The Foundations of Chemistry

A double bond consists of one sigma bond and one pi bond.

As a consequence of the sp^2 hybridization of C atoms in carbon–carbon double bonds,

each carbon atom is at the center of a trigonal plane. The porbitals that overlap to form

the bond must be parallel to each other for effective overlap to occur. This adds the

further restriction that these trigonal planes (sharing a common corner) must also be

coplanar.Thus, all four atoms attached to the doubly bonded C atoms lie in the same plane

(see Figure 8-6). Many other important organic compounds contain carbon–carbon double

bonds. Several are described in Chapter 27.

COMPOUNDS CONTAINING TRIPLE BONDS

One compound that contains a triple bond is ethyne (acetylene), C 2 H 2. Its Lewis formula

is

The VSEPR theory predicts that the two regions of high electron density around each

carbon atom are 180° apart.

Each triple-bonded carbon atom has two regions of high electron density, so valence

bond theory postulates that each is sphybridized (see Section 8-5). Let us designate the

pxorbitals as the ones involved in hybridization. Carbon has one electron in each sphybrid

orbital and one electron in each of the 2pyand 2pzorbitals (before bonding is considered).

See Figure 8-7.

The two carbon atoms form one sigma bond by head-on overlap of the sphybrid orbitals;

each C atom also forms a sigma bond with one H atom. The sphybrids on each atom are

180° apart. Thus, the entire molecule must be linear.

2 p^2 py^2 pz

sp

2 s

C[He] hybridize C[He]

S  N 
 A

 20 
 10  10 e
 shared

HHHCCH CC

8-14

The three porbitals in a set are
indistinguishable. We can label the one
involved in hybridization as “px” to
help us visualize the orientations of the
two unhybridized porbitals on carbon.

342 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

The unhybridized atomic 2pyand 2pzorbitals are perpendicular to each other and to the

line through the centers of the two sphybrid orbitals (Figure 8-8). The side-on overlap

of the 2pyorbitals on the two C atoms forms one pi bond; the side-on overlap of the 2pz

orbitals forms another pi bond.

A triple bond consists of one sigma bond and two pi bonds.

Figure 8-7 Diagram of the two
linear hybridized sporbitals (green)
of an atom. These lie in a straight
line, and the two unhybridized p
orbitals py(tan, cross-hatched) and pz
(tan, hatched) lie in the perpendicular
plane and are perpendicular to each
other.

py

sp sp

pz

180 °

C