The Foundations of Chemistry

SATURATED HYDROCARBONS

ALKANES AND CYCLOALKANES

The saturated hydrocarbons,or alkanes,are compounds in which each carbon atom

is bonded to four other atoms. Cycloalkanesare saturated hydrocarbons that

contain carbon atoms joined to form a ring.

Alkanes

Each H atom in a saturated hydrocarbon is bonded to only one C atom. Saturated hydro-

carbons contain only single bonds. Petroleum and natural gas are composed mostly of

saturated hydrocarbons.

In Section 8-7 we examined the structure of the simplest alkane, methane,CH 4. We

saw that methane molecules are tetrahedral with sp^3 hybridization at carbon (Figure

27-2).

Ethane,C 2 H 6 , is the next simplest saturated hydrocarbon. Its structure is quite similar

to that of methane. Two carbon atoms share a pair of electrons. Each carbon atom also

shares an electron pair with each of three hydrogen atoms. Both carbon atoms are sp^3

hybridized (Figure 27-3). Propane,C 3 H 8 , is the next member of the family (Figure 27-4).

27-1

The term saturatedcomes from early

studies in which chemists tried to

add hydrogen to various organic

substances. Those to which no more

hydrogen could be added were called

saturated, by analogy with saturated

solutions.

27-1 Alkanes and Cycloalkanes 1043

(a)

CH 4 or H C

H

H

H

H

H

H

C

H

1 s

sp^3

(b)

(c)

(d)

C

H

H H

H

(e)

Figure 27-2 Representations of a molecule of methane, CH 4. (a) The condensed and

Lewis formulas for methane. (b) The overlap of the four sp^3 carbon orbitals with the s

orbitals of four hydrogen atoms forms a tetrahedral molecule. (c) A ball-and-stick model,

(d) a space-filling model of methane, and (e) a three-dimensional representation that uses

the wedged line to indicate a bond coming forward and a dashed line to represent a bond

projecting backward.