interesting and complex shapes organic molecules can take on:
FIGURE 25.9
Complex organic structures.Aromatic hydrocarbons are a special subset of cyclic hydrocarbons. Although many "aromatic" compounds have
distinctive odors, this word is used very differently in organic chemistry than in everyday life. The benzene ring is
the foundational structure for most aromatic compounds:
The illustrations in theFigure25.10 give different perspectives on the actual structure of the molecule. The left-
hand illustration shows the six hydrogen atoms attached to the six carbons and indicates that there are three double
bonds in the ring, while the next structure shows this symbolically. A more realistic picture is given by the next two
models. The circle shows the reality of the bonding. The three pi bonds in the ring overlap one another and form
a cloud of electrons above and below the plane of the ring. Benzene and its derivatives do not undergo the same
reactions as most carbon-carbon double bonds, due to the special stability that is inherent in this type of interactive