Organic Chemistry

144 CHAPTER 4 Reactions of Alkenes

C

H

H

H

hyperconjugation

δC

δ+ +

H

H

+

H C

H

H

CH 3 CH 2 +

ethyl cation

+CH

3

methyl cation

Figure 4.1N
Stabilization of a carbocation by
hyperconjugation: The electrons of
an adjacent C H bond in the ethyl
cation spread into the empty p
orbital. Hyperconjugation cannot
occur in a methyl cation.

¬

How do alkyl groups decrease the concentration of positive charge on the carbon?

Recall that the positive charge on a carbon signifies an empty porbital (Section 1.10).

Figure 4.1 shows that in the ethyl cation, the orbital of an adjacent C H bond can

overlap with the empty porbital. No such overlap is possible in the methyl cation.

Movement of electrons from the bond orbital toward the vacant porbital of the ethyl

cation decreases the charge on the carbon and causes a partial positive charge to

develop on the carbon bonded by the bond. Therefore, the positive charge is no

longer localized solely on one atom, but is spread out over a greater volume of space.

This dispersion of the positive charge stabilizes the carbocation because a charged

species is more stable if its charge is spread out (delocalized) over more than one atom

(Section 1.19). Delocalization of electrons by the overlap of a bond orbital with an

empty porbital is called hyperconjugation. The simple molecular orbital diagram in

Figure 4.2 is another way to explain the stabilization achieved by the overlap of a filled

C¬H bond orbital with an empty s porbital.

s

s

sp^2

s

¬ s

empty p

orbital

filled σ

bond

CH

Energy

Figure 4.2N

A molecular orbital diagram

showing the stabilization achieved

by overlapping the electrons of a

filled C H bond with an empty p

orbital.

¬

Hyperconjugation occurs only if the bond orbital and the empty porbital have

the proper orientation. The proper orientation is easily achieved because there is

free rotation about a carbon–carbon bond (Section 2.10). In the case of the tert-

butyl cation, nine C H bond orbitals can potentially overlap with the empty p

orbital of the positively charged carbon. The isopropyl cation has six such orbitals,

and the ethyl cation has three. Therefore, there is greater stabilization through hy-

perconjugation in the tertiary tert-butyl cation than in the secondary isopropyl

cation and greater stabilization in the secondary isopropyl cation than in the prima-

ry ethyl cation.

CH 3

CH 3

CH 3

tert-butyl cation

C+ CH 3

H

CH 3

isopropyl cation

C+ CH 3

H

H

ethyl cation

most stable >>C+ least stable

¬ s

s

s