Organic Chemistry

30 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

H H

H HH

π bond π bond

σ bond

σ bond

a. b. c.

H

H

H

H

CC C C

σ bond formed

by sp^2 – s overlap

σ bond formed by

sp^2 – sp^2 overlap

C C

HH

H

atom is bonded to three atoms that define a plane, it is called a trigonal planar carbon.

The unhybridized porbital is perpendicular to the plane defined by the axes of the

orbitals (Figure 1.15).

The carbons in ethene form two bonds with each other. This is called a double

bond. The two carbon–carbon bonds in the double bond are not identical. One of the

bonds results from the overlap of an orbital of one carbon with an orbital of the

other carbon; this is a sigma bond because it is formed by end-on overlap

(Figure 1.16a). Each carbon uses its other two orbitals to overlap the sorbital of a

hydrogen to form the bonds. The second carbon–carbon bond results from

side-to-side overlap of the two unhybridized porbitals. Side-to-side overlap of por-

bitals forms a pi bond (Figure 1.16b). Thus, one of the bonds in a double bond is a

sbond and the other is a bond. All the p C¬Hbonds are bonds.s

(p)

C¬H

sp^2

(s)

sp^2 sp^2

sp^2

side view top view

p

120 °

sp^2

sp^2

sp^2

Figure 1.15N
An hybridized carbon. The three
degenerate orbitals lie in a
plane. The unhybridized porbital is
perpendicular to the plane. (The
smaller lobes of the orbitals are
not shown.)

sp^2

sp^2

sp^2

Figure 1.16

(a) One bond in ethene is a bond formed by overlap, and the bonds

are formed by overlap. (b) The second bond is a bond formed by side-to-side

overlap of a porbital of one carbon with a porbital of the other carbon. (c) There is an

accumulation of electron density above and below the plane containing the two carbons

and four hydrogens.

sp^2 – s C¬C p

C¬C s sp^2 – sp^2 C¬H

3-D Molecule:

Ethene

The two porbitals that overlap to form the bond must be parallel to each other

for maximum overlap to occur. This forces the triangle formed by one carbon and

two hydrogens to lie in the same plane as the triangle formed by the other carbon

and two hydrogens. This means that all six atoms of ethene lie in the same plane,

and the electrons in the porbitals occupy a volume of space above and below the

plane (Figure 1.16c). The electrostatic potential map for ethene shows that it is a

nonpolar molecule with an accumulation of negative charge (the orange area) above

the two carbons. (If you could turn the potential map over, a similar accumulation of

negative charge would be found on the other side.)

p

1.33 A°

1.08 A°

121.7°

CC116.6°

H

H

H

H

a double bond consists of

one σ bond and one π bond

ball-and-stick model

of ethene

space-filling model

of ethene

electrostatic potential map

for ethene