Organic Chemistry

302 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

3-D Molecules:

2,3-Pentadiene;

1,4-Pentadiene;

1,3-Pentadiene

Tutorial:

Orbitals used to form

carbon–carbon single bonds

1,4-pentadiene (an isolated diene) which, in turn, is greater than than that of 1,3-

pentadiene (a conjugated diene).

From the relative values for the three pentadienes, we can conclude that con-

jugated dienes are more stable than isolated dienes, which are more stable than cumu-

lated dienes.

Why is a conjugated diene such as 1,3-pentadiene more stable than an isolated diene?

Two factors contribute to the stability of a conjugated diene. One is the hybridization of the

orbitalsforming the carbon–carbon single bonds. The carbon–carbon single bond in

1,3-butadiene is formed from the overlap of an orbital with another orbital, where-

as the carbon–carbon single bonds in 1,4-pentadiene are formed from the overlap of an

orbital with an orbital.

In Section 1.14, you saw that the length and strength of a bond depend on how close

the electrons in the bonding orbital are to the nucleus:The closer the electrons are to

the nucleus,the shorter and stronger is the bond. Because a 2selectron is closer, on

average, to the nucleus than is a 2pelectron, a bond formed by overlap is

shorter and stronger than one formed by overlap (Table 8.2). (An orbital

has 33.3% scharacter, whereas an orbital has 25% scharacter.) Thus, a conjugat-

ed diene has one stronger single bond than an isolated diene, and stronger bonds cause

a compound to be more stable.

Electron delocalizationalso causes a conjugated diene to be more stable than an

isolated diene. The electrons in each of the double bonds of an isolated diene are

localizedbetween two carbons. In contrast, the electrons in a conjugated diene are

delocalized. As you discovered in Section 7.6, electron delocalization stabilizes a mol-

ecule. Both the resonance hybrid and the molecular orbital diagram of 1,3-butadiene in

Figure 7.9 show that the single bond in 1,3-butadiene is not a pure single bond, but has

partial double-bond character as a result of electron delocalization.

CH 2 CH CH CH 2

resonance hybrid

CH 2 CH CH CH 2

−+

CH 2 CH CH CH 2

+−

CH 2 CH CH CH 2

resonance contributors

delocalized

electrons

p

p

sp^3

sp^3 – sp^2 sp^2

sp^2 – sp^2

CH 2 CH CH CH 2 CH 2 CH CH 2 CH CH 2

1,3-butadiene 1,4-pentadiene

single bonds formed by

sp^3 −sp^2 overlap

single bond formed by

sp^2 −sp^2 overlap

sp^2

sp^3

sp^2 sp^2

conjugated diene

relative stabilities of dienes

>>isolated diene cumulated diene least

stable

most

stable

-¢H°

2,3-pentadiene

a cumulated diene

1,4-pentadiene

an isolated diene

CH 3 CH CCHCHCH 3 + 2 H 2 3 CH 2 CH 2 CH 2 CH 3

Pt

∆Hº=

∆Hº=

∆Hº=

−70.5 kcal/mol (−295 kJ/mol)

CH 2 CHCH 2 CH CH 2 + 2 H 2 CH 3 CH 2 CH 2 CH 2 CH 3

Pt

−60.2 kcal/mol (−252 kJ/mol)

1,3-pentadiene

a conjugated diene

CH 2 CHCH CHCH 3 + 2 H 2 CH 3 CH 2 CH 2 CH 2 CH 3

Pt

−54.1 kcal/mol (−226 kJ/mol)