Organic Chemistry

Section 4.11 Addition of Hydrogen • The Relative Stabilities of Alkenes 171

For the radical addition of HCl, the first propagation step is exothermic and the sec-
ond is endothermic. For the radical addition of HI, the first propagation step is en-
dothermic and the second is exothermic. Only for the radical addition of HBr are both
propagation steps exothermic. In a radical reaction, the steps that propagate the chain
reaction compete with the steps that terminate it. Termination steps are always
exothermic, because only bond making (and no bond breaking) occurs. Therefore,
only when both propagation steps are exothermic can propagation compete with ter-
mination. When HCl or HI adds to an alkene in the presence of a peroxide, any chain
reaction that is initiated is terminated rather than propagated because propagation can-
not compete with termination. Consequently, the radical chain reaction does not occur,
and all we have is ionic addition ( followed by or ).

4.11 Addition of Hydrogen • The Relative Stabilities

of Alkenes

In the presence of a metal catalyst such as platinum, palladium, or nickel, hydrogen
adds to the double bond of an alkene to form an alkane. Without the catalyst, the
energy barrier to the reaction would be enormous because the H H bond is so strong
(Table 3.1). The catalyst decreases the energy of activation by breaking the H H
bond. Platinum and palladium are used in a finely divided state adsorbed on charcoal
(Pt/C, Pd/C). The platinum catalyst is frequently used in the form of which is
known as Adams catalyst.

The addition of hydrogen is called hydrogenation. Because the preceding reac-
tions require a catalyst, they are examples of catalytic hydrogenation. The metal cat-
alysts are insoluble in the reaction mixture and therefore are classified as
heterogeneous catalysts. A heterogeneous catalyst can easily be separated from the
reaction mixture by filtration. It can then be recycled, which is an important property,
since metal catalysts tend to be expensive.

CH 3 CH

CH 3 CCH 2 + H 2

CH 3

CHCH 3 CH 3 CH 2 CH 2 CH 3

2-butene

2-methylpropene

cyclohexene cyclohexane

2-methylpropane

butane

Pt/C

+ H 2

Pd/C

+ H 2

Ni

CH 3 CHCH 3

CH 3

PtO 2 ,

¬

¬

(H 2 )

H+ Cl- I-

Mechanistic Tutorial:

Addition of HBr in the pres-

ence of a peroxide

Cl + CH 2 CH 2 ClCH 2 CH 2 ∆H° = 63 − 82 = −19 kcal/mol (or −79 kJ/mol)

Br + CH 2 CH 2 BrCH 2 CH 2 ∆H° = 63 − 69 = −6 kcal/mol (or −25 kJ/mol)

ClCH 2 CH 2 ++HCl ClCH 2 CH 3 Cl ∆H° = 103 − 101 = +2 kcal/mol (or +8 kJ/mol)

I + CH 2 CH 2 ICH 2 CH 2 ∆H° = 63 − 55 = +8 kcal/mol (or +33 kJ/mol)

ICH 2 CH 2 ++HI ICH 2 CH 3 I ∆H° = 71 − 101 = −30 kcal/mol (or −126 kJ/mol)

BrCH 2 CH 2 ++HBr BrCH 2 CH 3 Br ∆H° = 87 − 101 = −14 kcal/mol (or −59 kJ/mol)

exothermic

exothermic

exothermic

Roger Adams (1889–1971)was

born in Boston. He received a Ph.D.

from Harvard University and was a

professor of chemistry at the Univer-

sity of Illinois. He and Sir Alexander

Todd (Section 27.1) clarified the

structure of tetrahydrocannabinol

(THC), the active ingredient of the

marijuana plant. Adams’s research

showed that the test commonly used

at that time by the Federal Bureau of

Narcotics to detect marijuana was

actually detecting an innocuous

companion compound.