Organic Chemistry

bonded to its carbons, and the least stable of the three alkenes has only one alkyl
substituent (an isopropyl group) bonded to its carbons. Thus, it is apparent that
alkyl substituents bonded to the carbons of an alkene have a stabilizing effect on
the alkene. We can, therefore, make the following statement:The more alkyl sub-
stituents bonded to the carbons of an alkene, the greater is its stability.(Some stu-
dents find it easier to look at the number of hydrogens bonded to the carbons. In
terms of hydrogens, the statement is,the fewer hydrogens bonded to the carbons of
an alkene, the greater is its stability.)

PROBLEM 27

The same alkane is obtained from the catalytic hydrogenation of both alkene A and alkene

B. The heat of hydrogenation of alkene A is 29.8 kcal mol (125 kJ mol), and the heat of

hydrogenation of alkene B is 31.4 kcal mol (131 kJ mol). Which alkene is more stable?

PROBLEM 28

a. Which of the following compounds is the most stable?

b. Which is the least stable?

c. Which has the smallest heat of hydrogenation?

Both trans-2-butene and cis-2-butene have two alkyl groups bonded to their
carbons, but trans-2-butene has a smaller heat of hydrogenation. This means that the
trans isomer, in which the large substituents are farther apart, is more stable than the
cis isomer, in which the large substituents are closer together.

sp^2

CH 2 CH 3

CH 2 CH 3

CH 2 CH 3

CH 2 CH 3

CH 2 CH 3

CH 2 CH 3

CH 2

CH 2 CH 3

> >

> >

sp^2

sp^2

sp^2

sp^2

sp^2

sp^2

Section 4.11 Addition of Hydrogen • The Relative Stabilities of Alkenes 173

CC >

RR

R R

CC

RR

R H

> CC

RH

R H

> CC

RH

H H

relative stabilities of alkyl-substituted alkenes

most stable

least stable

CH 3 C CHCH 3

CH 3

∆H° = −26.9 kcal/mol

CH 2 CCH 2 CH 3

CH 3

Potential energy

∆H° = −28.5 kcal/mol

CH 3 CHCH CH 2

CH 3

CH 3 CHCH 2 CH 3

∆H° = −30.3 kcal/mol

CH 3

most stable

least stable >Figure 4.7

The relative energy levels

(stabilities) of three alkenes that

can be catalytically hydrogenated

to 2-methylbutane.

CC

H 3 CH

HCH 3

+ H 2

Pd/C

CH 3 CH 2 CH 2 CH 3

CC

H 3 C CH 3

HH

+ H 2

Pd/C

CH 3 CH 2 CH 2 CH 3

trans-2-butene

cis-2-butene

heat of hydrogenation kcal/mol kJ/mol

− 115

− 120

−27.6

−28.6

27.6

28.6

∆H°

The fewer hydrogens bonded to the

carbons of an alkene, the more stable

it is.

sp^2

Alkyl substituents stabilize both alkenes

andcarbocations.

Platinum and palladium are expen-

sive metals, so the accidental finding

by Paul Sabatier 1854–1941)that

nickel, a much cheaper metal, can

catalyze hydrogenation reactions

made hydrogenation a feasible

large-scale industrial process. The

conversion of plant oils to margarine

is one such hydrogenation reaction.

Sabatier was born in France and

was a professor at the University of

Toulouse. He shared the 1912 Nobel

Prize in chemistry with Victor

Grignard (p. 468).