Organic Chemistry

116 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

3.3 The Structure of Alkenes

The structure of the smallest alkene (ethene) was described in Section 1.8. Other

alkenes have similar structures. Each double-bonded carbon of an alkene has three

orbitals that lie in a plane with angles of 120°. Each of these orbitals overlaps an

orbital of another atom to form a bond. Thus, one of the carbon–carbon bonds in a

double bond is a bond, formed by the overlap of an orbital of one carbon with an

orbital of the other carbon. The second carbon–carbon bond in the double bond

(the bond) is formed from side-to-side overlap of the remaining porbitals of the

carbons. Because three points determine a plane, each carbon and the two atoms

singly bonded to it lie in a plane. In order to achieve maximum orbital–orbital overlap,

the two porbitals must be parallel to each other. Therefore, all six atoms of the double-

bond system are in the same plane.

It is important to remember that the bond represents the cloud of electrons that is

above and below the plane defined by the two carbons and the four atoms

bonded to them.

PROBLEM 6

For each of the following compounds, tell how many of its carbon atoms lie in the same

plane:

3.4 Cis–Trans Isomerism

Because the two porbitals that form the bond must be parallel to achieve maximum

overlap, rotation about a double bond does not readily occur. If rotation were to

occur, the two porbitals would no longer overlap and the bond would break

(Figure 3.1). The barrier to rotation about a double bond is Compare

this to the barrier to rotation about a carbon–carbon single bond

(Section 2.10).

(2.9 kcal>mol)

63 kcal>mol.

p

p

a. CH^3 d. CH^3

CH 3

c.

CH 3

b.

CH 3

p orbitals overlap to form

a p bond

H 3 C CH 3

H 3 C CH 3

CC

sp^2

p

the six carbon atoms

are in the same plane

CH 3

CH 3

H 3 C

H 3 C

C C

sp^2

p sp^2

sp^2

s sp^2

s

sp^2

sp^2

3-D Molecule:

2,3-Dimethyl-2-butene

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