Organic Chemistry

Section 4.2 Carbocation Stability 143

The greater the number of alkyl

substituents bonded to the positively

charged carbon, the more stable the

carbocation is.

The question now is, Why is the tert-butyl cation formed faster than the isobutyl
cation? To answer this, we need to take a look at the factors that affect the stability of
carbocations and, therefore, the ease with which they are formed.

4.2 Carbocation Stability

Carbocations are classified according to the number of alkyl substituents that are
bonded to the positively charged carbon: A primary carbocationhas one such sub-
stituent, a secondary carbocationhas two, and a tertiary carbocationhas three. The
stability of a carbocation increases as the number of alkyl substituents bonded to the
positively charged carbon increases. Thus, tertiary carbocations are more stable than
secondary carbocations, and secondary carbocations are more stable than primary car-
bocations. Notice that when we talk about the stabilities of carbocations, we mean
their relativestabilities: Carbocations are not stable species; even the relatively stable
tertiary carbocation is not stable enough to isolate.

Why does the stability of a carbocation increase as the number of alkyl substituents
bonded to the positively charged carbon increases? Alkyl groups decrease the concen-
tration of positive charge on the carbon—and decreasing the concentration of positive
charge increases the stability of the carbocation. Notice that the blue—recall that blue
represents electron-deficient atoms—is most intense for the least stable methyl cation
and is least intense for the most stable tert-butyl cation.

CH 3 CCH 2 +

CH 3

CH 3 CCH 3

CH 3

CH 3 CCH 3

CH 3

Cl

Cl−

Cl−

tert-butyl chloride

only product formed

tert-butyl cation

isobutyl chloride

not formed

isobutyl cation

CH 3 CHCH 2

CH 3

CH 3 CHCH 2 Cl

CH 3

HCl

+

+

electrostatic

potential map

for the tert-butyl cation

electrostatic

potential map

for the isopropyl cation

electrostatic

potential map

for the ethyl cation

electrostatic

potential map

for the methyl cation

R >>>

R

a tertiary

carbocation

R

C+ R

H

a secondary

carbocation

R

C+ R

H

H

a primary

carbocation

C+ H

H

H

methyl cation

C+

relative stabilities of carbocations

most stable least stable

Carbocation stability: 3 °> 2 °> 1 °

George Olahwas born in Hungary

in 1927 and received a doctorate

from the Technical University of

Budapest in 1949. The Hungarian

revolution caused him to emigrate to

Canada in 1956, where he worked as

a scientist at the Dow Chemical

Company until he joined the faculty

at Case Western Reserve University

in 1965. In 1977, he became a profes-

sor of chemistry at the University of

Southern California. In 1994, he re-

ceived the Nobel Prize for his work

on carbocations.